"You don't need to be confused, just understand it". What a quote! I rate these videos very highly but that was a classic jumble of unintended words. Apart from that keep up the good work. It is very enjoyable to watch and learn.
Thanks Dave, this is another one of your best posts ever. I'd always assumed the best engineers in the world were already building multimeters and it would be a lot harder than this to improve on their designs. It's great to see how you break down a complex problem and make the solution fit into a typical multimeter form factor. I'll be following the development of your new multimeter closely!
2:50 The reason for replacing the decimal point with "R" for ohms was because back in the day when making photocopies of schematics on low resolution photocopiers sometimes the decimal point would get lost in the copy. The same holds true for the "K" and "M" for Kilo-Ohms and Meg-Ohms. BTW I didn't even get a notification of this video and yes I checked the "Send Notification" box. I just stumbled across it looking for something else.
Brilliant, I have had to switch to the Amps range to measure circuits in the past or bumped up the power supply but I had never investigated the reason my 3.3v and 5v circuits were not behaving as expected when trying to measure current. Thanks for the video, another thing to keep in mind. Time to get another multimeter to measure the circuit voltage at the same time :-)
I think I have gone a bit overboard - I have 6 of the cheapest ones you can get that I just don't care if they go bang. Two intermediate quality for day to day use. Two decent flukes and two bench. I think I'm missing an AVO.
Or a multimeter with simultaneous voltage & current measurement! I'm thinking of making a handheld dual channel and dual measurement (for measuring efficiency) multimeter. Then you only need one in 99.99 % of the cases. And I'll probably incorporate an electronic resettable fuse with a small 10 A relay and an opamp. That way, the end user saves money on expensive HRC fuses since it can be reset pushing a button. Not to mention the much lower burden voltage!
18:10, the smaller the conductor, the lower the power can be passed through it (higher resistance). fuses work on a bit more precise version of that. for example 30 awg (copper) wire will have a higher resistance vs 12 awg per 50 ft (just shy of every 15¹/⁴ m), fuses use one or any combination of copper, zinc, silver or aluminum to get it's requirements.
I would like to see a multimeter that can do guarded resistance measurements. These work by applying a voltage on one side of a resistor, then using an opamp current to voltage converter on the other side of the resistor. The inverting terminal acts as a virtual ground if the non-inverting input is grounded. This allows you to cancel out parallel current paths on in-circuit resistor measurements by grounding (guarding) points in the circuit. In-circuit test equipment works this way. Keithly makes some good SourceMeters that do this, but these cost about $5000. A little $100 meter that had a third guard terminal for in-circuit measurements would be really handy for troubleshooting. I've seen some LCR meters that had a guard/shield plug, but I don't believe they use the opamp current to voltage (virtual-ground) current measurement technique.
Dave, your µCurrent saved my bacon last week. I found crappy Chinese MLCCs consuming 100* more battery power than the actual RTC/SRAM. Replacing them with quality Murata ones fixed the thing.
Hi Dave, Maybe as a follow up video you could explain that you can use the sense terminals of your power supply after your ammeter to get "rid" of the burden voltage, including disadvantages like the measuring current of the sense terminals and others. Also regarding the resistance of the HRC fuses: I *think* it is because HRC fuses are usually sand or ceramic filled, and because of the higher heat conduction of these, the wire needs to be thinner to break at a certain current, because the heat is transferred away from the wire. I would also assume the tempco of HRCs is lower because of that reason.
Hey Dave! Did you mean traps for young players in the description instead of traps for YOUR players? Another awesome video. I've been trying to get my college to use some of your fundamentals stuff in their entry level classes. For example, the oscilloscope video you just did is fantastic.
Hi Dave...Good "vidayo" on an improved "moldymeter"! Rather than using a mux for the different current ranges, could you switch that via the range selection switch, or, does that cause some potential isolation issues?
The burden voltage is exactly why you need to have at least one precision 0.1Ohm 4 terminal resistor box and a high input impedance digital multimeter on your bench. Fortunately a box like this it is not so hard to make. Apart from all the regular stuff you only need some proper connectors and nichrome wire. Then you can just hook it up and measure voltage drop across it when needed. The other good thing is that the resistor stays connected all the time your circuit operates. So you only need one quality multimeter to probe around.
I recently bought a Keysight U1242C. One of the reasons I bought it was that it seemed to have amongst the lowest resistance shunts I'd seen. It's 0R5 on the A range and 30R on the combined mA/uA range if I understand the datasheet correctly (I haven't actually measured it yet). At work we even managed to program a 3.3V circuit with the multimeter inline, while our other Flukes had to be switched form uA to the mA range in order to program. Well, actually, the combined uA/mA range meant it just autoranged to mA, but I tried with a 33R resistor and it worked... Apart from that it's a reasonable meter, albeit with a really spongy range switch. I'm not much of a poster, in fact I don't think I've ever posted anything on the forums, but if there's interest I could post some pictures. I've got a BM869s that I could compare it to, but I'm rather reluctant to open them up... I don't want to dissect my babies... even if opening them means i'm "taking ownership" of them... :(
EEVblog, are you ever gonna do more Old game system tear downs? The N64 Tear down is still one of my top fav vids from you, along with crap product tear downs.
The reason why they came up with the 0R01 is because on print the dot tend to dissapear when you make a photocopy or the paper is a bit worn. For 0R01 it's not too bad, you will read it as 001 and guess correctly that it is 0.01. However 1.1 ohms for example... 1.1 or 11? 1R1 is very clear!
I don't know exactly, but I guess because R is a normal letter of the alphabet (you also often see u instead of µ). However, to use fewer symbols would it be valid to write R01? I'm not sure but I think I've seen this before.
+Klaufmann R01 would be the number of one of the resistors on the diagram, better write the extra zero. 10m could get easily changed to 10M by a glitch, which would be a 1000 million times error between design and production. To me the big question is when and why someone changed the R from meaning ohms on typewriters to meaning any other decimal point, since those would previously be written as the appropriate unit, such as 3V3, 2A2 etc.
If the 600mA fuse has too high a resistance, why not replace it with three 200mA fuses in parallel? According to the chart shown at 15:45, 200mA fuses have a typical cold resistance of 1.83 ohms -> 0.61 ohms altogether. Burden voltage at 5000mA would drop from 500mV to 300mV. If that's not enough, can you get six 100mA fuses?
Adding an x10 Amplifier to the standard MM concept is, on my opinion, the easiest solution. That reduces burden voltage from 200/400/500 mV to 20/40/50 mV FS. That should be small enough...
I would favour a 0.01R / 0.09R / 0.9R / 9R / 90R divider chain, switching the mA/uA input to a tap in the chain, measuring from the 90R end (via x10 amp except for 10A scale). This avoids the multiway switch contact resistance from being sensed, and allows make-before-break on the range switch within the current section.
+EEVblog Hi Dave. I've always thought that top-of-the-line measurement instruments should use current transformers instead of shunts. What are your thoughts?
Hey Super Dave I think you need to re-cad the bridge rectifier there is nothing to protect against over negative milliamps on the current version It needs to turn 90 degrees to the right. for .7 volts peak to peak protection.
Shouldn't be much surprise that a fuse has some resistance, after all the power it drops over that is what makes it blow. Afaik HRC fuses typically need more power dropped due to the surrounding material being much more efficient in transporting excess heat away than just the air (or in some cases even vacuum) of glass fuses. You should open one fresh one, and one catastrophically failed one (HRC fuses that is). Given that the 300+ bucks range multimeters usually don't do these things make me think that the industry doesn't really need it though.
@EEVblog One thing that confuses me about the op amp portion of the current shunt (something I would love details on), how does something like the MAX4238 deal with potentially up to 1000V common mode voltage? Or does that only refer to the voltage difference range across the shunt?
Since the OPAmp only "measures" across the shunt in current mode, it doesn't matter. When measuring current, the meter gets put "in line" with the current-to-measure. So as far as the meter can tell, there is no 1000s of volts (referenced to ground). Because the meter isn't connected to ground.
If the fuses are a problem, what about using a MOSFET to isolate the probe and a dedicated high current sensor that would switch the MOSFET off? It wouldn't be much slower than the blow time in some of those fuses...
The biggest problem with that is that even power MOSFETs require power dissipation for relatively low currents of 10 A, something difficult in a handheld multimeter. Relays would be a better option.
Could you go with a remote sensing configuration like a hall sensor and remove burden voltage alltogether? Or are hall sensors just not sensitive enough or too noisey for measuring mA current levels (even with multiple wraps and/or magnetic concentrators)?
The voltage on the input takes time to settle to 0 after the input current changes. Using it to measure something like a microcontroller power rail would mean the rail voltage would jump all over the place.
I have build transimpedance amplifiers with a gain of 10^9 V/A for my Masterthesis. I used the OPA 129 which has a offset current of typically 100fA. Cant u suck a bit on the current that flows through the resistor and amplify that to a handleable Voltage? I mean takeing 10^-9 A froma 10^-6 A current wont matter that much. If needed the OPA 129 can handle 10^10 V/A in a single step at 300Hz bandwidth. Wouldnt that be enough for a multimeter?
It is a very very usefull , after i see this than maybe I'm going to rethink my circuit to measure the characteristics of transistors making it very easy. Thank's a lot Dave and sorry for may poor english i'am italian .. bye.
I was surprised when I noticed this problem when trying to fix one of my son's toys. Not that it mattered, I mostly needed to know there was power at all. Now I do have a question on some strange behaviour of the circuit I'm testing that I will need a diagram for. Ok, electronic wizzes here, where is the best forum to post the question with such a diagram (in DaveCAD of course)?
Ultra RC so if I stuck probes across a 1k resistor that's dropping 10V then all of a sudden my little 1R resistor has 10 Amps going through without protection. Obviously the fuse would blow, but pretend it's a value to small to blow the fuse but high enough to burn up the resistor. Well if you're new, check out VI curves of diodes. They drop about .7 volts once a certain current is reached. Now if I have 2 diodes like in a bridge, I drop 1.4V. If you know your circuit analysis, it "clamps" the voltage at 1.4 volts. That means the node (connection) between the fuse, diodes, and resistor is at 1.4V no matter what and the excess current is directed to ground. So now you have (1.4v-0v)/1ohm which is a much more manageable current. Also if the current is too crazy through the diode then the fuse will just blow and all you need is a cheap fuse to fix it. oh and the diode bridge is incase you have AC voltages. It clams both the negative and positive to +-1.4 V
You can also think about what would happen if there were 600 mA (rating of the fuse) going across the 100R sense resistor. P=I^2R, so that resistor would dissipate 36 watts. Even though the fuse wouldn't blow, the resistor would still burn up.
Dave, You show that (through that 'SPDT' switch action how the mA and uA ranges of the DMM switch to different shunt resistors for these ranges. However all my meters have NO switch for these two ranges--it has just mA and Amps, so how are different shunt resistors selected for each of these reading ranges? THANKS for these great videos--I am about halfway through ALL of them!!
They are mostly just a piece of wire. (The 0,... Ohm ones anyway) So it is hard to tell what the exact "specified" maximum power rating is. Depends on how hot the "resistor" is allowed to get.
An ACS712 has a typical resistance of 12mR and can survive up to 60A for a short time. You can then use a 50A fuse, which will be far lower than 1R. To add some safety, the input of the ACS is isolated of the rest of the multimeter.
my main question that all of this raises is if i had a shitty dmm with glass fuses, and i try to do the right thing and upgrade them to nice hrc fuses, your saying that i now just screwed up all of my future measurements with it because the calibration is now off? ?? ?
Well, My suspicion is that it would mean that a small over current that blew one of your instruments fuses would require you to send the instrument in for service, as it would require recalibration, and the fuse itself, being specially produced for the application would be even more expensive. The resistance of a fuse is part of the formula for it's rated current, so your instrument design would be constrained to the resistance of the available fuse materials, and it might be that none of the fuse materials that would have the required resistance would be linear and stable enough for precision resistors. There are resistors that are fusible, but I don't think I have seen any with less than 1% tolerance and they have rather too high resistances if the purpose is to reduce the burden voltage. So, my guess is that it simply more flexible, cheap and convenient to make an accurate instrument with a separate resistor and fuse than using a fusible resistor.
Well, It probably is possible to overcome any issue with making and using a high rupture capacity precision fusible current shunt, the real question is if it is possible to do so economically, safely, and without adding problems for the end user that would give your instrument a bad name. Of course, such things are not really decided by some random people discussing it in the comment section of some TH-cam video, it is ultimately the people who promote a new approach that has to show it is viable by actual research and experiment.
But it also only goes through the 11A rated resistor. In fact Dave's solution boils down to simply setting the switch one range higher to get less meter resistance at the cost of one digit of precision.
I am at 18 minutes, dont know it you'll do it later but it would be cool to log burden voltage over a fuse at half its rated current. I think i have done that with piss end fuses in the past and had really bad results.
I had a project where I've encountered burden voltage being a problem, but I didn't needed really fast measurement, so I solved that using a higher resolution ADC running on battery(to chancel noise as much as possible) hooked up to an arduino, and my PC using just the intermal amp of the ADC, and pretty low value current shunt. I've got pretty low burden voltage, and high enough resolution, but it's not portable and the refresh rate was terrible as well... But it worked... :)
What about making a special HRC fuse with a stable controlled resistance that can also serve as the current shunt, It will be expensive to change if it blows, but it won't be possible to be bridged.
I'm sure multimeter design engineers have thought about his in some form or variation but I guess the added cost of the extra components is not really worth the added value. Also having that combination of input jacks and ranges might seem confusing for the user. We could also consider there is a right tool for a every job and a handheld multimeter isn't really the right tool for precision low current measurements.
So, would your friends and Brymen actually make use of this? Maybe for the next version of EEVBlog multimeter? Or is this just yapping into the breeze? ;)
A current shunt resistor is simply a resistor used to measure the current in a circuit - by measuring the voltage drop across that resistor. If you know the value of the resistor you use, like 0.5 ohm, and you measure a voltage of 1.45 V across it, you can calculate the current going through it from V = I*R (V/R = I), in this case 1.45 V / 0.5 Ohm = 2.9 A. The microcontroller and analog-to-digital converter inside multimeters can only "read" voltage, not current.
Maybe I should have explained that term. The term (I believe) comes from the old school analog meters when a resistor was placed in parallel with the analog ammeter in order to measure higher current. The resistor would "shunt" most of the current away from the ammeter.
Why not use a high precision current transducer? Have multiple coils with a different number of turns for various ranges, have maybe a seperate coil.for specifically OCP, opening a relay/contactor and breaking the current path. Wkth good shielding, i cant see tbis being infeasable, though im nkt sure whag the manufacturing costs would be if you utilize custom parts with any kind of percisice specification
Or you could just actively sink or source current on the low current ranges with a servo loop that drives the burden voltage to zero. Protection is more difficult, but still possible, might be easiest to just make the input impedance large when the current source loop is railed (but zero when within compliance). A junkbox LM358 and a few resistors would be a vastly cheaper and simpler way to make a micro or nanoammeter than a chopper stabilised shunt amplifier.
Simple SMPS and you have up to 400VDC... I did little research nad there is no devices like that for above 32V :/ But these devices are also using mosfets, so my first idea is good.
That devices are build in way that there is almost no chances of fail at closed state. They even check SOA... I guess if there will be overvoltage mosfet will be opened and if it will be very high voltage mosfet will burn at open state.
had a problem trying to make a really simple Lipo charger. Buck-converter based on a 4.2V reference, an OpAmp and a power mosfet. While the open output-voltage is fine, and it works with small loads, i somehow don't get a strong enough output. The reference alone can output 400 mA with just a cheap 6V 600 mA supply (mobile-phone). But my own circuit only manages 200 mA -.- Mosfet is IRL3103D1S - 30V/64 Amps. LM 358 opAmp LM 317 reference capacitor-banks before and after the mosfet, and inductors (with diode protection) before and after the caps (don't want any too sharp rush in currents). Ideally i would want to generate a stable output of 4.1 V from an input of 20V all the way down to 4.5V, and with the bigger LiPos i have i would need output currents of up to 5 Amps. (I think at 5 Amps the burden-voltage would be quite a problem :P ) but still no clue why my simple design doesn't work.
The FET has a 33nC gate charge. The op-amp has a max current of 50mA. Unless you have put in some hysteresis what you have got is an oscillator at about 50kHz. You need about 1% hysteresis (minimum) 3% would probably be better and ideally an op-amp better suited to driving a FET.
"You don't need to be confused, Just understand it." - Love it!
Did I really say that? Not sure what I wanted to say there, but that just popped out!
Yeah it's sheer genius! I'm going to try that one with my kids. It's at 3:04
That should be on a T-shirt! :-)
Was going to write the same thing lol
"You don't need to be confused, just understand it". What a quote! I rate these videos very highly but that was a classic jumble of unintended words. Apart from that keep up the good work. It is very enjoyable to watch and learn.
I always love it when Dave gets really excited and says Volts instead of Ohms... :-)
A point for anyone interested: if you are measuring only low currents (say,
I hold my tongue to a circuit. If it hurts a lot there's more current flowing in comparison when it doesn't hurt that much.
I call that good enough.
Thanks Dave, this is another one of your best posts ever. I'd always assumed the best engineers in the world were already building multimeters and it would be a lot harder than this to improve on their designs. It's great to see how you break down a complex problem and make the solution fit into a typical multimeter form factor. I'll be following the development of your new multimeter closely!
2:50 The reason for replacing the decimal point with "R" for ohms was because back in the day when making photocopies of schematics on low resolution photocopiers sometimes the decimal point would get lost in the copy. The same holds true for the "K" and "M" for Kilo-Ohms and Meg-Ohms.
BTW I didn't even get a notification of this video and yes I checked the "Send Notification" box. I just stumbled across it looking for something else.
Called it on Twitter, ended up just as long (or longer) but tons of good information, learned a lot. Thanks Dave.
Brilliant, I have had to switch to the Amps range to measure circuits in the past or bumped up the power supply but I had never investigated the reason my 3.3v and 5v circuits were not behaving as expected when trying to measure current. Thanks for the video, another thing to keep in mind. Time to get another multimeter to measure the circuit voltage at the same time :-)
Gotta have two meters minimum!
I think I have gone a bit overboard - I have 6 of the cheapest ones you can get that I just don't care if they go bang. Two intermediate quality for day to day use. Two decent flukes and two bench. I think I'm missing an AVO.
Or a multimeter with simultaneous voltage & current measurement! I'm thinking of making a handheld dual channel and dual measurement (for measuring efficiency) multimeter. Then you only need one in 99.99 % of the cases. And I'll probably incorporate an electronic resettable fuse with a small 10 A relay and an opamp. That way, the end user saves money on expensive HRC fuses since it can be reset pushing a button. Not to mention the much lower burden voltage!
18:10, the smaller the conductor, the lower the power can be passed through it (higher resistance). fuses work on a bit more precise version of that. for example 30 awg (copper) wire will have a higher resistance vs 12 awg per 50 ft (just shy of every 15¹/⁴ m), fuses use one or any combination of copper, zinc, silver or aluminum to get it's requirements.
Useful as usual. It never hurts to see someone else's take on things :).
"You don't need to be confused. Just understand it". I love it.
I would like to see a multimeter that can do guarded resistance measurements. These work by applying a voltage on one side of a resistor, then using an opamp current to voltage converter on the other side of the resistor. The inverting terminal acts as a virtual ground if the non-inverting input is grounded. This allows you to cancel out parallel current paths on in-circuit resistor measurements by grounding (guarding) points in the circuit. In-circuit test equipment works this way. Keithly makes some good SourceMeters that do this, but these cost about $5000. A little $100 meter that had a third guard terminal for in-circuit measurements would be really handy for troubleshooting. I've seen some LCR meters that had a guard/shield plug, but I don't believe they use the opamp current to voltage (virtual-ground) current measurement technique.
I think my textbooks are poor quality; "bugger all" and "bob's your uncle" don't appear in the index at all.
"no need to be confused, just understand it!" Thank`s men!-)
Dave, your µCurrent saved my bacon last week. I found crappy Chinese MLCCs consuming 100* more battery power than the actual RTC/SRAM. Replacing them with quality Murata ones fixed the thing.
Nice!
They just made their own, or used a low external shunt and high precision meter.
using the sense terminal of the power supply after the multimeter maybe solve the voltage drop issue?
Hi Dave,
Maybe as a follow up video you could explain that you can use the sense terminals of your power supply after your ammeter to get "rid" of the burden voltage, including disadvantages like the measuring current of the sense terminals and others.
Also regarding the resistance of the HRC fuses: I *think* it is because HRC fuses are usually sand or ceramic filled, and because of the higher heat conduction of these, the wire needs to be thinner to break at a certain current, because the heat is transferred away from the wire.
I would also assume the tempco of HRCs is lower because of that reason.
This series will be an interesting one indeed!
Hey Dave! Did you mean traps for young players in the description instead of traps for YOUR players?
Another awesome video. I've been trying to get my college to use some of your fundamentals stuff in their entry level classes.
For example, the oscilloscope video you just did is fantastic.
Enjoying this, please continue....
Thanks Dave very informative. I smell the multimeter project is revived.
Hi Dave...Good "vidayo" on an improved
"moldymeter"!
Rather than using a mux for the different current ranges, could you switch that via the range selection switch, or, does that cause some potential isolation issues?
Designing a Multimeter, I am eagerly looking forward to Multimeter designed by Dave Jones
Love the whiteboard tutorials / designing videos, good stuff.
The burden voltage is exactly why you need to have at least one precision 0.1Ohm 4 terminal resistor box and a high input impedance digital multimeter on your bench. Fortunately a box like this it is not so hard to make. Apart from all the regular stuff you only need some proper connectors and nichrome wire. Then you can just hook it up and measure voltage drop across it when needed. The other good thing is that the resistor stays connected all the time your circuit operates. So you only need one quality multimeter to probe around.
Just to let you know, this didn't appear in my sub-box! IDK why it didn't, but I thought I'd let you know.
Yep, something is wrong, not sure why.
Looks like it just appeared in my sub-box. Who knows what's going on.
It says "you can't set up e-mail alert for this subscription" wtf? :( Not enough subscribers I guess?? :(
WTF? TH-cam is broken again.
I was able to do it with no probs...could it be country sensitive?
I recently bought a Keysight U1242C. One of the reasons I bought it was that it seemed to have amongst the lowest resistance shunts I'd seen. It's 0R5 on the A range and 30R on the combined mA/uA range if I understand the datasheet correctly (I haven't actually measured it yet). At work we even managed to program a 3.3V circuit with the multimeter inline, while our other Flukes had to be switched form uA to the mA range in order to program. Well, actually, the combined uA/mA range meant it just autoranged to mA, but I tried with a 33R resistor and it worked...
Apart from that it's a reasonable meter, albeit with a really spongy range switch. I'm not much of a poster, in fact I don't think I've ever posted anything on the forums, but if there's interest I could post some pictures. I've got a BM869s that I could compare it to, but I'm rather reluctant to open them up... I don't want to dissect my babies... even if opening them means i'm "taking ownership" of them... :(
Oops, correction, it's got 30R, 0R5 and 0R01 shunts, respectively...
Nice video Dave, could you make a video talking about how to use a high frecuency transformer and how to properly filter a switchmode psu?
EEVblog, are you ever gonna do more Old game system tear downs? The N64 Tear down is still one of my top fav vids from you, along with crap product tear downs.
If I come across one.
Love your videos! Keep up the good work m8!
Greetings from Croatia
The reason why they came up with the 0R01 is because on print the dot tend to dissapear when you make a photocopy or the paper is a bit worn. For 0R01 it's not too bad, you will read it as 001 and guess correctly that it is 0.01. However 1.1 ohms for example... 1.1 or 11? 1R1 is very clear!
I don't know exactly, but I guess because R is a normal letter of the alphabet (you also often see u instead of µ). However, to use fewer symbols would it be valid to write R01? I'm not sure but I think I've seen this before.
Also on schematics or BoM's when you see xRx or xKx you instantly know thats a resistor without having to put an ohms symbol ^_^
+Klaufmann R01 would be the number of one of the resistors on the diagram, better write the extra zero. 10m could get easily changed to 10M by a glitch, which would be a 1000 million times error between design and production. To me the big question is when and why someone changed the R from meaning ohms on typewriters to meaning any other decimal point, since those would previously be written as the appropriate unit, such as 3V3, 2A2 etc.
Because R stands for radix.
en.wikipedia.org/wiki/Radix_point
Smudger Dave That explain why they use R for inductors and capacitors too...
Great video! Came just in time for a design I am struggling with.
Sweet
I was the one that called into the Amp Hour about measuring current. I may try using the MOSFET option and see how that goes.
If the 600mA fuse has too high a resistance, why not replace it with three 200mA fuses in parallel?
According to the chart shown at 15:45, 200mA fuses have a typical cold resistance of 1.83 ohms -> 0.61 ohms altogether. Burden voltage at 5000mA would drop from 500mV to 300mV.
If that's not enough, can you get six 100mA fuses?
Adding an x10 Amplifier to the standard MM concept is, on my opinion, the easiest solution. That reduces burden voltage from 200/400/500 mV to 20/40/50 mV FS. That should be small enough...
"small enough"
Yes, small enough as you'll never eliminate it.
Great video Dave! I enjoyed a lot of examples as pictures along the narration.
Why to use multiplexers? Isn't it better to use the range switch?
what about magnetic ways to sense the current, like LEM probes? More generally, why to they stick to shunts resistors in multimeters?
As I am looking for a good second meter, I am even more tempted to get a Gossen now.
Gossen is great, expensive but great german quality.
A really informative and helpful video! - thanks for posting.
amazing..I am always learning new things :)
your uncle Bob will be happy
I would favour a 0.01R / 0.09R / 0.9R / 9R / 90R divider chain, switching the mA/uA input to a tap in the chain, measuring from the 90R end (via x10 amp except for 10A scale). This avoids the multiway switch contact resistance from being sensed, and allows make-before-break on the range switch within the current section.
Does the fuse need resistance so that it will get hot as current goes up and, when the current gets too high, blow?
What if you put a amplifier before the shunt for the low currents?
how long until the eevblog multimeter to blow fluke out of the water? ;)
+EEVblog Hi Dave. I've always thought that top-of-the-line measurement instruments should use current transformers instead of shunts. What are your thoughts?
CTs don't do DC.
Rs do DC and AC just fine ...
@@youdonotknowmyname9663 You're right, of course.
Hey Super Dave I think you need to re-cad the bridge rectifier there is nothing to protect against over negative milliamps on the current version It needs to turn 90 degrees to the right. for .7 volts peak to peak protection.
Shouldn't be much surprise that a fuse has some resistance, after all the power it drops over that is what makes it blow. Afaik HRC fuses typically need more power dropped due to the surrounding material being much more efficient in transporting excess heat away than just the air (or in some cases even vacuum) of glass fuses. You should open one fresh one, and one catastrophically failed one (HRC fuses that is).
Given that the 300+ bucks range multimeters usually don't do these things make me think that the industry doesn't really need it though.
Dave, are you going to make some more uCurrent Gold adapters? Thanks.
@EEVblog One thing that confuses me about the op amp portion of the current shunt (something I would love details on), how does something like the MAX4238 deal with potentially up to 1000V common mode voltage? Or does that only refer to the voltage difference range across the shunt?
Since the OPAmp only "measures" across the shunt in current mode, it doesn't matter.
When measuring current, the meter gets put "in line" with the current-to-measure.
So as far as the meter can tell, there is no 1000s of volts (referenced to ground).
Because the meter isn't connected to ground.
If the fuses are a problem, what about using a MOSFET to isolate the probe and a dedicated high current sensor that would switch the MOSFET off? It wouldn't be much slower than the blow time in some of those fuses...
The biggest problem with that is that even power MOSFETs require power dissipation for relatively low currents of 10 A, something difficult in a handheld multimeter. Relays would be a better option.
Could you go with a remote sensing configuration like a hall sensor and remove burden voltage alltogether? Or are hall sensors just not sensitive enough or too noisey for measuring mA current levels (even with multiple wraps and/or magnetic concentrators)?
Yep, I liked it. Good fundamentals in this in the areas of problem solving, engineering and electronics principles. Darned good stuff :)
What about a transimpedance amplifier for the lower current ranges?
The voltage on the input takes time to settle to 0 after the input current changes. Using it to measure something like a microcontroller power rail would mean the rail voltage would jump all over the place.
5mV + meh
Ha. Gotta be the best value specification ever
Interesting stuff. Any catch using analog mux'es ? They add a few Ohms, but the input impedance to the x10 amp is high enough to ignore it ?
Correct.
Would a polyswitch in parallel (or series?) with the HRC fuse do anything useful to solve the Hi-R problem?
Is this the reason most multimeter ma and ua ranges only go up to 200ma / 200ua ?
Great video Dave! :)
@EEVblog what about measuring the voltage drop over the multimeter and calculate the loss? two or three multimeters will srt this out.
How about a review of that U1733 LCR-meter?
Instead of a single 1ohm 1000V HRC fuse, could you use 4 * 250V 0.1ohm glass fuses in series (in an armored box built into the case)?
Can you suggest a design for a DYI chopper amp for microvolts measurements?
I have build transimpedance amplifiers with a gain of 10^9 V/A for my Masterthesis. I used the OPA 129 which has a offset current of typically 100fA. Cant u suck a bit on the current that flows through the resistor and amplify that to a handleable Voltage? I mean takeing 10^-9 A froma 10^-6 A current wont matter that much. If needed the OPA 129 can handle 10^10 V/A in a single step at 300Hz bandwidth. Wouldnt that be enough for a multimeter?
It is a very very usefull , after i see this than maybe I'm going to rethink my circuit to measure the characteristics of transistors making it very easy.
Thank's a lot Dave and sorry for may poor english i'am italian ..
bye.
I was surprised when I noticed this problem when trying to fix one of my son's toys. Not that it mattered, I mostly needed to know there was power at all. Now I do have a question on some strange behaviour of the circuit I'm testing that I will need a diagram for. Ok, electronic wizzes here, where is the best forum to post the question with such a diagram (in DaveCAD of course)?
My brain hurts if I try to think about what you're saying.
I got confused, what is the reason for the bridge rectifier on the mA and uA range?
It is for clamping the voltage across the current sense resistor. th-cam.com/video/jx0dryLmUEQ/w-d-xo.htmlm50s
Ultra RC so if I stuck probes across a 1k resistor that's dropping 10V then all of a sudden my little 1R resistor has 10 Amps going through without protection. Obviously the fuse would blow, but pretend it's a value to small to blow the fuse but high enough to burn up the resistor. Well if you're new, check out VI curves of diodes. They drop about .7 volts once a certain current is reached. Now if I have 2 diodes like in a bridge, I drop 1.4V. If you know your circuit analysis, it "clamps" the voltage at 1.4 volts. That means the node (connection) between the fuse, diodes, and resistor is at 1.4V no matter what and the excess current is directed to ground. So now you have (1.4v-0v)/1ohm which is a much more manageable current. Also if the current is too crazy through the diode then the fuse will just blow and all you need is a cheap fuse to fix it. oh and the diode bridge is incase you have AC voltages. It clams both the negative and positive to +-1.4 V
+Robert Maxson thanks for the replies, I'll do a little more digging
You can also think about what would happen if there were 600 mA (rating of the fuse) going across the 100R sense resistor. P=I^2R, so that resistor would dissipate 36 watts. Even though the fuse wouldn't blow, the resistor would still burn up.
WOOO! I've been waiting for this video
Dave, You show that (through that 'SPDT' switch action how the mA and uA ranges of the DMM switch to different shunt resistors for these ranges. However all my meters have NO switch for these two ranges--it has just mA and Amps, so how are different shunt resistors selected for each of these reading ranges? THANKS for these great videos--I am about halfway through ALL of them!!
Also, Are there typical Wattage ratings for the three (100, 1, 0.01 Ohm) resistors?
They are mostly just a piece of wire.
(The 0,... Ohm ones anyway)
So it is hard to tell what the exact "specified" maximum power rating is.
Depends on how hot the "resistor" is allowed to get.
What's about using hall effect current sensors in multimeters?
Still we will have 1R at fuse...
An ACS712 has a typical resistance of 12mR and can survive up to 60A for a short time. You can then use a 50A fuse, which will be far lower than 1R. To add some safety, the input of the ACS is isolated of the rest of the multimeter.
my main question that all of this raises is if i had a shitty dmm with glass fuses, and i try to do the right thing and upgrade them to nice hrc fuses, your saying that i now just screwed up all of my future measurements with it because the calibration is now off? ?? ?
the higher resistance makes sense if the cheapie is slow blow and the hrc is fast blowing
the heat has to come somewhere to melt the wire in the fuse
Are you looking at restocking the μCurrent?
Shortly
Why don't we make precision fuses and use them as sense resistors?
And that is exactly what a fuse is.
Well, My suspicion is that it would mean that a small over current that blew one of your instruments fuses would require you to send the instrument in for service, as it would require recalibration, and the fuse itself, being specially produced for the application would be even more expensive.
The resistance of a fuse is part of the formula for it's rated current, so your instrument design would be constrained to the resistance of the available fuse materials, and it might be that none of the fuse materials that would have the required resistance would be linear and stable enough for precision resistors.
There are resistors that are fusible, but I don't think I have seen any with less than 1% tolerance and they have rather too high resistances if the purpose is to reduce the burden voltage.
So, my guess is that it simply more flexible, cheap and convenient to make an accurate instrument with a separate resistor and fuse than using a fusible resistor.
Wow! This is actually a brilliant Idea
even with temp probe it still be much more accurate across a range than the current method, simpler
Well, It probably is possible to overcome any issue with making and using a high rupture capacity precision fusible current shunt, the real question is if it is possible to do so economically, safely, and without adding problems for the end user that would give your instrument a bad name.
Of course, such things are not really decided by some random people discussing it in the comment section of some TH-cam video, it is ultimately the people who promote a new approach that has to show it is viable by actual research and experiment.
Is your multi-meter 121GW based on this design? How does it deal with the burden voltage??
how about using a Flux gate magneto meter as a current sensor? cost would be very high but it would have spectacular specs!
Is it a problem that the 500mA range is not fused properly? It goes through the 11A fuse...
But it also only goes through the 11A rated resistor. In fact Dave's solution boils down to simply setting the switch one range higher to get less meter resistance at the cost of one digit of precision.
No, because the shunt is designed for that current.
Nice!!! So, when will we be able to buy the burden-updated EEVBlog multimeter? :)
I am at 18 minutes, dont know it you'll do it later but it would be cool to log burden voltage over a fuse at half its rated current. I think i have done that with piss end fuses in the past and had really bad results.
I had a project where I've encountered burden voltage being a problem, but I didn't needed really fast measurement, so I solved that using a higher resolution ADC running on battery(to chancel noise as much as possible) hooked up to an arduino, and my PC using just the intermal amp of the ADC, and pretty low value current shunt. I've got pretty low burden voltage, and high enough resolution, but it's not portable and the refresh rate was terrible as well... But it worked... :)
Also no fuse was used, but it wasn't a high power application...
Thanks Dave!
What about making a special HRC fuse with a stable controlled resistance that can also serve as the current shunt, It will be expensive to change if it blows, but it won't be possible to be bridged.
I'm sure multimeter design engineers have thought about his in some form or variation but I guess the added cost of the extra components is not really worth the added value. Also having that combination of input jacks and ranges might seem confusing for the user. We could also consider there is a right tool for a every job and a handheld multimeter isn't really the right tool for precision low current measurements.
So, would your friends and Brymen actually make use of this? Maybe for the next version of EEVBlog multimeter? Or is this just yapping into the breeze? ;)
You have to have relatively high resistance because fuses use resistance to acquire enough energy to melt before the current fries you or your tool.
What is a current shunt resistor again?
A current shunt resistor is simply a resistor used to measure the current in a circuit - by measuring the voltage drop across that resistor. If you know the value of the resistor you use, like 0.5 ohm, and you measure a voltage of 1.45 V across it, you can calculate the current going through it from V = I*R (V/R = I), in this case 1.45 V / 0.5 Ohm = 2.9 A. The microcontroller and analog-to-digital converter inside multimeters can only "read" voltage, not current.
Maybe I should have explained that term. The term (I believe) comes from the old school analog meters when a resistor was placed in parallel with the analog ammeter in order to measure higher current. The resistor would "shunt" most of the current away from the ammeter.
Why not use a high precision current transducer? Have multiple coils with a different number of turns for various ranges, have maybe a seperate coil.for specifically OCP, opening a relay/contactor and breaking the current path. Wkth good shielding, i cant see tbis being infeasable, though im nkt sure whag the manufacturing costs would be if you utilize custom parts with any kind of percisice specification
Or you could just actively sink or source current on the low current ranges with a servo loop that drives the burden voltage to zero. Protection is more difficult, but still possible, might be easiest to just make the input impedance large when the current source loop is railed (but zero when within compliance). A junkbox LM358 and a few resistors would be a vastly cheaper and simpler way to make a micro or nanoammeter than a chopper stabilised shunt amplifier.
What about using mosfet instead of fuse?
Interesting component.
But its up to 32V, we should look for at least 1kV.
Can you give me link to mikeselecticstuff vid about that component?
Simple SMPS and you have up to 400VDC...
I did little research nad there is no devices like that for above 32V :/
But these devices are also using mosfets, so my first idea is good.
What happens when the MOSFET fails as a short? A fuse is guaranteed to open when limits are exceeded.
That devices are build in way that there is almost no chances of fail at closed state.
They even check SOA...
I guess if there will be overvoltage mosfet will be opened and if it will be very high voltage mosfet will burn at open state.
Dave Jones: "You don't need to be confused, just understand it."
Nice vid, who's going to implement it first?
So is most of Dave's background in instrumentation design?
+Erick Miller a decent amount of it
I think they designed the HRC fuses with higher resistance than the cheaper one to blow up safely.
o my... 6.5, 7.5, 8.5 digit multimeter projects are several steps above than the norm 🧐 thanks a lot 🥳🏄♂🌲🥠☕
DMM-B-11A, I have popped so many, I have it memorized.
Saw the plastic film on the screen of that Keithly meter... That film would never survive in my presence! :P
Nice to see the micro Current again... Are you going to be selling it again soon ? Thank you for the video ! tjl
When are you going to start uploading in 4k?
I don;t have a 4K camera, and the added file size and bandwidth is ridiculous.
ok no worries
had a problem trying to make a really simple Lipo charger.
Buck-converter based on a 4.2V reference, an OpAmp and a power mosfet.
While the open output-voltage is fine, and it works with small loads, i somehow don't get a strong enough output.
The reference alone can output 400 mA with just a cheap 6V 600 mA supply (mobile-phone).
But my own circuit only manages 200 mA -.-
Mosfet is IRL3103D1S - 30V/64 Amps.
LM 358 opAmp
LM 317 reference
capacitor-banks before and after the mosfet, and inductors (with diode protection) before and after the caps (don't want any too sharp rush in currents).
Ideally i would want to generate a stable output of 4.1 V from an input of 20V all the way down to 4.5V, and with the bigger LiPos i have i would need output currents of up to 5 Amps.
(I think at 5 Amps the burden-voltage would be quite a problem :P )
but still no clue why my simple design doesn't work.
The FET has a 33nC gate charge. The op-amp has a max current of 50mA. Unless you have put in some hysteresis what you have got is an oscillator at about 50kHz. You need about 1% hysteresis (minimum) 3% would probably be better and ideally an op-amp better suited to driving a FET.