OP Amp power supply question

[Zero999]

Here's the circuit I made a long time ago, while I was still a student. It doesn't have any compensation capacitor and it's perfectly stable. There are a couple of other things about the design which are marginal and I wouldn't built is exactly like this again, but frequency compensation isn't one of them.

[Frankentronics]

Thanks again.

I made some revisions (all shown in red).

...C8 should go between +5 volts and ground and does not need to be nearly that large; 10uF would be better.

I think you misread my handwriting. That was a 470n cap, not 470u. So, I left it 470n. That value is OK, right?

C7 and D6 should go between the positive output and ground.

I fixed that.

U3 usually requires a 30pF capacitor between pins 1 and 8 but might work without it in this case.

I made that revision, too. Could you please check if my pin numbers are correct? Thanks...

[David Hess]

Here's the circuit I made a long time ago, while I was still a student. It doesn't have any compensation capacitor and it's perfectly stable. There are a couple of other things about the design which are marginal and I wouldn't built is exactly like this again, but frequency compensation isn't one of them.

The 741 has the same relatively slow performance as a 301A with its standard 30pF compensation capacitor so further compensation is not needed in this circuit.  The precision version of this circuit might use a slow 308 or OP-07 also without further compensation.  The LF351 shown in the original circuit is much faster so requires the 10uF output capacitor or something else.  When I used the fast OP-27 or LT1007 in a precision low noise design, the external compensation became pretty complex.

Usually it is not really necessary for the output to go all the way down to zero volts; most loads are not going to draw much current at +/-1.25 volts.

[David Hess]

I think you misread my handwriting. That was a 470n cap, not 470u. So, I left it 470n. That value is OK, right?

Yes, that will be fine.

U3 usually requires a 30pF capacitor between pins 1 and 8 but might work without it in this case.

I made that revision, too. Could you please check if my pin numbers are correct? Thanks...

Those are correct; you can always check the LM301A datasheet.  The 30pF capacitor between pins 1 and 8 makes the LM301A duplicate the performance of a 741 which has this capacitor built in.

StillTrying brought up a good point about the voltage across C6 going to negative -1.25 volts.  In this circuit with the LM301A and its 30pF compensation capacitor, C6 should not even be needed.  If the negative side is unstable without C6, then the value of C11 can be increased to gain the same effect which is something that an operational amplifier with a built in compensation capacitor like the 741 cannot do.

I have a funny story about the 741 and 301A involving power supplies.  Tektronix used the 741 in their PS500 series power supplies but I have a very late PS503A which came stock from the factory with 301As replacing the 741s and the needed 30pF compensation capacitors soldered to the back of the board.  I have no idea why this was done and this change does not show up in any of the detailed service documentation they provided.  Maybe they ran out of 741s that day?

[Frankentronics]

Wow! I already got a lot more from this post than I expected. I should end up with a very decent power supply thanks to all your help.

I made yet another revised schematic.

1 - C6 is now dashed, in case a different OP Amp is used.

2 - I added fine and coarse output voltage adjustments, based on another schematic I've seen on the internet. Are those values correct?



Also, thanks for the PDF (it came in as I was posting my previous reply).

[StillTrying]

I wouldn't think course and fine pots were really worth the bother on a 0-15V supply.

I've often thought that for a 5k pot, you could use a dual 10k with the tracks in parallel to increase the stability and resolution, I bought a dual pot to test the theory a while ago, but never got a round tuit, as usual.

[David Hess]

I prefer fine and coarse single turn potentiometers to 10-turn potentiometers unless extra stability is required; wirewound 10-turn potentiometers have better absolute and ratiometric stability.  In most applications, just a coarse single turn potentiometer is sufficient.

[Frankentronics]

I've often thought that for a 5k pot, you could use a dual 10k with the tracks in parallel to increase the stability and resolution...

That sounds like a neat idea, also to keep in mind for any future reference.

...I bought a dual pot to test the theory a while ago, but never got a round tuit, as usual.

Sounds like you and I have something in common. I got quite a few project bins filled with parts I never got around assembling.

[Frankentronics]

First of all, I want to thank everyone for taking the time to reply to my questions. This was all very helpful.

I ordered the components and will be posting some photos when I put it all together. I'll be experimenting with different pots to see the effects, but I guess I'll settle on a 4.7K multi turn pot that I found at a very good price.

One last question (I think important) if you don't mind.

Should my output ground be connected to the same ground as the ground from the AC outlet? In other words, are the transformer center tap, the transformer core and body, the enclosure, the heat sinks and the output ground all connected?

Thanks...

[Richard Crowley]

Should my output ground be connected to the same ground as the ground from the AC outlet? In other words, are the transformer center tap, the transformer core and body, the enclosure,

Typically, no.  If you examine the "common practice" you will note that  the "ground"/0V node is NOT connected to mains green-wire safety ground.  It is typical to find the mains ground connected to a green terminal on the power supply front panel so that the user has the option to connect circuit ground to mains ground and ultimately to the crust of the planet if it is appropriate for the use-case.

the heat sinks and the output ground all connected?

NO! Pay very careful attention to where the heat-sink tab is connected in the regulator components! The data sheet shows that the LM317 tab is connected to the output node. So it should most certainly be isolated from ground.  And the LM337 tab is connected to the input node.  There are some fixed, positive voltage regulators whose heat-sink tabs are connected to ground, but most other regulator chips must have their heat sink tabs isolated from circuit ground.

[Zero999]

As mentioned above, it's better not to connect the 0V rail to mains ground.

As far as the heat sink is concerned: an insulating tab should be used, otherwise the two devices should be on separate heat sinks, which are not connected to anything else.

[Frankentronics]

Got it! Thank you both for taking the time to explain.