# Toggle battery polarity across capacitor

Discussion in 'The Projects Forum' started by pafrazier, Jul 26, 2013.

1. ### pafrazier Thread Starter Active Member

Jun 4, 2011
57
0
Hi,

Can someone point me in the right direction.

I need a circuit to toggle the polarity of a battery across a capacitor.

The positive switch open, negative switch closed.

The negative switch open, positive switch closed.

One is open when the other is closed.

Any help much appreciated.

Thanks

Jul 18, 2013
10,836
2,510
DPDT? double pole double throw switch?
Max.

3. ### wayneh Expert

Sep 9, 2010
12,379
3,232
How quickly do you need to do this? 1Hz, 1MHz ? How big a capacitor?

It would help to know why you want to do this.

4. ### pafrazier Thread Starter Active Member

Jun 4, 2011
57
0
Hi Max and Wayneh,

I need the circuit to flip flop back and forth with out manual switch.

The frequency to be 20khz min.

Capacitor is 200 pf +/-.

I am experimenting with some alt. energy.

Thanks

5. ### wayneh Expert

Sep 9, 2010
12,379
3,232
Do you truly need reverse polarity, or just charge and discharge? The latter could be done with a simple 555 timer IC. The former is a bit trickier but not bad.

6. ### pafrazier Thread Starter Active Member

Jun 4, 2011
57
0
Hi Wayneh,

It is not charge - discharge.

Let me put it this way. I need to expose the positive terminal of the capacitor to the positive terminal of the battery-closed switch, while not exposing the negative terminal of the capacitor to the negative terminal of the battery-open switch. Then flop it the other way negative exposed to negative, while positive not exposed. There is not a reversal. The terminals are never connected at the same time. Does that help?

Thanks

7. ### wayneh Expert

Sep 9, 2010
12,379
3,232
You're losing me. What is the "unconnected" end of the capacitor connected to? Changing the potential of one side of a capacitor won't change its store of energy unless the circuit is completed somehow.

But I suppose you could accomplish this with the 555 output directed to 2 diodes, one forward and one reverse biased. The low side of the capacitor would see a path to ground when the timer output goes to ground, and the high side would see the 555 output voltage (less the diode drop) when the timer goes high. The capacitor would quickly achieve a steady state charge, though, so again I don't see the point.

8. ### pafrazier Thread Starter Active Member

Jun 4, 2011
57
0
Thanks Wayneh. I will check it out.

Thanks Again

9. ### wayneh Expert

Sep 9, 2010
12,379
3,232
I can't put my finger on it but this smells like another perpetual motion project. I hope you're not wasting (y)our time on such a thing.