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Charging a Capacitor with High Frequency DC voltage 
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#19
Jan1913, 02:03 PM

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#20
Jan1913, 04:20 PM

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#21
Jan1913, 04:24 PM

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#22
Jan1913, 05:03 PM

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#23
Jan2013, 01:52 PM

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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 lets 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... 


#24
Jan2013, 04:32 PM

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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'? 


#25
Jan2013, 06:55 PM

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#26
Jan2013, 07:41 PM

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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 oneway charging current then the equilibrium state would be at peak voltage. No averaging required, RMS or otherwise. 


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Jan2013, 10:52 PM

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#28
Jan2113, 05:36 AM

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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 'nonmains' frequency involved, is there any difference from what we find in every conventional power supply?



#29
Jan2113, 06:13 AM

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#31
Jan2113, 06:27 AM

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#32
Jan2113, 06:50 AM

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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. 


#33
Jan2113, 07:28 AM

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#34
Jan2113, 12:33 PM

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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 nonzero 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. 


#35
Jan2113, 01:26 PM

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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. 


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