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Capacitor as a power supply

This is more of a thought experiment triggered by handling of a huge cap. Would it be possible to charge a big cap and discharge it through a load resistor without dropping voltage much over a significant portion of the discharge time?

One way I could think of is to use a Zener followed by an emitter follower to act like a current source. However the Zener needs a load resistor and non-zero current which means energy (charge) will be wasted there.

A linear regulator would also have the same power wasting problem. Switching regulator? DC-DC converter?

Reply to
M. Hamed
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I'm not sure what line you draw between a switching regulator and a DC-DC converter.

Yes, you could use a big cap followed by a switching regulator. But you might find it a worthwhile exercise to take a cap as big as your hand and calculate the amount of energy you can store in it (using its rated voltage, and W = C * v^2/2). Then take a NiMH cell or similar the size of a finger, and do the same calculation (using rated capacity and nominal voltage).

I'm not sure where you'll hit energy parity, but when you do the cap will be much larger than the battery.

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www.wescottdesign.com
Reply to
Tim Wescott

If the energy source is a charged capacitor, value C and if the applied load is accurately characterized as a resistor, value R

Then the system as described 'doesn't drop voltage much' only during discharge times which are a small fraction of R*C.

When one wants constant voltage until the energy source is nearly empty, batteries are preferred.

Reply to
whit3rd

A boost/buck switching regulator could apply the constant desired voltage to the resistor, as the capacitor discharges. The C voltage would droop slowly at first, and then faster and faster as the cap discharges, because the load resistor would be running at constant power.

Once the cap gets down to, say, 20% of its initial voltage, there's not much energy left to extract. So a buck switcher could be good enough. Or a boost, depending on the voltages.

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John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

to me a switching regulator is more sophisticated. the other day I came upon a 2 transistor inverter circuit. Put a rectifier afterward it will become a DC-DC converter. I wouldn't dare call it a switching regulator.

I understand but it's still more fun to see steady voltage coming out of a capacitor

Reply to
M. Hamed

Shouldn't this be dependent on how much voltage is required to operate the switching regulator rather than how much percentage of the capacitor voltage?

Reply to
M. Hamed

John was speaking of basic physics. When a cap's voltage is down to 20% of it's initial voltage, the amount of energy stored is down to 4% of the initial energy.

This is REALLY BASIC STUFF for anyone willing take the trouble to actually do some REALLY SIMPLE MATH instead of just letting their fingers dribble all over a keyboard.

--
www.wescottdesign.com
Reply to
Tim Wescott

There are all sorts of possibilities. It would help to have a specified requirement.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

I'm glad I posted to the right group!!

Reply to
M. Hamed

I know the math. I just don't usually think of caps in terms of energy. It just doesn't come to me automatically.

Reply to
M. Hamed

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