Charging a Capacitor with High Frequency DC voltage

[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

 

[nasu]

What is the voltage rating of the capacitor?

 

[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), which 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.

 

[DocZaius]

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

 

[gedfire]

What is the voltage rating of the capacitor?

500 volts.

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

Pulsed DC @30khz.

Thanks for the interest.

Ged

 

[gedfire]

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), which 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.

Thanks for replying,

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?

 

[gedfire]

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

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

 

[Low-Q]

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

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

 

[Drakkith]

500 volts.

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

Pulsed DC @30khz.

Thanks for the interest.

Ged

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)

 

[gedfire]

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

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

 

[Drakkith]

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

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?

 

[davenn]

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?

Agreed

MODS, can you please assess this thread for safety reasons

Thanks
Dave

 

[gedfire]

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?

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 want to know if I can use a GDT or zener to limit the amount of voltage the cap can charge to?

Ged

 

[Drakkith]

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.

 

[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:


Please read the comments after.Lots of good stuff here.







Type in youtube and supercaps and see more.

Ged

 

[sophiecentaur]

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 want to know if I can use a GDT or zener to limit the amount of voltage the cap can charge to?

Ged

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.

 

[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?

 

[Drakkith]

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?

 

[sophiecentaur]

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?

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.

 

[BruceW]

Sophie, since this is pulsed DC, should some sort of filter be used to smooth it out? Would that help at all?

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

 

[BruceW]

Other factors also could become relevant, though. The RMS current can also be relevant to how much stress a capacitor is having to take.

yeah. I am too much in the habit of thinking of circuit components to always act in the 'ideal' way. I'd guess kHz frequency is nice enough though.

 

[Drakkith]

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

I don't think so. From his description and linked videos I think he wants to use the capacitor as an actual power source once charged. I don't know why, as a battery would seem to be a much better choice, but it's not my project.

 

[BruceW]

ok. i think you are right. I am trying to get my head around it. Ok, so supposing he does charge the capacitor, then discharges it through an incandescent bulb, it will take of the order of RC (about 1 second) for the first capacitor he mentioned to discharge. (assuming ideal resistance). I guess this should be long enough for him to see the flash of light.

I am not so sure about how he is going to charge it using pulsed dc though. pulsed dc will vary from zero voltage to the peak voltage, right? So if he let's the capacitor charge, then disconnects it from the pulsed dc, then the voltage 'stored' in the capacitor could be anything from zero volts to the peak voltage...

 

[sophiecentaur]

The nearest thing that I can think of to this is a camera flash circuit. But the load is of a specific type - well fitted to using the energy stored in a capacitor. Anything that requires a particular operating voltage would just not be suited.
Or is this "load" something slightly 'mischevious'?

 

[Drakkith]

I am not so sure about how he is going to charge it using pulsed dc though. pulsed dc will vary from zero voltage to the peak voltage, right? So if he let's the capacitor charge, then disconnects it from the pulsed dc, then the voltage 'stored' in the capacitor could be anything from zero volts to the peak voltage...

It would be the average or RMS voltage or something like that I believe.

 

[jbriggs444]

It would be the average or RMS voltage or something like that I believe.

If you model it as an ideal capacitor in series with a voltage source and some internal resistance then I would expect it to be an exponentially decaying weighted average. The time constant on the exponential decay would be determined by the capacitor rating in farads divided by the power supply resistance in ohms. In the limiting case of an ideal voltage source with negligible internal resistance, BruceW has it right -- the final charge is an instantaneous sample of the final supply voltage.

This model means that the charging current would end up being AC, not pulsed DC.

If you were to use a diode or similar as a "ratchet" to enforce one-way charging current then the equilibrium state would be at peak voltage. No averaging required, RMS or otherwise.

 

[Drakkith]

If you model it as an ideal capacitor in series with a voltage source and some internal resistance then I would expect it to be an exponentially decaying weighted average. The time constant on the exponential decay would be determined by the capacitor rating in farads divided by the power supply resistance in ohms. In the limiting case of an ideal voltage source with negligible internal resistance, BruceW has it right -- the final charge is an instantaneous sample of the final supply voltage.

This model means that the charging current would end up being AC, not pulsed DC.

If you were to use a diode or similar as a "ratchet" to enforce one-way charging current then the equilibrium state would be at peak voltage. No averaging required, RMS or otherwise.

You're saying that without some kind of resistance in the circuit the capacitor charges and discharges so fast that it follows the pulsed DC almost exactly?

 

[sophiecentaur]

We have taken 27 posts, just talking around how a normal rectifier circuit with a reservoir capacitor works. Apart from the relatively high voltage and the 'non-mains' frequency involved, is there any difference from what we find in every conventional power supply?

 

[jbriggs444]

You're saying that without some kind of resistance in the circuit the capacitor charges and discharges so fast that it follows the pulsed DC almost exactly?

Yes. This does also suppose zero inductance.

 

[Drakkith]

We have taken 27 posts, just talking around how a normal rectifier circuit with a reservoir capacitor works. Apart from the relatively high voltage and the 'non-mains' frequency involved, is there any difference from what we find in every conventional power supply?

Some of us like to take the "scenic" route!

 

[BruceW]

We have taken 27 posts, just talking around how a normal rectifier circuit with a reservoir capacitor works. Apart from the relatively high voltage and the 'non-mains' frequency involved, is there any difference from what we find in every conventional power supply?

no, as Drakkith was saying, the OP'er is not using the capacitor to smooth out the pulsed dc. He wants to charge up the capacitor, then discharge it separately. So the capacitor is not being used as a reservoir capacitor (which is what I initially assumed, too).

 

[Dale]

Place the capacitor in series with a diode to keep the capacitor from discharging. If you cannot get a voltage regulator then place that in parallel to a zener to set the max voltage. Place that in series with a resistor to limit peak current.

The capacitor will basically charge like a normal RC circuit, but the time constant will have to be scaled up by a factor of the duty cycle of the source.

 

[sophiecentaur]

Some of us like to take the "scenic" route!

HAHA

no, as Drakkith was saying, the OP'er is not using the capacitor to smooth out the pulsed dc. He wants to charge up the capacitor, then discharge it separately. So the capacitor is not being used as a reservoir capacitor (which is what I initially assumed, too).

So now it's like a camera flash? Also very much a 'known art'.

 

[BruceW]

yeah, the tricky bit is thinking of what the pulsed dc would do. A camera flash uses normal dc to charge the capacitor I would guess. But for the pulsed dc, even though the voltage is always in the same 'direction' across the capacitor, it would generally increase and decrease.

If the pulsed dc had very low frequency compared to the time constant, the voltage across the capacitor would equal the instantaneous voltage of the pulsed dc at all times (since the capacitor can 'catch' up, before the pulsed dc voltage can change to another value). So in this limit, the voltage stored in the capacitor will be any value of the instantaneous voltage of the source.

And in the limit of very high source frequency compared to the time constant, I would intuitively think that the voltage stored in the capacitor would tend to some non-zero value which is approximately constant with time. I haven't done the calculation though.

jbriggs and dalespam have the interesting idea of using a diode to ensure that current only travels in one direction. This seems like it might work... I'm guessing it means that a lot of the power from the pulsed dc is going to be 'wasted' in resistance of the diode. But maybe this is the best way.

 

[sophiecentaur]

yeah, the tricky bit is thinking of what the pulsed dc would do. A camera flash uses normal dc to charge the capacitor I would guess. But for the pulsed dc, even though the voltage is always in the same 'direction' across the capacitor, it would generally increase and decrease.

If the pulsed dc had very low frequency compared to the time constant, the voltage across the capacitor would equal the instantaneous voltage of the pulsed dc at all times (since the capacitor can 'catch' up, before the pulsed dc voltage can change to another value). So in this limit, the voltage stored in the capacitor will be any value of the instantaneous voltage of the source.

And in the limit of very high source frequency compared to the time constant, I would intuitively think that the voltage stored in the capacitor would tend to some non-zero value which is approximately constant with time. I haven't done the calculation though.

jbriggs and dalespam have the interesting idea of using a diode to ensure that current only travels in one direction. This seems like it might work... I'm guessing it means that a lot of the power from the pulsed dc is going to be 'wasted' in resistance of the diode. But maybe this is the best way.

That whistle you hear is the inverter - which produces pulses, surely, after rectification. How would th voltage on the capacitor decrease except through the load?
Until the capacitor is loaded, the time constant will be very long.
The forward voltage of a diode is hardly relevant to power dissipation in this sort of circuit.

I don't understand why there is so much arm waving on this thread. Put the values of the components into a simulator (if the sums are 'too hard') and see what emerges. Of course, the whole thing depends totally on the values of the critical components- like the capacitor, the operating frequency. If the OP can supply them then it can all be solved with standard tools.

Perhaps this should be on the Electrical Engineering Forum.