I have a circuit that requires 12-14VAC. I have tried 12VDC and it does not work. I'm thinking about reusing a toroidal transformer I have around to build a power supply. But the transformer is 120VAC in and 18VAC out. How do I drop the voltage further to 12-14VAC and regulate if possible? Thanks.

 

Well, if you know the resistance of the circuit you can use a resistor to drop some voltage assuming the current is pretty constant and not too great.

 

You say you tried 12V and it doesn't work. Does your supply maintain 12V, or does it drop to less than 12V when you attach your circuit? How much current does your circuit draw? And just for my curiosity...what is your circuit.

ken

 

KMoffet. He tried 12V DC on a AC circuit. Thats why it didnt work.

 

AAAAH...missed that.

Ken

 

I think a zener clamp with a resister should work. Thanks.

 

use a voltage divider!

As long as the 18v is Peak to Peak then you can use a voltage divider depending on how much current you need! A voltage divider looks something like the attachment. I used V- and V+ out of lack of thinking anything better.
well the first thing you need to do is figure out how much current you need for your load circuit, call this I. We need to also figure out what the voltage drop across R1. Since we are dealing with 18v and need 12v-14v we can say our voltage drop is 5v. We then use ohms law to calculate R1. Lets use an example, we need 12v @ 100ma. The voltage drop is 18v-12v=6v. Using ohms law I = E/R and substituting the voltage and current we calculate R1.

.100ma = 6v/R1.
R1 = 60ohms

Our next step is to calculate R2 using the simple voltage division formula.

Vo = Vi*(R2)/(R1+R2)

Vo is the voltage we need out while Vi is what is coming in. In our example we need 12v out and 18v is coming in. We also calculated R1 to be 60ohms so calculating R2 would be easy.

R2 = 120ohms.

You now how a voltage divider the outputs 12v @ 100ma the last step would be to calculate the power rating of each resistor. knowing the voltage drop and current across you can use P = I x V