Discharging those big old capacitors - diyAudio
Go Back   Home > Forums > Design & Build > Construction Tips

Construction Tips Construction techniques and tips

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 30th November 2010, 02:10 PM   #1
diyAudio Member
 
Join Date: Nov 2010
Location: Wiltshire UK
Default 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.
  Reply With Quote
Old 30th November 2010, 03:42 PM   #2
ArtG is offline ArtG  United States
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.
  Reply With Quote
Old 30th November 2010, 06:23 PM   #3
diyAudio Member
 
Join Date: Nov 2010
Location: Wiltshire UK
Just the ticket, thanks.
  Reply With Quote
Old 15th December 2010, 07:55 PM   #4
diyAudio Member
 
kstagger's Avatar
 
Join Date: Dec 2003
Location: Grand Rapids
nice thread of the tube amp forum
Safety Practices, General and Ultra-High Voltage
  Reply With Quote
Old 15th December 2010, 08:01 PM   #5
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
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 = 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
  Reply With Quote
Old 15th December 2010, 10:05 PM   #6
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
  Reply With Quote
Old 15th December 2010, 10:12 PM   #7
diyAudio Member
 
Join Date: Dec 2008
Location: Victoria, BC
Quote:
Originally Posted by tomchr View Post
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....
  Reply With Quote
Old 17th December 2010, 04:32 AM   #8
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?
  Reply With Quote
Old 17th December 2010, 07:32 PM   #9
diyAudio Member
 
Join Date: Jun 2010
wear heavy duty rubber dish washing gloves and keep your trainers on, take all jewellry off too...
  Reply With Quote
Old 18th December 2010, 12:31 AM   #10
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?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Discharging Cap Question skid20 Everything Else 7 11th March 2010 02:52 AM
FS - Big Capacitors mtflycaster Swap Meet 0 12th April 2008 11:43 PM
Discharging capacitors pjpoes Parts 10 3rd June 2007 07:54 AM
Big capacitors in PSU? Higo Chip Amps 5 5th August 2003 12:32 PM
How big do the capacitors have to be? MikeW Pass Labs 60 23rd November 2002 01:36 PM


New To Site? Need Help?

All times are GMT. The time now is 06:52 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2