Hi there,

Does anybody know if there is any way of reducing the output voltage of a plug in wall adapter?

At the moment it has a DC 12v 1A output.
I would like to reduce the output to DC 9v (1A current not critical.)

The two red left hand wires are the 240v input.

The component in the TO-220 package is a STK0260 MOSFET.

Is there any way to adjust the regulation to get a 3v drop or is it set by the transformer before finer regulation.

Any help appreciated.

Regards NM

 

Hard to say, without measuring the voltage out of the transformer, if there is additional voltage reduction or not. It would be an inefficient method of attaining the 12V. However, not all wall adapters are made with the same technical quality.

A simple way, although not the most efficieny, would be to regulate it again with a 7809 voltage regulator. But if you do not already have these parts on hand you will spend as much $$ as purchasing the correct wall adapter (9V 1A).

 

Hello,

It looks like a smps to me.
Those can be hard to be changed.

Bertus

 

How about a few 1A rectifier diodes in series with the +. Each one will drop the voltage. I've done it before, and it was just fine.

 

Yes,
On further inspection the output does not appear to be rectified. Its output is taken direct from the secondary of the transformer. With a 470uf smoothing cap across it.

Could the switching frequency be altered to drop the voltage?

 

Hello,

You could also use a LDO behind the adapter to lower the voltage to the wanted 9 Volts.
A LM2940CT-9.0 could do the job:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM2940CT-9.0-ND

Bertus

 

Its an off-line switch mode power supply and I doubt it would be safe for you to work with it, let alone modify it. I suggest closing the case back up and just adding a 78M09 linear regulator to the output. 12 volts is a very good voltage with which to power a 78M09, plus the 78M09 will give you excellent regulation along with current and over-temperature limit.

Pick a datasheet from the URL below and have a look. Very easy, almost fool-proof and gives excellent performance.

http://www.datasheetdir.com/78M09+Regulators

(Apparently Bertus was posting the same time I was...great minds work alike...)

 

Thanks for the help and links everyone.

doug08 good idea had some rectifier diodes lying around and added them in series and will work perfect for what I want to run off it.

I never thought of diodes!

Thanks Again,
Regards NM

 

NM,

One simple way is to connect the + output of the adapter to the + of 5 or 6 diodes that are wired in series that will drop down the voltage to the load. However, since many of these adapters are a rectified sine wave you will be changing the output wave shape and ultimately reducing the actual projected working voltage since the "ON" time of the sine wave is reduced. Therefore, get a 12 volt 10 watt halogen lamp, like the kind for track lighting, and use it for a test load. Measure the output voltage across the light with an analog meter instead of a digital one. Use a jumper cable with alligator clips to short out the diodes to pull up the voltage to 9 volts. Put one alligator clip on the positive side of the light, which is also connected to the negative of the diodes in series. Short out the diodes increasing one at a time working away from the positive of your light with the light positive/diode negative connection being a constant alligator clip connection. Watch the meter and the light glow until you get 9.0 volts. When you get 9.0 volts remove the unneeded diodes from the circuit.

We don't know what your load is but if it is mostly resistive this should work fine.

Danny