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Electrolytic Capacitor in Voltage Multiplier?

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Saz43

Tue Mar 15 2011, 05:42AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

I've seen many voltage multipliers built with electrolytic capacitors, and even multiplier schematics with the electrolytic symbol used, but isn't that a problem since the first capacitor (C1) sees full AC?

I'm talking about the standard circuit here:

This is important for my project because my design has to be compact, and if I can use a lytic it will be about a fourth of the size. I've searched the forum and Google and can't find anything conclusive. Thank you to anyone able to provide clarification.

Daedronus

Tue Mar 15 2011, 10:33AM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

The fist cap does not see the full AC, D1 is in series with it.
The fist cap sees the full rectified DC, every other caps sees 2x DC

Patrick

Tue Mar 15 2011, 07:38PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Yes Daedronus is right in his explanation. Perhaps Saz43, you should use a simulator to convince yourself it is so. If you build a good approximation inside the computer, you can avoid many common problems. Also if your CW does anything funny you can go back to the simulator and investigate.

I was very near a CW, when a student mis-assembled it and it exploded. (but this was a direct 120v socket input with no transformer.)

Saz43

Fri Mar 18 2011, 03:12PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Thanks for the replies! Here's another question from someone who hasn't worked much with AC before:

I have a converted computer PSU powering an inverter (car portable type) driving a voltage doubler to charge a capacitor. The PSU also supplies 12V to some control circuitry. I want the capacitor to have the same ground reference as the PSU/control circuitry so I can fire it with a mosfet later. How do I do that?

While being charged by the multiplier, the capacitror negative terminal has either 24VDC or 70VAC referenced to the PSU ground (I'm not sure which value is accurate). When charging stops, the difference falls to 2-5VDC. Trying to connect the two nodes shorts out the PSU and trips the overcurrent shutoff. What's going on here?

A crude drawing to illustrate my question:

Daedronus

Fri Mar 18 2011, 03:43PM

Registered Member #2329 Joined: Tue Sept 01 2009, 08:25AM
Location:
Posts: 370

Try to connect the 12V psu ground to the other pin coming out of the car inverter.
One of the output pins of the inverter should be the ground...but not necessarily the one you think it is....

Saz43

Sat Mar 19 2011, 12:15AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Daedronus wrote ...

Try to connect the 12V psu ground to the other pin coming out of the car inverter.
One of the output pins of the inverter should be the ground...but not necessarily the one you think it is....

Hmm, connecting the PSU ground to any of the three pins of the inverter seems to causes a short and shuts off the PSU.

klugesmith

Sat Mar 19 2011, 06:16PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

> ...any of the three pins on the inverter...
Huh?

Last year I reverse-engineered and repaired a similar inverter, I think 300 watt rating.
Its AC output does not have one pin common with the DC input.
I would hope it's isolated, but I don't remember if it's strictly so.
Four MOSFETS switch the DC input into a transformer whose secondary is rectified to 170 V DC.

The AC output lines are driven by an H-bridge,
which alternately connects each side to secondary GND or +170V.
(with intermediate phases where both output lines are connected to GND).

Saz43

Sat Mar 19 2011, 10:50PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Klugesmith wrote ...

> ...any of the three pins on the inverter...
Huh?

Last year I reverse-engineered and repaired a similar inverter, I think 300 watt rating.
Its AC output does not have one pin common with the DC input.
I would hope it's isolated, but I don't remember if it's strictly so.
Four MOSFETS switch the DC input into a transformer whose secondary is rectified to 170 V DC.

The AC output lines are driven by an H-bridge,
which alternately connects each side to secondary GND or +170V.
(with intermediate phases where both output lines are connected to GND).

Thanks Klugesmith, that makes perfect sense then, looking at the output waveform

Is there any way then to ground the output common to the input DC voltage?

I want to charge a capacitor and then switch it into my coil with IGBTs, which I don't think will trigger if the control and driver circuitry is all referenced to DC ground and the capacitor isn't.

Everyone, your help is hugely appreciated, thank you so much.

Saz43

Tue Mar 22 2011, 02:31AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Ok, my inverter is the same, transformer putting out 86VAC, rectified, then output through an H-bridge. I'm getting some weird behavior when I play with it now:

When I bypass the output H-bridge and try to draw current straight from the transformer secondary, it shuts off instantly for overcurrent, even though its feeding the exact same load that it happily supplies via the H-bridge. Also, grounding 1 leg of the secondary (common to PSU) seems to stop all oscillation and there's no voltage on the secondary at all. Ideas??

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