# I need a quick fix - help please

Discussion in 'General Electronics Chat' started by Gavhogg, Sep 23, 2008.

1. ### Gavhogg Thread Starter New Member

Sep 23, 2008
1
0
Hi Folks

I need help to reduce 28vdc down to 12vdc using resistors, but I'm not sure of the formula.

Can anyone help?

Many Thanks

Gavin

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
E = IR. There is no fixed formula for your application, as you don't indicate what current you wish to flow. Using fixed resistors means that the actual voltage changes with the load. Or you could get a 7812 regulator.

3. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
Vout = Vin ( R2 / (R1 + R2))

Vin -> R1 -> R2 -> GND

Vout / Vin = (R2 / (R1 + R2))
12/28 = R2 / (R1 + R2)

One equation with two variables, bad news. As beenthere says, you need to specify current. Any current flowing out via 12V will cause the voltage to change. If the current out from the divider is much less than the current flowing through R1 and R2, then it can work. You will have a lot of power loss as you bring the resistances down to accomplish this. This is only practical for high impedance inputs, although the temperature can effect the circuit.

Steve