Capacitor Discharge Probe: How to make, and how to determine resistor value
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- เผยแพร่เมื่อ 5 ม.ค. 2014
- Warning: Either my brain or my calculator was not working properly. At 23:41 for example, the discharge time for a 470 is .47 seconds (not .235), for a 1ohm, it should be .001 (not .0001). Serves me right for making these videos at 3am. Either way, it does not matter, the point is that the 470 ohm resistor is the best compromise between discharge time and maximum current. Regards, Brian
Thank you for your efforts to teach us and keep us safe. Much appreciated! Yours is the only video which explains the determination of the value of the resistor. Excellent!
Man, I searched forever for a good video on this. Couldn't remember my physics well enough. Saw some AP Physics C stuff that was pretty good, but too in depth and didn't address my need. This was exactly what I needed. Thank you for explaining how to make the probe, how to safely use it, and how to pick the correct resistor. Highly appreciated! And thank you for actually having good video quality and pretty good audio quality :)
+pasta0328 Glad I could help, that's why I make'em. Regards.
+stereopolice You briefly mentioned this in the video and didn't end up covering it, but how do we connect to something grounded? Specifically what would be grounded and how do we check? Do I need turn off power to and then connect to the ground wire in a power outlet? Are there better things to connect to with easier access? That's the one other part I wouldn't want to screw up. As long as I can find ground, I'm confident at this point. I'm just not sure exactly how to go about that since I haven't done it before.
+pasta0328 If I were to do that, I don't see how that would work since it wouldn't be a circuit (I don't) so there'd be no electricity flow. Let's say I have a disconnected power board. Is there something on the board I can connect to safely?
+stereopolice You also said your power receptacle was connected to earth, but I could have sworn you said your circuit in the video was disconnected from your device. It looks like it's just sitting on a counter to me. Can you clarify? Sorry for the barage of questions now. I just wish you had covered finding a ground point to connect to the alligator clip to :)
Thanks for the attention to detail. I was about to ask "Why wouldn't we want the resistor closer to the probe? " Then I figured I'd finish watching. Thanks again!
Awesome!!!
This is the best video I cold find after searches in my spare time over the last few weeks.
All the info I needed to get confidence in tackling the capacitor discharge. I've got a few devices to fix so I'm a lot more confident now.
I only had to watch it once but watched it twice as I'm stoked that it had the info I needed.
Thank You for spending time on it.
2014 video is now helping me in 2019! Thank you for putting this on and that I found it helpful in my life today!
Your addendum just answered the question I was gonna write in the comments. Perfect video, thank you.
GREAT VIDEO....forgot about the 5RC for discharge time etc.....glad you added the quick note about the .47 secs (not .235s)!!! I checked it x3 and thought it was my sinus medicine I had to take...
Hi, just to say many thanks for your helpful video, including the maths part and reasoning for choosing a particular value of resistance.I am building such a unit for the safe discharge of microwave oven capacitors, and yes, I know we are in a different ball game there, but the principles are similar and a great emphasis on safety needs to be followed to the letter.Cheers from the UK.
Thank you! Best of these videos I’ve found so far
dude this video is so cool I love learning about this stuff, this is the stuff I wish I learned in school! thanks!
Thank you for putting up this video and sharing your knowledge and experience sir. I am grateful for you passing the knowledge.
Very good. How about using the interior metal ink writer of old ball point pens to make the probe. Wouldn't the plastic exterior of the pen be enough insulation.
ive been looking for such information for few days thanks alot
Well done! Appreciated. The addendum was a good idea too! Cheers!
Really helpful and just what I have been looking for. Thanks also for explaining the theory. is
Glad to hear that. Thanks for the positive comment.
Nicely done, thank you.
Thanks for the precisely detailed video. Just one more question: what should be the gauge of the wire used for the probe?
Great video! Thanks for sharing such valuable information!
That said, I have a device I made also but with a 100 ohm 25w. Never used it yet. Does that mean that I should make another one with your values?
I'm starting to check for tube amps. Thanks !
Good depth to vid - thx. Glad you moved resistor to the front. Pushing the envelope with a 2,300 V, 1.0 uF cap ... ? Seems like it needs a resistor proportional to current, gold plated nail, and LED idiot lights, and some beeping noises. Then maybe some dice hanging off the rear-view ;)
what about adding an LED in the probe so you can visually see when the capacitor is done discharging?
Thanks for sharing!
Question : what's the resistance of your nail ?
It does not act as a resistor from an extended point of view ?
Thank you !
Thanks for the information, it is really helpful.
If I may ask, where did you come up with number "5", in your equation T=5*RC?
Thanks again, your work is much appreciated.
In case anyone else was wondering the same, after about 5x the time constant (i.e., RC), approx. 99% of the capacitor will be charged/discharged, which is useful enough for basic calculations and practical usage. For example, in a case where the time constant is 1: 1-e^(-5*1)=0.9933.
Thank for detail information. But for 6000v cap what should be the wattage of resistor.
Nice to know someone knows 👍
The core reason for the one hand in the pocket is "if you have your left or right hand touching chassis ground and you manage to accidentally contact a charged device with the other ,, the current path will flow on the path of least resistance straight across your chest and "through your heart" and out the other arm.. Electric shock from low voltage current with amps as low as 60 mA can cause ventricular fibrillation in the heart. Ventricular fibrillation, or Vfib, is the most serious form of heart arrhythmia, and it prevents the heart from pumping blood. This results in cardiac arrest. at the very least it will cause severe involuntary muscle action throwing you and anything that gets in the way with huge force across the room.
Right on.
I don’t understand this? A 9 volt battery provides 500mA, a AA maybe 50mA. But if you were to lick either it’s not going to give a heart attack..? Can you explain.
I like your style.
And I appreciate kind words. Cheers ....
Hello, at 24:24 you stated that the peak current of a 900V capacitor with resistance of 470 ohm = 2.12A, but that is incorrect. The peak current would be 1.91A
video police on point
So I ordered the resistor, got alligator clips, soldered, and made a
very, very nice probe, just to find out that the microwave cap has a
built in resistor! I guess it';s good to have for working on computers
or other.
where did you buy resistor and clips?
@@chris8383: I bought these same resistors on Amazon. The alligator clips i bought at a local hardware store.
I'm looking for resistors to buy and a doubt occurred me. Does it need to be wirewounded like those rectangular ones or it can be metal oxide (those normal looking ones with color bands). I've found both with 470ohm and 5w.
Thanks for the helpful video! Is it possible to add an indicator light to the probe so that it's clear when current is being discharged?
We're dealing with a broad spectrum of voltages here, and I believe in keeping things simple, and adding a light would add unnecessary parts and complexity. You just have to trust your probe. However, better than a light, if you really need to know, is a voltmeter. You can check the cap voltages after the fact on the DC voltage setting, ensuring that your particular meter can handle the voltages. I suppose too you could rig a system that measures the voltage across the probe's resistor in real-time, and when there is no voltage across the resistor (v = 0), there is no danger. I think that might be better than a light. Regards ...
I have a question, could we realistically drain capacitors on a circuit or a motherboard by using a multimeter? By running the voltage test and use the internal resistance of the multimeter to drain the charge in the capacitors.
mohammad faour normal dmm resistance is 10 million ohms so no but you could use a dmm with a low z function
very nicely done!
Would it not be more productive and efficient to draw the power from the capacitor and then redirect that into a battery for later use.
Nice info for novice like me. One thing I want to know, if I use 500 ohm 5w resistor for 400v capacitor, then max current can handle the cap is 400/500 i.e 0.8 amp. Again capacity of the cap is s.r of 5w/r = s.r of 5/500 = 0.1 amp which is less than 0.8 amp. Don't it (max current) fried up the resistor of 5w 500 ohm? Kindly comment. Thanks.
Check out the "Capacitor Discharge Calculator" at www.learningaboutelectronics.com/Articles/Capacitor-discharge-calculator.php to calculate how much time is required for a safe discharge given the size of the cap and the resistance of your discharge tool.
That said, I like how amperage is a consideration in your calculation!
Very cool, thanks...👍😎🇺🇲
A nice simple cheap discharger...how about adding a bulb into the device for visual assurance?
Not a math person, what resistor value should i use to discharge a 450v 3300uf Cap (that is on a plasma tv power supply)
Hi there, this probe looks to be handy in discharging capacitors in stereo receivers and other old electronics I have. However, say I'm working on my desk, where can I ground the alligator clip to?
Discharge across the cap leads
Thanks!
The addendum you added might be better to put first in case somebody watches and doesn't make it to the end.
After thirty years, it’s finally hit you. Just kidding. Thanks for the video.
Much needed video. Great content! Have one question, in the example with 200 micro farad cap and the 470 ohm resistor, peak instantaneous current 0.85 A (IIRC, can't see video while I type from smartphone). If the cap potential is at 500 V the instantaneous power through resistor will be P = U I = 500 V * 0.85 A = 425 W
Is that safe? I bet that's why some people use 10 k ohm (which would end up at max 25 W)
Thanks
// Eric, Sweden
Sorry for the late reply. Using a 10K resistor, it would take about 10 seconds to fully discharge a .0002F capacitor. That's just too long to be acceptable; you run the risk of not fully discharging just by virtue of getting bored :). The power dissipation of the resistor is not that relevant, because the discharge period will be a very brief period of time. Power is, recall, the amount of work done over a period of time. In our case, the period of time is so small so as to be inconsequential. So, "power" and "instantaneous power" (as you referred to above; which is kind of a non sequitur btw) are not considerations per se. However, what we do get with higher powered resistors is heaver gauge wire which can better handle the peak current during the initial discharge period. 5W resistors would have no problem handling a few amps over a short period of time. Having said that, if you want to spend 10 seconds holding a probe to each cap (using a 10k) then, well, that's fine, but I believe that 470 is the best compromise. Regards.
@@stereopolice The correct formula to use is: Vc=V0e-T/RC to include the starting voltage potential. This for example results in a 220uF capacitor charged to 500V potential fully discharging in around 15 seconds with a 10k resistor. With a 100k resistor which is what is traditional and what you should be using for HV circuits it is about 120 seconds. In a reasonable equivalent transistor circuit let's use 50V and 2200uF. 10k resistor. Again about 120 seconds. If you can't find something else that needs doing on the project or straighten up tools on the bench for a couple or 5 min or answer an email, I don't know what to say.
I was also curious about HV. In the video he says voltage doesn’t matter, but I didn’t understand that. Why does your provided equation include V and his: t=5rc does not? I’m trying to build a discharge tool for crt television capacitors. People recommend 1M ohm for tube anode cap but I figure for the capacitors it would require a much smaller resistor?
excellent video. one thing is the grounding of the probe. If you have disconnected the amp from all power sources then grounding the probe to the amp ground doesn't allow the cap to discharge correct? so the ground should be a actual surface connected to earth ground. someone else asked about this discrepancy but you haven't answered
When you 'connect one end of the resistor to ground', what you are actually doing is connecting one end of the resistor, via the chassis, to the negative terminal of the capacitor. When you touch the other end of the resistor to the positive terminal you are completing a circuit consisting of the capacitor and the resistor. The voltage on the capacitor appears across the resistor and current flows in it (I = V/R). If a voltage is causing a current to flow, then power must be dissipated (P = V x I). All resistors dissipate power by converting electrical energy into heat. The charge on the capacitor is converted to heat in the resistor and then lost into the atmosphere. 'Ground' plays no part in the discharging procedure.
@@silasfatchett5693 ground gives another path of current to flow known as a safe path also isolation
May I use a 10W x 2K2 ceramic cement resistor? TNX
I used to work in a repair shop one time my co-worker forgot to discharge the caps in a microwave oven, he was standing up at the time. the shock threw him backwards up against the wall, he was stunned but otherwise not hurt. It really scared me.
cat 11 He was lucky enough to survive. A co-worker's husband was killed trying to fix a $10 garage sale microwave. He touched the capacitor and was also thrown backwards...but did not survive. Please, please be careful when around capacitors. Long story, she had to raise two fatherless children. So if you don't know what you are doing leave it to the professionals. Some things are not worth trying to save money on. If I didn't know about my co-worker's story I might have been a casualty too trying to save money. Educate yourself.
@@michaeljames2183 I used to manage a repair shop...worked on tv sets microwaves I left those alone.
It's all a luck of the draw. If the shock occurs at just the right, or rather wrong time, when your heart valves are in the closed position during its beat... ☠️
hello
how do you know that you have discharged toe capacitor?
is there a way to measure voltage pre discharge, then after?
thanks
john
You could measure the DC voltage across the cap, before and after, with a multimeter.
why is it a waste of time to discharge a 0.1UF 50V capacitor its connected to a 5v DC motor using Arduino, isn't it best practice to discharge it to be safe? is this correct? 5 * 470Ω * 0.1µF = 2.35 seconds
Just curious... why not use a clip grabber (insulated handle, of course) to attach to one post on the capacitor, and then use the probe on the other post? (Rather than using the alligator clip attached to a grounding point on the device?) If you're doing repairs, I don't know that it's safe to assume the circuit is properly grounded - that might be the problem. I'm getting back into electronic repairs (after 30 year hiatus) and watching your video to jog my memories, and if I recall correctly that's the way I was taught to do it and for that reason. Was I taught or remembering it wrong? Oh, and what I was hoping to find in one of these videos is a way to add some sort of indicator - LED light or buzzer - that lights/buzzes while it's discharging but stops when you get into a safe voltage range.
Wondering the same thing
To calculate the resistor value we should consider the momentary voltage across the cap Vt, the time t in which the capacitor should be discharged, the resistance used for discharge R, the initial voltage Vo and the capacity C: Vt=Vo*e^(-t/R*C). Which gives us the bleeding resistor R=t/C*ln(Vo/Vt).
Or you can use this XLS calculator if you like: goo.gl/dupnvg
I was also curious about HV. In the video he says voltage doesn’t matter, but I didn’t understand that. Why does your provided equation include V and his: t=5rc does not? Your calculator link is now dead by the way. I’m trying to build discharge tool for crt television capacitors. People recommend 1M ohm for tube anode cap but I figure for the capacitors it would require a much smaller resistor?
How do you calculate the size of a resistor to discharge a CRT? I know that they can carry a charge of up to 30k volts.
Use a high voltage probe to discharge a cathode ray tube second anode is preferred.I have always jumpered a screwdriver by means of aligator clips to chassis and have never heard of anything bad for the picture tube. Static voltage is high, but nut much current.
That’s fine for the anode out of the tube. But discharging capacitors on the crt tvs board is a different story I believe.
how can you tell if the resistor is 5 watts when its not marked on the resistor
The same way you determine the resistance value and tolerence value.
Did you see the prettty colored bands on the resistor? Thats' resistor shorthand to tell you all that information.
How would you add a LED to the tool?
Richard Girou put a parallel resistor with a series led. Just make sure the current flow doesnt exceed the LEDs max current
For Mr. Carlson's Lab project? :-) Neat Patreon project if you haven't seen.
Hi, And thanks for the info. Not to crazy about touching both contacts with a screw driver..LOl
But can you post a link as too which 470 Ohm 5 watts resistor should i get ? There's so many and not sure which one . Thanks
It doesn't matter. Any 470 ohm 5W resistor will be OK.
a cement resistor
Great vídeo apart from the fact you saying that you would not speak about the reason of the video and then a minute or two later..: Bam! Why!!!
Lets say I have 1400uF cap, at 400V, the discharge time for a 11k resistor would be 77 seconds. The initial wattage would be 11watts, how to calculate a safe plot for a resistor wattage? Because I have a 0.25w resistor, and it's fucking tempting. lol