Subject:
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Re: NiCd Charger
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Sat, 14 Nov 1998 14:09:44 GMT
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Original-From:
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FTHOMPSON9@AOL.spamcakeCOM
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Viewed:
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1799 times
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Russ:
In a message dated 98-11-13 04:48:38 EST, you write:
<< ...
power rating of resistor = 15Volts * 200mA = 3Watts
Series resistor needed = 75 Ohms @ 3 Watts.
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The 75 Ohms bit seems reasonable to me but 3Watts must be way to much.
could anyone show me where i have gone wrong.
>>
Your trouble with this is you have assumed the only thing across the power
supply is the resistor. It is in series with the 12 Volt battery. So for the
resistance you need:
R = (15 - 12) / 200mA = 3 / .2 = 15 Ohm
For power you can use either the 3 volts across the resistor times the .2 amps
through it, or the current squared times the resistance (E = I * R and P = E *
I ->
P = I * R * I = I^2 * R).
Power = current squared times resistance = 0.04 * 15 = 0.6 W
The only concern I would have here is verifying the 15V supply. Take a
capacitor and place it across the output, turn on the supply and measure the
voltage. I'll bet you get more than 15 (probably 19 or 20), which is the peak
voltage. It's up to you if you use this new (peak) voltage, or stick with the
RMS. Either way I would not leave the batteries in the charger more than 20
hours. And touch them every now and then, if they feel warm, they're drying
out and that's nto good.
Good Luck
Pherd
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