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What size resistor to change power supply from battery CR2032 to 12vdc Car battery?
Wiki User
∙ 16y ago
It doesn't quite work that way. At least not in theory. A resistor inserted in the circuit between a 12v battery and a consuming device will not reduce the voltage (to the expected 3V of a CR2032) in a predictable manner by itself. In order to "drop" the 12-3=9 volts over the resistor you would need to know the exact current consumed by the device; then Ohm's law would give you the required resistance in ohms as R=U/I=9/required current in amps. However, a coin cell supplied device is likely needing minute currents, so you might need an unrealistically large resistor, and even then, if the device's current consumption changes, you're out of spec. A safer way would be to use not a resistor but a simple and cheap 3-pin 3v linear regulator, such as the LP2950CZ-3.0. You tie one pin to the 12 battery (+), one pin to the chassis (-), and one pin to the device's power input (+), also tying the device's (-) pin to the chassis. If you can't be bothered to use a regulator, an even simpler and cheaper Zener diode (for 3 volts) will serve you equally well, for such a small load. In this case, you DO need a resistor (around 10...20Kiloohms) between the battery and the device, but you also connect the Zener diode directly to the supplied device's pins. Warning: the polarity of the Zener matters. Tie the cathode (marked with a line usually) to the (+) pin.
Wiki User
∙ 16y ago
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Yes
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