how to reduce voltage in power supplys?

[Marcos - Munky]

One friend have a 12V power supply, but for some reason he need to reduce the voltage output to between 1V and 1,5V. Wich is the simplest way to do this? Maybe a zener, but wich value?

[Ansil]

regulator safe easy reliable.

[Peter Snowberg]

If it's a regulated supply, you could use a combination of zener and regular power diodes to drop the voltage, but I would use an LM317 regulator instead. The LM317 is able to deliver voltages down to 1.2, and it can be run from an unregulated supply. If he needs to go lower than 1.2, just place a diode in series with the output and that will allow for voltages down to approximately 0.7. The circuit would require the LM317, two resistors, and two small capacitors.

http://cache.national.com/ds/LM/LM117.pdf

I hope that helps,
-Peter

[gez]

The method I use to get low voltages is the one I used for the autowah mods on your site Marcos - a Mosfet source follower, biased by a trim pot.  It's a variation of a series pass circuit, you just move the wiper up from ground till you measure the voltage you want at the source.

So long as your supply is regulated it's pretty rock solid, and you can go as low as you like!  

[gez]

PS Although it's possible to use Zeners to drop voltage (I've done so many a time), low power ones aren't that consistent - they require a certain amount of current to flow through them before they do what it says on the tin.

[Marcos - Munky]

Wich is the simplest way? He wants something that use less parts possible. Hey Peter, wich value should I connect to the output to get 0,7V?

[Peter Snowberg]

I don't have any LM317s around to try this, but I believe that you can get 1.2 volts out just by connecting the adjustment pin directly to ground and not using the resistors shown in the datasheet above. If you connect the output through a silicon power diode like a 1N4001, that will drop an additional 0.5 to 0.6 volts from the 1.2 volts.

I hope that helps,
-Peter

[Marcos - Munky]

Thanks, Peter. I will tell him about this.