# Help with 12V DC Step Down resistor values

#### dsmith

Joined Apr 25, 2007
1
Replacing AC power supplies with a 12V DC power supply (car battery).

Not sure what resistor/watt value I need for each circuit?

Circuit 1: 12V DC to 5V DC drawing 1.5A
Circuit 2: 12V DC to 5.8V DC drawing 500ma
Circuit 3: 12V DC to 9V DC drawing 1100ma
Circuit 4: 12V DC to 8V DC drawing 200ma
Circuit 5: 12V DC to 5V DC drawing 700ma

I got these values from the AC power supplies.

Thanks,

#### thingmaker3

Joined May 16, 2005
5,084
The load resistance in each case will be equal to voltage divided by current. The sum of dropping resistor plus the load divided by the load alone must equal your 12 volt supply divided by the desired voltage. Watt rating of the dropping resistor must be not less than the square of the dropped voltage divided by the dropping resistor resistance.

Example: 12V/5V = (Rd+(5V/1.5A)/(5V/1.5A)), so Rd = 4.6 Ohms. ((12-5)^2)/4.6=10.65 Watts. So you would want a 4.6 Ohm 11 Watt (or higher) resistor.

You may have tremendous difficulty finding the resistors you need, and they may run several dollars each.

An easier route might be a linear voltage regulator such as the good-ol' LM317:. http://cache.national.com/ds/LM/LM117.pdf