DISCHARGE HV CAPACITORS. - UK Vintage Radio Repair and Restoration Discussion Forum

23rd Jul 2003, 9:08 pm

Hi.
I don't want to use a screw driver; what is the best way to discharge caps?

Mat.

Paul Stenning · 23rd Jul 2003, 10:48 pm

For the HT caps in radios (250-350V DC) a 25W mains light bulb with a couple of flying leads is effective. The bulb initially flashes bright and quickly extinguishes. It limits the current and gives a nice visual indication that the caps were discharged.

I don't know what you'd do up at EHT levels. You could make up a chain of high voltage resistors (standard resistors are only rated at a few hundred volts), perhaps. I thought a screwdriver was the usual method of discharging EHT, but I do accept that the current surge is not good for the capacitors.

Duke_Nukem · 23rd Jul 2003, 11:08 pm

Particular care is needed with EHT voltages. Just shorting with a screwdriver can cause a high current pulse which can be picked up in other wiring ; a proper valve telly won't notice (in fact drawing an arc from EHT to see if it was working was standard practice in the good old days) but the surges would cretainly risk destroying transistors.

TTFN,
Jon

23rd Jul 2003, 9:08 pm	#1
Mat Guest Posts: n/a	Discharging HV Capacitors. Hi. I don't want to use a screw driver; what is the best way to discharge caps? Mat.

23rd Jul 2003, 10:48 pm	#2
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,071	Re: DISCHARGE HV CAPACITORS. For the HT caps in radios (250-350V DC) a 25W mains light bulb with a couple of flying leads is effective. The bulb initially flashes bright and quickly extinguishes. It limits the current and gives a nice visual indication that the caps were discharged. I don't know what you'd do up at EHT levels. You could make up a chain of high voltage resistors (standard resistors are only rated at a few hundred volts), perhaps. I thought a screwdriver was the usual method of discharging EHT, but I do accept that the current surge is not good for the capacitors. __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

23rd Jul 2003, 11:08 pm	#3
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: DISCHARGE HV CAPACITORS. Particular care is needed with EHT voltages. Just shorting with a screwdriver can cause a high current pulse which can be picked up in other wiring ; a proper valve telly won't notice (in fact drawing an arc from EHT to see if it was working was standard practice in the good old days) but the surges would cretainly risk destroying transistors. TTFN, Jon