I have a 12 volt battery which I need to get 1.5 volts out of. How do I calculate the necessary ohms resistor for this baby? Do I need to know the amperage of the battery to figure this? I've forgotten so much of this from physics.
THank you,
jomamameister

 

You need a 'voltage regulator'. A simple resistor will only work if you know your _load_ exactly, eg if you are running an LED.

Search online for voltage regulator circuits.

-Jon

 

You will probably require a heatsink to go with the voltage regulator, to prevent it from burning out.

To test if you have an adequate heat sink -- assuming there is nothing of higher voltage than the 12-volt supply in your circuit : Run your device after hooking it up and touch the regulator or heat sink. It will be warm, but if you can stand to keep your fingers in contact with it, you have an adequate heat sink. Do this both right after turning your device on (if the regulator is going to overheat, you might be able to turn it off in time before burning it out), and also after running it for a minute or two (after things have heated up to their steady operating temperature).

It would be good to buy 2 or 3 regulators (they can be fairly cheap), to have a spare in case you burn one out.

Also, if you let us know how much current or power your device uses (in terms of amps, mA, or watts), it would help us know better how big a heat sink is required.

 

Use 13.5 volts in your calc. A 12 volt battery is really 13.5 volts.
resistance = volts/amps. The volts in this case would be 12 since want 1.5 volts left for you load.
watts = amps x amps x resistance.
If your load is constant current then use these formula to find the resistor and wattage required.