Electrolytic Cap Voltage

Feb 11, 2025 Last reply: 1 year ago 1 Replies
U

I replaced a set of 500V / 100uF caps used in a HV doubler off a power supply that in the end generates about 900V from the typical input voltage.



I had a set of EPCOS (TDK) 450V / 120uF caps and installed them just to see if they were in the limit.



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In initial power up they seem to have about 478V on them with no load, with a small load they head down to 470v and under normal operations in the product they are as low as 430V.



We are about 7.8% over the 450V when the math is done.



The product is a ham radio transciever, so while listening the voltage on the caps is about 480V max. Transmit def brings the voltage below its rating.



I have seen test equipment designed with caps that are right at the operating voltage of the power supply and have the max voltage on them last about 35 yrs before failing, so I guess there is more wiggle room near the top in some cases?



In this case, they have a 0.01uF disc cap plus a 470k bleeder resistor in parallel with each cap, which I also replaced new. I believe this will help to smooth out the initial power spike the bigger caps would see?


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E

The .01's bypass RF; typically "initial power surge" refers to the high current a totally discharged cap draws when voltage is firsts applied. If so equipped, a "slow start" circuit keeps that "initial power surge" much lower. "wiggle room", well ummm I guess it's your transceiver so you can take whatever chances you want.

Ed

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