# Charging a Capacitor with High Frequency DC voltage

1. Nov 21, 2012

### gedfire

Hello All,

Could you please explain what would happen if I charge a capacitor with say 30 khz DC 500 volts with a 10000 uf cap. Would a gas discharge tube, varistor, triac of 300 volts connected across the capacitor help to preventing it from exploding?

Ged

2. Nov 21, 2012

### nasu

What is the voltage rating of the capacitor?

3. Nov 21, 2012

### willem2

The reactance of the capacitor is

$$\frac {1} { 2 \pi f C }$$ = 5.3 * 10^(-4) ohm,

If you connect it to a voltage source V(t) = 250 + 250 sin (2 pi f T), wich varies between 0 and 500 V.

You'll get an alternating.current of 250 / (5.3 * 10^(-4) = 4.7 * 10^5 A.

This is not a project that will fit in your garage.

Last edited by a moderator: Nov 21, 2012
4. Nov 21, 2012

### DocZaius

High frequency DC voltage? How can DC Voltage have frequency?

5. Jan 14, 2013

### gedfire

500 volts.

and another one, a super cap 12 volts @ 50Farads.

Pulsed DC @30khz.

Thanks for the interest.

Ged

6. Jan 14, 2013

### gedfire

Not clear why you said not a project that will fit in my garage.Please clarify.

Do you mean the massive amps and potential danger?

7. Jan 14, 2013

### gedfire

Thank you for replying.Yes you can have pulsed DC and it can be at any frequency I think.But heck, I am but a novice here...

Ged

8. Jan 14, 2013

### Low-Q

DC means Direct Current. The current flows only one way. Does not matter if it is pulsed. Continous Direct Current is something else. No frequency, but still DC.

However, using the DC for voltage might not be the correct terminology.

Vidar

9. Jan 14, 2013

### Drakkith

Staff Emeritus
Why would it explode if the voltage rating meets your applied voltage? (Obviously the 12 volt one should probably not have 500 volts applied to it)

10. Jan 14, 2013

### gedfire

Awesome man.

Thanks!

BTW could I just DC pulse a capacitor say though a diode then put a safety device to prevent it from going past its stated voltage? What would i use to do that, varistor, gdt etc? How could I pulsed the DC with a MOSFET etc to cap?

Ged

11. Jan 14, 2013

### Drakkith

Staff Emeritus
Hold on. I feel this thread is going to go nowhere unless you give us specific details of why you want to charge a capacitor to 500 volts at 30 KHz pulsed DC. Your questions lead me to suspect you may not have the appropriate training to safely accomplish this. I mean your original post is asking "what would happen if". That doesn't sound very good. What are you wanting to do here? Do you have any basic electronics training or knowledge?

12. Jan 14, 2013

### davenn

Agreed

Thanks
Dave

Last edited: Jan 15, 2013
13. Jan 18, 2013

### gedfire

I want to charge a capacitor to a certain voltage, keep it from overcharging then using that charge to power a load.

I do have basic electrical training and have actually made a capacitor charger using a flyback and self feedback oscillator made with resistors , a transistor, and microwave capacitors.Does work, dangerous sparks etc.But I wear good insulating electrician gloves, and thick wire on a chicken stick to discharge eme safely.I also used a meter to monitor charging.

So again, I wanna know if I can use a GDT or zener to limit the amount of voltage the cap can charge to?

Ged

14. Jan 18, 2013

### Drakkith

Staff Emeritus
What kind of load are you planning on using? Is a capacitor going to be capable of powering it? Is the load going to be able to handle the change in voltage as the capacitor discharges?

To my understanding, and I admit that my knowledge of electronics is mostly from a basic electronics course I just finished, capacitors are generally used as filters in power supplies and RF circuits, not as power sources.

15. Jan 18, 2013

### gedfire

What kind of load are you planning on using

Flourescent light bulb, LEDs, etc.

Is a capacitor going to be capable of powering it?

Oh yes depending on the voltage and capacity ratings of the capacitor.

Is the load going to be able to handle the change in voltage as the capacitor discharges?

More capacity means a longer time to discharge again depending on the load.

BTW I am fully aware of and exercise great care in handling high voltages.

See have a clearer idea of what I am talking about please view the following:

Type in youtube and supercaps and see more.

Ged

Last edited by a moderator: Sep 25, 2014
16. Jan 19, 2013

### sophiecentaur

Using a zener is not a good way to limit the volts on the capacitor (when you're talking in terms of 'power supply'. If you want to charge the capacitor quickly, you want a high current and this current would be dumped by a shunt zener diode - wasting energy and getting things hot. You need a voltage regulator, which will deliver no current once the desired voltage has been reached. This kind of regulator is widely available as a package. In general, shunt regulators are not a good idea. They are ok as a 'crowbar' protection against over voltage but not for your application.

17. Jan 19, 2013

### BruceW

I read somewhere online that it is good practice to only use half the voltage rating of the capacitor. So if your peak input voltage is 500v, then why not just send half of it somewhere else? Or make less in the first place?

18. Jan 19, 2013

### Drakkith

Staff Emeritus
This seems like a pretty simply problem. You want to charge a capacitor with pulsed DC current to a certain voltage. Sounds like a simple voltage regulator would work as Sophie said.
Sophie, since this is pulsed DC, should some sort of filter be used to smooth it out? Would that help at all?

19. Jan 19, 2013

### sophiecentaur

It would all depend upon the details of the way the capacitor is rated. I agree it would be best to chose one that is rated a bit higher - as a matter of 'conservative principle' but you could find yourself paying more money than you need if you went for twice the voltage rating. Other factors also could become relevant, though. The RMS current can also be relevant to how much stress a capacitor is having to take.

20. Jan 19, 2013

### BruceW

I assumed this is exactly what he is using the capacitor to do. (simply to smooth out the pulsed dc a bit).