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Question about constructing a capacitor discharge tool

Dony

Dony

AK Subscriber
Subscriber
Hello All,

I have a question about constructing a capacitor discharge tool. In a couple of articles I read it indicates to use a 20k OHM 5w resistor for large electrolytic capacitors used in power supplies, electric motor start circuits and camera flash circuitry. I'm wondering if I could use a 15k OHM 5w resistor instead as locally it is easier to source?

I am a newbie to this so I don't want to make a rookie mistake and end up getting zapped because I didn't make the right decision.

Thanks

Don
 
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Good question for a newbie as safety is an issue with the type of capacitor applications you mention. The larger the capacitor the greater the potential for a painful shock or even electrocution.
  • I suggest you google "capacitor discharge tool" and read Amazon reviews for general information including effect of resistor values; basically, resistors delay the time it takes to drain a capacitor. Some circuits incorporate a "bleed" resistor to ensure the charge is removed when power is turned off or unit is unplugged. Presumably you are concerned with caps in audio equipment, as in a power supply where the caps are always charged to smooth out ripple, as opposed to a photoflash unit where the cap delivers one big pulse (i.e., mucho amps) for a fraction of a second.
  • It appears that a "discharge tool" in the form of a pen/probe has LEDs to indicate if a voltage (charge) is still present. I would use a simpler method--just use a screwdriver or piece of wire across the cap terminals to short it out, then check it with a volt meter (VOM or DMM) to see if any charge remains.
  • Wearing rubber gloves would not be a bad idea if there is any chance of accidentally touching any part of the circuit.
  • When I was 14 and ignorant of the danger, a well-meaning photographer gave me an old photoflash unit for a laser project. It had a 2,500 volt 30 mfd cap the size of a small shoe box, with a spring steel strap bolted to one terminal and extending over and above the other terminal; when pressed down it shorted the cap and made a crack like a .22 rifle. Once when playing around my left thumb came into contact with two "hot" wires about a quarter inch apart; it completely paralyzed my entire hand and at first I was afraid to look for fear my fingers had been burned off. My whole hand was totally numb for about five minutes and I still have a little scar there (I'm 72). If it had been from one hand to the other it most likely would have stopped my heart.
  • Several people have told me about their electronics teacher who played a little joke by tossing a charged cap at them and saying "Catch!", which of course elicited an "Ouch!" from the catcher. Nowadays I suppose the teacher would be fired and sued for abusing his students.
So good luck and BE CAREFUL!
 
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What's the purpose of the discharge tool? In, for example, a valve amp power supply it's a good idea to have a resistor in parallel with the large electrolytic power supply caps so they're discharged after use, but they don't need to be discharged particularly quickly (ten seconds is plenty quick enough) so a high value resistor is fine. A low value resistor will discharge the caps quickly but will consume power and will increase ripple, so don't have it too low. A high value resistor will take longer to discharge the caps but dissipate less power and won't introduce much ripple.
If you intend to discharge caps quickly so they're safe to handle when you're working on the bench the same thing applies - a low value resistor will do the job quickly but will dissipate some power, whereas a high value resistor will do the job slowly but won't dissipate as much power.
The crudest discharge tool is a screwdriver - just short the cap's terminals with it - but the sparks you'll get can be quite alarming and you could do some damage to yourself or the cap.
 
Heathkit always just supplied a 100 ohm, 2 watt resistor to discharge capacitors in their kits. Works fine. You just hang onto it by the resistor body, touch the leads to the cap,.and voila

Doug
 
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And with the mention of 100 ohm resistor to dissipate, this would be a good time to point out that a temporary bleeder resistor values would differ greatly depending on if your talking typical voltages in a solid state amp or a tube amp.

The voltages are so different that going one way, low value, could get exciting in a HV tube amp power supply. And going the other way, a high value resistor, could drop voltage so slowly that you might get impatient and not get it fully drained.

A 100 ohms across a 400v supply for instance would attempt to pass 4 amps and the resistor to do so would need to be 1600 watts. I'm thinking spark and danger the story there! Put that same 100 ohms onto a 15 volt supply and you'll pull 150mA and need 2 1/4 watts dissipation. Very different story as any ohms-law calculator will tell you!

I constructed different bleeders for HV and lower voltage supplies. Separated a clip lead, inserted resistor in the middle, put heat shrink over all exposed points for safety and on a HV one, added extra test points so I could add a volt meter without having to have hands to touch anything so I could watch the number drop as it knocked it down. Seems like it was something like a 50K and 5 watt for HV.

Safety first!

EV3
 
evsentry3 said:
And with the mention of 100 ohm resistor to dissipate, this would be a good time to point out that a temporary bleeder resistor values would differ greatly depending on if your talking typical voltages in a solid state amp or a tube amp.

The voltages are so different that going one way, low value, could get exciting in a HV tube amp power supply. And going the other way, a high value resistor, could drop voltage so slowly that you might get impatient and not get it fully drained.

A 100 ohms across a 400v supply for instance would attempt to pass 4 amps and the resistor to do so would need to be 1600 watts. I'm thinking spark and danger the story there! Put that same 100 ohms onto a 15 volt supply and you'll pull 150mA and need 2 1/4 watts dissipation. Very different story as any ohms-law calculator will tell you!

I constructed different bleeders for HV and lower voltage supplies. Separated a clip lead, inserted resistor in the middle, put heat shrink over all exposed points for safety and on a HV one, added extra test points so I could add a volt meter without having to have hands to touch anything so I could watch the number drop as it knocked it down. Seems like it was something like a 50K and 5 watt for HV.

Safety first!

EV3
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Hi EV3,

Thanks for your reply, I think if I try to obtain a 20K ohm - 5 watt (or closest to that value) resistor to build my capacitor discharge tool to work on solid state amps then I should be fine.

AK forum is a great place to learn!

Don
 
I use a Resistor substitution box loaded with 1 watt resistors. I start it on the Highest resistance and crank it down until fully discharged. I measure the DC voltage while using and it has been full proof for many years. No sparks or arcs.
 
I remember when I was a kid watching my father check the tubes when our TVs would act up. After he opened the back of the TV, he'd just use a big screwdriver to short out the caps and a big spark would ensue. I'm surprised he knew how to do that because he wasn't particularly DIY inclined, but he did take a "radio electronics" class when in the army during WWII. Every since I saw him do that I just do the same thing when working on audio gear. I usually don't get much of a spark, and I've never had any problem doing it that way.
 
I went ahead and obtained a 20k ohm resistor rated at 5 watts. Soldered some 18 gauge wire to each end of the resistor and added alligator clips. Ready to discharge caps properly now.
 
Dony said:
I went ahead and obtained a 20k ohm resistor rated at 5 watts. Soldered some 18 gauge wire to each end of the resistor and added alligator clips. Ready to discharge caps properly now.
Click to expand...


I think 20k is too large. For my Yamaha B1 they say to use a 100 watt 10 ohm resistor for the main 15,000 80 volt caps .

And I also found that larger resistance takes much longer for caps to discharge. I used a multimeter and different resistor values out of curiosity once and found the lower values discharge quicker , but wattage needs to go up.

Athanasios
 
Dony said:
I went ahead and obtained a 20k ohm resistor rated at 5 watts. Soldered some 18 gauge wire to each end of the resistor and added alligator clips. Ready to discharge caps properly now.
Click to expand...
Ready to discharge caps slowly now.
Nashou66 said:
I used a multimeter and different resistor values out of curiosity once and found the lower values discharge quicker.
Click to expand...
Should be obvious, really, shouldn't it?
 
rothwellaudio said:
Ready to discharge caps slowly now.

Should be obvious, really, shouldn't it?
Click to expand...

Well, I don't mind it being slow as long as it is safe. I will also check status with a multimeter and once the readings are low enough then I will use a insulated screwdriver to complete the discharge of the capacitor. All is good.

Thanks

Don
 
Dony said:
Well, I don't mind it being slow as long as it is safe. I will also check status with a multimeter and once the readings are low enough then I will use a insulated screwdriver to complete the discharge of the capacitor.
Click to expand...
That sounds a bit like driving the 20 miles from one side of town to the other at 5mph for the first 19 miles, then doing the last mile at 90mph with your eyes shut.
 
You would need to know the safe voltage at which discharging the cap using shorting by a screwdriver is safe for you, for the cap, as well as for the connected circuits. To me, it is more difficult than deciding whether 20k or 15k is a good choice.
 
gslikker said:
You would need to know the safe voltage at which discharging the cap using shorting by a screwdriver is safe for you, for the cap, as well as for the connected circuits. To me, it is more difficult than deciding whether 20k or 15k is a good choice.
Click to expand...

Hi gslikker,

So from my understanding and what I have read, 50V DC or less and or getting the voltage to less than 50% of the rating of the capacitor should make it safe to touch. So thus if I put on my discharge tool and run the capacitor down to below 50V DC or less than 50% of the rating of the capacitor and then use an insulated screw driver to complete the discharge then I should be safe and the risk of damaging the connected circuits would be minimized.

I'm not worried about the state of the capacitor itself as I am intending to replace the capacitors with a new ones and thus is the reason why I am discharging it.

Thanks

Don
 
rothwellaudio said:
That sounds a bit like driving the 20 miles from one side of town to the other at 5mph for the first 19 miles, then doing the last mile at 90mph with your eyes shut.
Click to expand...

Hi rothwellaudio,

So I'm a little confused here, what are you recommended to be done?

You stated previously "The crudest discharge tool is a screwdriver - just short the cap's terminals with it - but the sparks you'll get can be quite alarming and you could do some damage to yourself or the cap."

So if I don't use a discharge tool I could damage to myself or the cap, but if I do use it and then use the screwdriver at the end when there is minimal voltage left then that is not good either. Can you clarify what your recommendation actually is?

Thanks

Don
 
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