MadSci Network: Engineering

Re: Can a capacitor be discharged slowly? (like a battery)

Date: Fri Oct 13 15:35:25 2000
Posted By: Donald Howard, Staff, Nuclear Engineering, Retired
Area of science: Engineering
ID: 970876892.Eg

I am trying to use capacitors instead of batteries to run electrical 
equipment. Let us say I am storing many thousands of amps on a capacitor. 
This amount of charge will destroy (and has destroyed) anything I connect 
it to. Is there anyway to discharge this electric potential slowly so as 
to be useful?

This has been done in VCR's for years.  If you move a VCR, and reconnect 
it to a power source within a limited amount of time, say, five to ten 
minutes, its memory is intact and it will remember the time and the 
programming.  NO battery is involved.  A capacitor based circuit is used 
to store the power needed to do that.  So the short answer to your 
question is, yes, it can be done.

What you have is a control problem.  And, though the equipment may not 
exist to apply same technique used in low voltage circuits to a high power 
application, on paper, at least, it can be done.

The voltage across a discharging capacitor follows an exponential curve.  
So one possibility is to use a variable resistor in series with the 
capacitor, with an appropriate control system programmed to decrease the 
resistance as needed to maintain a constant current drain from the 
capacitor.  A drawback to using such a control is that for high current 
flows, a resistor would dissipate significant energy in the form of heat.

It might be feasible, if the application can tolerate an AC supply to 
provide a variable inductor in parallel with your capacitor.  Properly 
sized and controlled, the inductor - capacitor pair would set up a "tank" 
circuit that could be designed to oscillate at whatever frequency is 
tolerable.  Here again, controlling the value of the variable inductor 
would be the difficult part.  But the inductor would have a much lower 
heat dissipation level than the resistor.

When you say that whatever you connect the capacitor to is "smoked," that 
indicates the need for proper insulation and components rated for high 
voltage service.  As far as I know, the highest working voltage for power 
transmission lines is 750,000 Vac, and "lightening" test facilities go to 
higher voltages than that.  That type of equipment isn't readily 
available, and it is expensive.  But if your budget permits, you can 
accomplish the results you desire.

Again, the control of the variable components is the key.

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