Hi,

Is it OK to use a higher than rated voltage to charge a capacitor (limiting the current), and stop when a threshold voltage is reached (below the breakdown voltage of the capacitor) ?
For instance, charging a 400v capacitor with 600v dc and stopping at 350v..

Thks

 

Hi,

Is it OK to use a higher than rated voltage to charge a capacitor (limiting the current), and stop when a threshold voltage is reached (below the breakdown voltage of the capacitor) ?
For instance, charging a 400v capacitor with 600v dc and stopping at 350v..

Thks

Yes and no. Yes it is safe to the component because the capacitor doesn't see what voltage is on the supply side of your current-limiting circuitry (resistor).

No (in some cases) because there is no safety feature to stop the charging of the cap if your logic is not perfect or a single component fails. A redundant system should be used to cut off power. Also, there are more specs to a cap that WVDC. Make sure you comply with all specs - especially at those voltages because a capacitor certainly goes out with a bang and cloud of who-knows-what when they are over charged or other over-stressed conditions.

 

I'm with Gopher. While it's theoretically safe, it would be poor design practice at the voltages you mention. I'm trying to think if I ever saw a capacitor charged from 600 volts that was only rated for 400, and nothing comes to mind.

Supercapacitors are an exception, because with today's technology, there is no choice except a low voltage rating (maybe 2.5 volts), and the high capacitance can justify additional support circuitry to limit voltage.

 

I'm guessing your using some sort of RC circuit and a timer? I agree with the two posters above, theoretically it seems ok but why do it? I had a tiny 25V .1µf cap electrolytic cap blow one time, scared the hell out of me.

 

I was thinking about it because I have a 415Vac (590V peak) coming from an isolation transformer, so I wanted to directly charge a capacitor, using an SCR before the transformer to control the current. Then, I was probably gonna cut off the power using a voltage divider/comparator. But indeed, if a single component would fail, this would be pretty dangerous.
I guess I should go with the SMPS buck converter.

 

What is the capacitor for? Is it just to get a cheapo, unregulated power supply?

I get very nervous around big voltages like you have. I've always designed products using 24VDC/15VDC/5VDC from AC line using a SMPS. It's also a lot easier to get UL/CSA/IEC approvals, if that matters to you.

 

the application would be to charge a bank capacitor, so I don't need a stable DC output. SMPS would be a solution, but I never designed one, so I was looking at the other possibilities. In fact, I can't really think of another solution which wouldn't involve another transformer, a voltage divider (too much power wasted) or linear regulator (same, too much power wasted).

And as it will stay a personal project, I don't need specific approvals, even though safety has to be part of my circuit.