Dropping 12vAC down to 9vAC

LaceSensor · Post by **LaceSensor** » 13 Jul 2011, 14:12

Hi

Anyone know of a simple method to drop AC from 12v down to 9v?
I have a new PSU that outputs 12vAC from one of the slots, but I need 9vAC.
I searched and came up empty. Lots of threads about doubling up

thanks

DrNomis · Post by **DrNomis** » 13 Jul 2011, 14:19

LaceSensor wrote:Hi

Anyone know of a simple method to drop AC from 12v down to 9v?
I have a new PSU that outputs 12vAC from one of the slots, but I need 9vAC.
I searched and came up empty. Lots of threads about doubling up

thanks

You could try using a suitably rated 5 Watt Wirewound resistor, you only need to drop about 3V if my maths is right, how much current can your power supply put out?...

LaceSensor · Post by **LaceSensor** » 13 Jul 2011, 14:29

~The AC power outs are rated 250mA

DrNomis · Post by **DrNomis** » 14 Jul 2011, 05:16

LaceSensor wrote:~The AC power outs are rated 250mA

Okay, so you could probably use Ohm's Law to calculate what the value of the resistor should be:

E=Volts.

I=Current.

R=Resistance (in Ohms).

P=Power (in Watts).

So, we want to drop 3V across a resistor, current is .25A (250mA), in Ohm's Law R=E/I (Volts divided by Current).

Therefore :

R= 3V/.25A

= 12 Ohms

And 12 Ohm/5 Watt resistors are avalible, 5Watts should be more than adequate....hope that helps....

P=I X V

P = .25A X 3V

= 0.75 Watts

Hides-His-Eyes · Post by **Hides-His-Eyes** » 14 Jul 2011, 09:22

Ah, but the outputs are UP TO 250mA; you need to know the actual DRAW of the pedal.

LaceSensor · Post by **LaceSensor** » 14 Jul 2011, 09:36

Hey

Whats the best way to do this? Can a DMM be used?

Thanks

DrNomis · Post by **DrNomis** » 14 Jul 2011, 09:40

Hides-His-Eyes wrote:Ah, but the outputs are UP TO 250mA; you need to know the actual DRAW of the pedal.

Good point Hides-His-Eyes....

Hides-His-Eyes · Post by **Hides-His-Eyes** » 14 Jul 2011, 09:58

LaceSensor wrote:Hey

Whats the best way to do this? Can a DMM be used?

Thanks

yes! connect up the power supply using crocodile clips or similar and put your multimeter on "AC amps" setting somewhere in series along the power supply (e.g. between the battery and the circuit). Just make sure it's in SERIES.

That should give you the current draw, roughly.