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9 Volt Battery Chargers
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Mods for the TRS-80 Color Computer

9 Volt Battery Chargers

0 - 160 mA Adjustable Charge Rate Battery Charger

0 - 160 mA Adjustable Charge Rate Battery Charger

This circuit uses a potentiometer and an op-amp to make a 0 - 160 mA adjustable constant current source. A transistor is used to boost the output current of this circuit.

The current flowing through the transistor is limited only by the control signal from the op-amp.

R5 is not a current limiting resistor. R5 is a sense resistor.

R5 is far too small to limit current to protect the transistor.

Divide the voltage drop across R5 to get charging current.

Use 1 ohm or 10 ohms or 100 ohms for the value of R5. This makes for very easy calculation.

VOM Reading R5 I
Charging Current
1 volt1 ohm 1 amp
1 volt10 ohm 100 ma
1 volt100 ohm 10 ma

I used a 2N2222A NPN transistor from Radio Shack. Any NPN transistor will work if it can handle the voltage, current and power.

Salvage a large high voltage transistor complete with large heat sink from a dead TV.

Use star grounding.

There are a lot of ways to destroy the transistor.
  • Connecting the Emitter to ground.
  • Connecting V+ to Base.
  • Opamp control circuit applying V+ to the Base .
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