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LRC Circut with a polarized cap

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Marauder709

Thu Jan 01 2009, 03:47AM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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Posts: 22

I have a polarized cap bank (Aluminum electrolytic) discharging through an underdamped LRC circuit. How do I prevent the capacitor form acquiring a reverse polarity?

rp181

Thu Jan 01 2009, 03:48AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Put a diode in antiparalell.

Marauder709

Thu Jan 01 2009, 05:15AM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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A diode in anti-parallel will not stop the cap from reversing polarity

Voltage and Current are out of phase in inductor circuit (2/pi radians for a perfect LC circuit I think). The capacitor does not reverse polarity because the voltage and/or current reverses.

The inductor provides an EMF to oppose the change in current. In the case of this circuit, the current is decaying and so the inductor applies an EMF to stop the current decay. However, the current is still moving in the same direction. The inductor then functions to prolong the pulse from the capacitor and in doing so it the capacitor moves from say +50 V through 0 and back to -50 V (in an ideal situation).

Maybe there is some part of this I am missing. As far as I can work out however, simply putting a diode in anti parallel wont cut it. Does anyone have a schematic, perhaps that will help me see exactly what you mean.

Myke

Thu Jan 01 2009, 05:45AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

The diode in antiparallel with the cap will stop the voltage reversal. If the voltage tried to charge the cap in reverse, the diode will conduct preventing the cap from charging.

badastronaut

Thu Jan 01 2009, 07:41AM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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I think Myke has the right idea. The diode basically removes the capacitor from the circuit after the cap voltage goes to zero. What's left is an RL circuit since the diode is shorting the cap. MEOW. I think they call them crowbar diodes.

El_Roberto

Thu Jan 01 2009, 09:59AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
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MEOW? wat?

Marauder709

Thu Jan 01 2009, 03:47PM

Registered Member #1895 Joined: Thu Jan 01 2009, 03:12AM
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Posts: 22

I would agree with the anti-parallel argument if there was a voltage reversal. My understanding of LC circuts however is that while there is a reversal of voltage in the capacitor there is not reversal of current.

When the capacitor voltage begins to fall the inductor attempts to maintain the also falling current. To do this it applies an EMF in the same direction as the capacitor voltage. The current therefore continues to flow after the capacitor voltage has reached zero. This continuation of current is the problem. Because of this continuation of current, the capacitor develops a reverse voltage. Is any of this wrong?

My capacitors have a polarity rating and cannot charge with a reverse bias. A diode in anti-parallel will not prevent the capacitor charging in reverse (as far as I know).

big5824

Thu Jan 01 2009, 03:54PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

im too hungover to think.....but most of us use an antiparallel diode over the coil, and it works for us, my caps havent exploded yet :)

rp181

Thu Jan 01 2009, 04:17PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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My thinking is that there will be a very short pulse in the capacitors as it takes the diode time to turn on (that is why i use ultrafast diodes).

badastronaut

Thu Jan 01 2009, 05:29PM

Registered Member #222 Joined: Mon Feb 20 2006, 05:49PM
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Posts: 96

Well, you're half right. At first there is no reversal of current; this is why the cap charges in reverse, but if you wait long enough, the reverse charged cap will discharge into the inductor causing a reverse current. This oscillation will continue until the energy has run out.

I think you're missing what they mean by antiparallel. Someone draw up a diagram.

You are right in that the cap will charge in reverse because the inductor forces the current to continue. If you put that diode in parallel with the cap, what will happen is that inductor current will flow through the diode instead of the cap, so the cap voltage will be zero.

If you still don't think this is true, you can probably download a free circuit simulator and play around with it. I think analog devices has a download for multisim.

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