How long to warmup/ form new capacitors with a variac?

[rh2music]

Hi, How long should I warmup/form new capacitors with a variac? I have read 2 different articles, as short as 5 to 15 minutes or as long as 10 to 12 hours? Should the time be increased by the number of capacitors? Thanks

[Shannon Parks]

Hi, How long should I warmup/form new capacitors with a variac? I have read 2 different articles, as short as 5 to 15 minutes or as long as 10 to 12 hours? Should the time be increased by the number of capacitors? Thanks

Forget about time - it could be long or short depending on how much reforming is needed. The key is to reform at a rate where the capacitor won't start heating up enough to impede the reforming process. And you don't even need a Variac! You can get by with a high voltage source and maybe a 100K and 10K resistor.

Example
- Need a 500V source voltage
- 100K 2W resistor
- 10K 2W resistor
- Dynakit Quadcap (four 450V rated sections)

Tip: Try to keep leakage current below 10mA at all times.

I jumper all the sections that are rated the same voltage together. I have received much better reforming results this way versus reforming each section individually. Connect the Quadcap to the 500V through the 100K resistor. The resistor limits the voltage and current naturally. Clamp a DMM on either side of the 100K resistor. A voltage drop of 400V means 4.00mA leakage current. So eventually you might end up with the 100K resistor dropping too much voltage. If the leakage current doesn't really improve after one hour, then drain everything and switch to the 10K resistor. I think I shoot for about 500uA leakage current per section at 450V.

Last time I reformed one, I think it took about 4-6 hours. Make sure it is safely secured as you may forgot about this exposed voltage lying on your bench. Using the 100K resistor may seem a little like overkill, but a nice slow start keeps the process on track at the end.

If one does have a Variac, I would maybe just skip the 100K resistor and use a 10K but start with a smaller voltage - say 1/3 rated power. After no change for 1H then 2/3 rated, etc. But let the resistor and the leakage current do the work.

Tip: A high voltage current source can be used to simply reform your caps and you can start gently at a 4mA setting.

This can be done in a ST70, but disconnect the driver power lines from the cap. You can remove the 5AR4 and use a 1N4007 to get a little higher voltage if you want. You'll need to add a temporary input cap, too, before the resistor.

[dhuebert]

new capacitors

Why would you "form" new capacitors?

Don

[erichayes]

Hi All,

NOS caps should be handled the same as in-circuit caps of the same vintage, i.e., reformed.

If one is going to make a career out of reforming old caps, an old capacitance bridge such as a Sprague telOmike can make life easier. These can usually be found at hamfests and flea markets for a reasonable price.

But then 45s and 2A3s used to be found for reasonable prices, too . . .

[rh2music]

Hi, what I mean is to warmup or burn in the caps, even though they are all new. Like burning in new tubes.. The amp is an old RCA tube PA from the late 50's. :) Thanks,

[erichayes]

Hi All,

The current techniques and materials used in making electrolytic caps require no breaking in period for them as long as you're operating them within their voltage rating. If you're planning on deliberately operating them outside their parameters, you'll need a power supply capable of inflicting the desired voltage on the cap, and a thermcouple type thermometer to precondition the cap before putting it in service.

Securely fasten the thermocouple to the cap; use a small dab of silicone heatsink compound between the TC and the can for better thermal transfer.

Use the technique described by Shannon, but monitor both leakage current and temperature. If the temp rises to 40°C, regardless of the leakage current, the cap is not a candidate for supervoltage operation.