MFOS Dual Power Supply

ronmexico · Joined: Mar 05, 2012 Posts: 3 Location: Cincinnati

I building the MFOS dual power supply.....and I have run in to a problem.

I am trying to use the design to give me an adjustable dual power supply in the 3V-9V range (roughly). Instead of a 24V transformer, I am using a 13V center tapped transformer such that I get about +- 8V when rectified.

I have also swapped out the 1K resistors @ R3 and R7 for 240ohm resistors.

So everything has gone well but I am having a issue that is driving me crazy.

The rectification/filter "stage" of the board works fine... one side puts out ~8V+ and ~8V-. However the adjustable voltage regulator "stage" doesn't seem to be working.

I am turning the 2K trim pots and no matter what I do, the output voltage only changes by +- ~.5V e.g. between 7.25 and 7.75. That is to say, I cannot get the voltage down to say 4V.

This problem is true of both the + and - supply "side" of the board.

I have gone so far as to model the circuit in LTSpice to make sure my resistor changes work as they are supposed to.... and Spice tells me as the resistance of R4/R8 drops, so does the voltage.

I have gone over the board again and again.... verifying the values of the resistors/capacitors, no short circuits, diode orientation etc....

The circuit is so simple, I feel stupid for having to ask for help but I hope someone can give me a pointer to think of something I haven't yet.... probably something rather obvious Smile

.

Thanks in advance!

Sound · Joined: Jun 06, 2006 Posts: 842 Audio files: 1

Osal · Joined: Aug 16, 2011 Posts: 144 Location: Here

You should obtain within 2.5V and 11.9V with this configuration. To obtain roughly 3 to 9V (2.5 to 8.9) you could use 330 resistors instead 240

The transformer is insufficient to output 9V. The minimum transformer to output +/-9V should be a 24VCT.

What is the maximum current output you desire to output?
What is the value of filter capacitors?

prgdeltablues · Posted: Tue Mar 06, 2012 4:49 am Post subject:

A few thoughts..

Are the trimmers wired correctly -centre pin connected to one end - it sounds as if you are not actually changing the resistance when you turn them.

Check the pinouts of the LM317/337 note they are different.

Have you checked the ground is properly connected (voice of experience)

The positive regulator should work without a load, but the negative one likes a load - try connecting an led with say 1k series resistor between V- out and ground.

Peter

ronmexico · Joined: Mar 05, 2012 Posts: 3 Location: Cincinnati

prgdeltablues · Posted: Tue Mar 06, 2012 9:32 am Post subject:

Could you measure the voltage between the output pin and the adjustment pin - it should be about 1.25 V (that's the reference voltage) - ie across R1.

The readings you're getting do sound suspiciously like CR1 and CR3 are in backwards - that would provide a path from input V to ouput V with a diode drop of 0.6V - about 8V down to 7.4V. You could try disconnecting those and CR2 and CR4 - they are all good for protection but are not essential to the operation of the regulators.

Post a picture?

ronmexico · Joined: Mar 05, 2012 Posts: 3 Location: Cincinnati