I'm trying to make a digitally adjustable power supply. The range would be 0-20 volts and up to 75 ma. I tried a MCP4725 D/A connected to a L272 op amp. The OP amp is connected to a 20 volt supply and the gain is set to 4. It worked great command the D/A to put out 2.5 volts and I get 10 volts out of the OP amp. When a put a 50 ma load on the OP amp heats up to hot to touch. The OP amp is rated at 0.7 amp o/p but I guess it just can't get rid of the power. Its a DIP package with no heatsink. Is there a better way to do this.

Thanks
Geoff

 

Yes, the maximum power dissipated in the op amp will be 1.5W which is too much for a DIP package with no heat sink. You need to add an emitter-follower bipolar power transistor with a small heat sink at the output of the Op Amp. Just about any small power BJT should work.

Take the op amp feedback from the emitter.

Note that with this configuration the maximum typical output will be about 18.3V.

 

Thanks, I'll give that a try. What about if I take some feedback from the DC output and scale it down by 4 ( the gain of the opamp) feed it back into the i/p of the opamp and just have the DAC set a reference voltage.

 

Thanks, I'll give that a try. What about if I take some feedback from the DC output and scale it down by 4 ( the gain of the opamp) feed it back into the i/p of the opamp and just have the DAC set a reference voltage.

I don't understand the purpose of doing that over whatever you did in the original circuit.

Post a schematic.

 

I'm trying to make a digitally adjustable power supply. The range would be 0-20 volts and up to 75 ma. ////but I guess it just can't get rid of the power. Its a DIP package with no heatsink. Is there a better way to do this.

Thanks
Geoff

Yes. Build a simple voltage regulator and use the output of the DAC as the reference voltage for the error amplifier.

 

Yes. Build a simple voltage regulator and use the output of the DAC as the reference voltage for the error amplifier. An op amp is not a good voltage regulator.

Why is an op amp not a good voltage regulator if it has a buffer output? It makes a very good error amplifier.

 

Why is an op amp not a good voltage regulator if it has a buffer output? It makes a very good error amplifier.

Mainly because it has very little power capability if it is the regulator. If it is just an amp and the power is dissipated in an external transistor, no worries.

FRom the L272 data sheet:

 

Try using a switching regulator, setup for your nominal voltage. Then use a DAC from your micro-controller, through a resistor, tied into the feedback signal. This lets you adjust the feedback and change the output voltage.

Same idea as the person that suggested adjusting the error amp reference. But sometimes the error amp reference is not available at an external pin.

Do not try to open the feedback loop and control the voltage entirely with the DAC. You'll never get it to work open-loop like that. Close the loop as normal, but insert the DAC to adjust it. Should work well.

 

Thanks guys. Sorry for the delay in reply.
Crutschow - I think I misunderstood what you meant. I thought you just meant to take the output of the op amp through a emitter follower but when I re read you mentioned to take feedback off the emiiter, which is essential what I was wondering in the post following yours.


I tried this circuit and it works but only up to 7.5 volts out. I do some more searching unless you can see whats wrong.

 

Found a circuit online that is probably what crutschow meant and I misunderstood. Works fine.

Thanks to all for your input and time.

Geoff