In article <1993Jan22.025009.20035@a.cs.okstate.edu>, worley@a.cs.okstate.edu (WORLEY LAWRENCE JA) writes:
|> I like to discharge 550mA nicads at their rated current, or about 1/2Amp.
|> This prevents any damage that can occur at higher discharge rates. The way
|> to calculate the size of your power resistor is as follows:
|> Resistor-value(in ohms) = Pack-voltage/Max.current(in Amperes). So, if you
|> have a 4-cell pack, it's "resting" voltage is around 4.8-5.5 volts, depending
|> on its state of charge. Plug in R=5/.5 (500mA=.5A), you get 10 ohms. So,
|> my discharger for the receiver pack uses a 10 Ohm power resistor. You must
|> also make sure your power resistor can dissipate sufficient heat. This is
|> calculated as: power-needed = Pack-voltage x discharge-current. In this
|> case, 5volts x .5amps = 2.5 Watts. Now, if you used a 2.5 watt power resistor,
|> (assuming there was such a thing), it would get REAL hot. I use the equivelant
|> of a 60 watt, 10-ohm resistor, to keep the generated down. This is accomplishedby using four 10-ohm resistors, wiring two sets of two in parallel, and then
|> those two parallel sets in series. By wiring two 10-ohm resistors in
|> parallel, you get a 5-ohm resistor that can dissipate twice as much power.
|> Wire two of these in series and you get a 10-ohm resistor that can handle
|> four times the power of just one of these resistors. I use 15-watt 10-ohm
|> resistors.
One point that was overlooked here is that the Zener diode will dissipate most
of the power, not the resistors. Also you have to subtract the Zener voltage from the
bettery voltage to properly calculate the resistor value. As a result you really
want about a 4 ohm resistor. The resistor will dissipate only one watt, no big deal.
But you will need a two-watt zener diode, which might be a little bigger that
what Radio Shack, etc. carries. You could use a couple of smaller zener diodes in series.
--
"There is no human problem which could not be solved if
people would simply do as I advise." -- Gore Vidal
Doug Braun Intel Design Technology
408 765-4279
dbraun@scdt.intel.com
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