Discharging those big old capacitors

revoli · 30th November 2010, 02:10 PM

Advice please. I want to make some mods to my valve amp. Clearly there are some big voltages kicking around and some capacitors that I wish to change include 100uF 450V. What are the safety precautions I need to take here. I have nice thick gloves goggles et al but am not clear on safe discharge and storage of capacitors.

ArtG · 30th November 2010, 03:42 PM

First, disconnect the amplifier from the AC line. If you have a voltmeter, clip the negative lead to the chassis, or negative side of the capacitor, and using one hand only, check the voltage at the positive terminal of the capacitor. If the circuit uses bleeder resistors, the capacitor voltage will drop to nearly 0V in a couple minutes on it's own. If not, solder a couple leads with clips to a resistor with at least a 2 watt rating. The value of the resistor isn't critical, but 10K will discharge a 450V 100mfd cap in less than five minutes. Clip the resistor across the leads (again using one hand) to discharge it, and remove the capacitor from the circuit. Resist the temptation to discharge the cap by shorting the terminals with a screwdriver or something similar, as the high current "jolt" can permanently damage the cap.

Leave the resistor connected to the capacitor for a day or so after it is removed, since dielectric absorption will cause electrolytic caps to "charge up" again after being apparently fully discharged. Examine the removed cap for any signs of bulging or and leakage at all. If any is suspected, discard the cap.

revoli · 30th November 2010, 06:23 PM

Just the ticket, thanks.

kstagger · 15th December 2010, 07:55 PM

nice thread of the tube amp forum
Safety Practices, General and Ultra-High Voltage

tomchr · 15th December 2010, 08:01 PM

Eh.... 10 kOhm across 450 V will cause 20 W to be dissipated in the resistor. With high voltages it's really easy to get multiple watts of dissipation in resistors that are considered "high value" in the "sand" world. Do the P = E^2/R math and use a safety factor of at least 2 (for prototypes) or 4~5 (for permanent installation).

As an estimate, you can calculate the amount of time needed to fully discharge the capacitor as t = 5*R*C (R in Ohm, C in Farad, t in seconds).

In addition, note that many tube rectifiers have max specs for the amount of capacitance they can drive. Check the spec sheets or use the same value as was in the original equipment.

~Tom

VictoriaGuy · 15th December 2010, 10:05 PM

One variation on the 'resistor with two leads and clips' theme is to solder one (insulated) lead to the resistor. Then, use heat-shrink to fasten the resistor to the end of a wooden or plastic chopstick so that the resistor lead is protruding 1/4" or so. Clip the lead to chassis ground, and hand-hold the 'probe' against the + point on the capacitor.
I don't like sticking my fingers into a chassis which may have charged caps. 'Grabber' adapters on the voltmeter are handy as well - lets you monitor how the voltage is dropping.

John

VictoriaGuy · 15th December 2010, 10:12 PM

Quote:

Originally Posted by tomchr

As an estimate, you can calculate the amount of time needed to fully discharge the capacitor as t = 5*R*C (R in Ohm, C in Farad, t in seconds).

Thanks, Tom.
That formula shows that for common tube PS rectifier sizes you can increase the resistance quite a bit, thus lowering the watt-rating required on the resistor.
For instance, the 10k resistor would discharge a 40uF cap in 2 seconds, by my calculation. (And the voltage would be below dangerous levels before that..)
So, a higher ohm-value resistor and a bit of patience will do the trick....

Destroyer OS. · 17th December 2010, 04:32 AM

And what about on capacitors that are on the AC side? What do you guys recommend for that? I had a piece of equipment that used a 2 million ohm unit that was probably only 1 watt. Would a higher like 5-50w at any range work?

chokesrule · 17th December 2010, 07:32 PM

wear heavy duty rubber dish washing gloves and keep your trainers on, take all jewellry off too...

Destroyer OS. · 18th December 2010, 12:31 AM

I assume one of these would be safe since they are rated for it? Just use a 50kohm to discharge AC capacitors?

30th November 2010, 02:10 PM	#1
revoli diyAudio Member Join Date: Nov 2010 Location: Wiltshire UK	Discharging those big old capacitors Advice please. I want to make some mods to my valve amp. Clearly there are some big voltages kicking around and some capacitors that I wish to change include 100uF 450V. What are the safety precautions I need to take here. I have nice thick gloves goggles et al but am not clear on safe discharge and storage of capacitors.

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30th November 2010, 03:42 PM	#2
ArtG diyAudio Member Join Date: Nov 2006 Location: Central PA	First, disconnect the amplifier from the AC line. If you have a voltmeter, clip the negative lead to the chassis, or negative side of the capacitor, and using one hand only, check the voltage at the positive terminal of the capacitor. If the circuit uses bleeder resistors, the capacitor voltage will drop to nearly 0V in a couple minutes on it's own. If not, solder a couple leads with clips to a resistor with at least a 2 watt rating. The value of the resistor isn't critical, but 10K will discharge a 450V 100mfd cap in less than five minutes. Clip the resistor across the leads (again using one hand) to discharge it, and remove the capacitor from the circuit. Resist the temptation to discharge the cap by shorting the terminals with a screwdriver or something similar, as the high current "jolt" can permanently damage the cap. Leave the resistor connected to the capacitor for a day or so after it is removed, since dielectric absorption will cause electrolytic caps to "charge up" again after being apparently fully discharged. Examine the removed cap for any signs of bulging or and leakage at all. If any is suspected, discard the cap.

30th November 2010, 06:23 PM	#3
revoli diyAudio Member Join Date: Nov 2010 Location: Wiltshire UK	Just the ticket, thanks.

15th December 2010, 07:55 PM	#4
kstagger diyAudio Member Join Date: Dec 2003 Location: Grand Rapids	nice thread of the tube amp forum Safety Practices, General and Ultra-High Voltage

15th December 2010, 08:01 PM	#5
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	Eh.... 10 kOhm across 450 V will cause 20 W to be dissipated in the resistor. With high voltages it's really easy to get multiple watts of dissipation in resistors that are considered "high value" in the "sand" world. Do the P = E^2/R math and use a safety factor of at least 2 (for prototypes) or 4~5 (for permanent installation). As an estimate, you can calculate the amount of time needed to fully discharge the capacitor as t = 5RC (R in Ohm, C in Farad, t in seconds). In addition, note that many tube rectifiers have max specs for the amount of capacitance they can drive. Check the spec sheets or use the same value as was in the original equipment. ~Tom

15th December 2010, 10:05 PM	#6
VictoriaGuy diyAudio Member Join Date: Dec 2008 Location: Victoria, BC	One variation on the 'resistor with two leads and clips' theme is to solder one (insulated) lead to the resistor. Then, use heat-shrink to fasten the resistor to the end of a wooden or plastic chopstick so that the resistor lead is protruding 1/4" or so. Clip the lead to chassis ground, and hand-hold the 'probe' against the + point on the capacitor. I don't like sticking my fingers into a chassis which may have charged caps. 'Grabber' adapters on the voltmeter are handy as well - lets you monitor how the voltage is dropping. John

17th December 2010, 04:32 AM	#8
Destroyer OS. diyAudio Member Join Date: Nov 2005	And what about on capacitors that are on the AC side? What do you guys recommend for that? I had a piece of equipment that used a 2 million ohm unit that was probably only 1 watt. Would a higher like 5-50w at any range work?

17th December 2010, 07:32 PM	#9
chokesrule diyAudio Member Join Date: Jun 2010	wear heavy duty rubber dish washing gloves and keep your trainers on, take all jewellry off too...

18th December 2010, 12:31 AM	#10
Destroyer OS. diyAudio Member Join Date: Nov 2005	I assume one of these would be safe since they are rated for it? Just use a 50kohm to discharge AC capacitors?