From: James Meyer
Subject: Re: Limiting total wattage for a constant-current dummy load?
References: <firstname.lastname@example.org> <email@example.com>
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Date: Sun, 10 Nov 2002 13:40:03 GMT
NNTP-Posting-Date: Sun, 10 Nov 2002 08:40:03 EST
On Sat, 09 Nov 2002 20:54:20 GMT, jmuchow@SPAMMENOTcamlight.com (John Muchow)
>That's the way I'm doing it now and I typically prefer the "hands-on"
>method myself, especially if the alternative involves a new circuit at
>a higher cost in $$ and time. But, I also have 2 friends who want to
>use this device to test their R/C airplane battery packs and I don't
>trust their math abilities at all. :-)
OK. And I'm surprised that someone hasn't suggested it before now. How
about a cheap PIC microcontroller? Didn't you say you were a digital sort?
A PIC, simple keypad, and an LCD should allow you to have a digital
input for the current setpoint. Then let the PIC use its built in A/D converter
to measure the battery voltage. The PIC calculates the power and if its OK then
it turns on the current controlled load using its PWM D/A output facility. If
the power is too high then an appropriate message is displayed on the LCD and
the current load isn't activated.
If you wanted too, you could program the PIC to shut off the load when
the battery drops to it's lowest safe operating voltage. And since you, or the
PIC, knows the current, voltage, and time, you could let it calculate and
display both the AmpHour and WattHour rating for the battery when it's finished.
Parts cost should be $25.00 or so and it would be quick and easy to do.
Jim "With a calculator right beside the slide rule." Meyer