piclist 2001\02\19\133249a >
Thread: Capacitor Charge
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face BY : David VanHorn email (remove spam text)



At 10:07 AM 2/19/01 -0800, James Paul wrote:
>  Andy,
>
>  If you charge at a constant current, the voltage will rise linearly
>  as a function of time.  So use your 1A source, and charge until the
>  terminal voltage gets to your cutoff point.  To get the cutoff, you
>  could through trial and error measure the terminal voltage of the
>  cap as it is charging, and see how long it takes to get there.
>  Do this several times to get a good feel for the time interval, and
>  then just charge that long before switching.


I did this in a different scale for a thermal printer.
A current-limited boost SMPS charged 1000uF of output caps as fast as it
could, to 32V, from whatever it could get on the input. The PS then signals
the micro when it's ok to print the next dot-line. That requires as much as
19A from the caps.

This let me run the printer WAY down on line voltage, and still get good
printing. Final operating range was 70-140VAC into the wall-wart, using a
20W transformer.

Even at very low line, the current limiting kept me from overheating the
transformer.
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