How to Make a Capacitor Discharge Tool
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- เผยแพร่เมื่อ 1 ก.ค. 2024
- This is an essential tool if you are going to be working on valve guitar amplifiers. Those sneaky HT caps can hold a lethal charge even with the amp switched off. Use this tool to discharge them safely and quickly.
An interesting and informative video that teaches how to discharge capacitors and protect against electrocution, or electric shock! Thanks Stuart this has provided a useful way to also check for leaky compactors in my valve amp.
Compactors ??
Very good, i will try to make it. Nice sharing sir......
Bonus tool! Thanks. Gonna make one just for the joy of it.
Excellent!
+ve side of electrolytic cap can be on chassis/ground if supply line is negative voltage, eg in op-amp output stages.
Hi ANdrew YEs also for -ve bias supply on most valve amps.
Just for the record I didn’t believe a word of what’s in your box. I was thinking of building one that has a built in voltage indicator. What’s the highest voltage you come across in a valve amp ? I’m usually playing with valves radios so more typically 250-350 for my stuff
This is very nifty little device, Stuart. I saw you using it on the Fender HotRod DeLuxe Hum video, and I was happy to see that you had taken the time to make this little tutorial! Thank you! I do have two questions before I build my own -- 1. Was there a particular reason for using a 100 mfd cap, or would 80 mfd work just as well? 2. Would a radial capacitor of the same rating work just as well?
Hi I had 100uF to hand. 80uF is fine. Yes radial or axial. All the best
Apologies in advance if this is a dumb question, but I'm a bit of a newbie at electronics (which is why I so appreciate all I'm learning from watching your videos). If the capacitance rating is simply the amount of energy stored in a capacitor, and, for the purposes of this tester, we're not concerned with the amount of energy stored (after all, we're going to discharge the capacitor anyway when the test is done), then it seems like even a 20uF capacitor would would work, if that's what I happen to have lying around. Is that right?
The purpose of these large elctrolytic caps is to smooth out the AC component and produce DC. The bigger the cap, the better it does that. If the cap is too small (and/or open circuit) you'll perceive this as hum on the output (essentially, the unsmoothed 50Hz AC). So yes a 20uF would 'work' but a larger cap will smooth out more of the AC.
The prupose of this tool is to check if one of the in-circuit HT caps is not working 9e.g. ipen circuit). We're 'dabbing' a good cap across the suspected bad cap. May as well dab a decent size cap, that way we can hear if the hum changes. Hum before you use the tool and no hum after? It's a bad HT cap. Value not critical but I think I'd go 47uF as a minimnum.
That makes perfect sense, Stuart. Thanks for explanation and the quick reply.
Hi Stuart. Great bit of handy equipment.
On the case next to the switch there is a screw on your tester, what is the function of the screw.
Hi Ernie. Nothing exotic! It just holds a bit of tag board inside to solder the compnents to.
@@stuartukguitarampguy5830 Thank you for the quick reply.
I was viewing the video in a poor resolution. 1080P makes it a lot clearer.
9.00 PM here in Australia.
@@ernielazdins1348 Ah, that's because the TH-cam signal has to get right through the core of the planet to reach you....
Excellent design! This may be a dumb question, but does the internal capacitor matter as to how its rated? For instance, if I'm going to probe a 500v capacitor, does that internal cao need to match it or be greater than the cap I'm probing? Because many Fender amps seem to have at least one 500v cap, but your internal testing cap is only 450v... Just wanted to make sure. Thanks so much for this video!
Hi Felix. You make a good point and in retrospect I should have used a 500V cap. BUT... no real harm as although some caps are rated at 500V (in amps) they rarely, if ever, have 500V on them so the 450V cap will be fine. Also it's only connected in circuit for a few secs. You'd need sustained over voltage for a sustauined period to get an HT cap to fail. But if you're making one, bung a 500V cap in if you have one.
Aha, ok that makes sense. And yeah I don't think I've ever had a reading anywhere near the full 500v on one of those filter caps but thought I would ask your thoughts on that. Thanks for the response and again, great video! There is a lot of info and methods to drain caps out there, but making a two for one tool like this is just genius! I can't tell you how many times I wish I had a way to know which cap was my problem and would just end up changing all of them even if I probably didn't really need to.
@@stuartukguitarampguy5830 Stuart, I have one last question... It looks like the probe goes straight to the switch where it meets the positive side of the internal capacitor. And the alligator clip goes straight to ground... How exactly does the resistor get wired to everything? It's really hard to see in the video how everything should be connected. I have zoomed in best I can, but it's just a blur basically where the switch and the resistor/ground point is. Any assistance on that would be greatly appreciated!
@@LeviBulger Hi Felix at 6.45 there's a schematic! Just follow that.
Stuart another great video. You didn't mention the function of the nut and bolt next to the switch.
Thnaks robert. It probably holds a capacitor clip or something - can't remember!
Ah yes I do recall. I'd used the box for something else in the past and it had a hole there. This was a simple way of blocking it up!
While shopping for momentary switches, I noticed that they all have an amp/volt rating (usually 6 amp/120V, 3 amp/240V). Do I need a particular rating for the switch on this tool?
Hi. The current is quite low so you don't need to worry about that. I'd go for 250V. The DC voltage can actually be a bit higher than that but it should be fine.
HI Stuart, In a later video, it looks like you modified this box to change out the 100 ohm 2 watt resistor for a 10K resistor?
Yes I think I did.
Now wouldn't it be a great idea to not just waste that energy inside the capacitors but to collect it instead in a rechargeable battery for later use?
The only downside I can see right now is the amount of energy being collected, so it might take quite some time to charge a battery.
What do you think?
H ah, yes probably not worth doing. You can calculate it from Q=CV. So say 400V and 100uF I make that 0.04 of a coulomb of charge.
Energy = V x Q/2 = 0.02 x 400 = 8 joules. A watt is a joule per second so this will give you 8 watts for one second!
what if we add a couple of diodes to secure the polarity?
Hmm you have to be a bit careful here. If you're making JUST a dsicharge tool, then do not add a diode! You could fool yourself into thinking you had discharged the cap, when really the diode was preventing it. My tool also had a 47uF 500V cap inside it so I can quickly just add that across an existing possibly bad cap to see if that helps. I guess there is a slight danger of putting the probes the wrong way around and hence the internal cap getting reversre voltage. In that case a diode in series ONLY with the internal cap, would prevent that. I hope that makes sense.
Mr Internaut, it's a jig to discharge capacitors and to place it in parallel to look for bad capacitors that cause humming sound. 😅 Don't make it complicated.
Am I right STUART?
@@dennischarles68 Yes that's right. In retrospect I'd add a 100K 2W resistor in parallel with the internal capacitor. That's just to discharge it when you have finished, otherwise high voltage could linger on the probes.
Hi 👋 Mr Stuart . I thought it was a 100 ohm resistor you mentioned in that video ? So I have to add another 100K ohm resistor in parallel to the capacitor in my discharge tool ?
Cheers ✌🏼
@@dennischarles68 YEs that would work. It won't affect the operation but after you have finished it will gradually discharge the internal capacitor.
Hi 👋 sir. I have a question. I made one myself. Followed your instructions. When checking for humms from an amplifier, must the amplifier be switched on or off ? Because I had a large spark when I put the probes in parallel with the Capacitor 🤔🙊🙊
The amp should be on. The spark is the internal capacitor charging instantly when it is put across an existing capacitor.
Remember to discharge the internal capacitor after use as otherwise the probes will still have a voltage on them.
Hi 👋 sir. Thank you for your reply. How do you do that ? Shorting the the capacitor in my discharge tool ? By shorting the probes together and pressing the button for discharged ?
@@dennischarles68 Exactly yes.
@@stuartukguitarampguy5830 Thanks sir . Noted. Will short the probes and press the discharge button as well on that capacitor discharge tool once work has been done/ complete.
Cheers.
This would have been even better with an indicator light.
What would the indicator light show though?
@@stuartukguitarampguy5830
The indicator would glow so long as there is a charge and then dim out and off once the charge is depleted.
For peace of mind.......
To be sure, to be sure.
@@jayytee8062 OK, not THAT easy to do actually. You need to arrange for the bulb to glow at 450V but also to have a perceptible glow at say 50V and no glow below say 30V. Also you'd need a 10W power resistor minimum. Doable I guess but not immediately trivial.
@@stuartukguitarampguy5830
Pitty it wasn't easier to engineer. Would have been awsome.
Thanks for the reply either way.
This is a dangerous tool. You could have an internally charged capacitor and not know it. Plus you are breaking the 'one hand' rule by needing two to use it. Separate the tools.
I have added an internal discharge resistor. Most tools are dangerous in the wrong hand! Disagree about the 2-hand rule. You clip the croc clip on earth and then probe (one hand) with the other.
@@stuartukguitarampguy5830 Good idea to add the discharge resistor. Regarding the 2-hand rule, with one hand doing the probing ... which hand operates the switch?
@@colinellicott9737 Ok agreed but the probe is insulated and also the switch so VERY low chance of any problems. All the best.