Greetings all

I would be really grateful if any of you could shed some light on a couple of small issues.

Now I know this PSU kit has a number of design problems, not least the Vcc supply to the TL081s being too much when supplied from a 24v AC transformer. To avoid this, I am running from an 18v transformer but it is still running close, however it works and supplies what I need.

Nevertheless, for £5, this kit has been a really great learning experience. I must have read virtually every text on the net about it.

My problem is I cannot null U2 TL081 with RV1 100K, twiddling has no effect.

The TL081s that came with the kit were unmarked, so I replaced them but the issue remained. Out of desperation I tried a 741 for U2, and was shocked to find that actually worked, I was able to null the output to 0V.

The other curiosity is when the current limiter becomes active, the DC out waveform is a little ugly on the scope, by that I mean it is slightly modulated (but not AC ripple AFAICT). I am still investigating this.

The only mods I have made so far are to beef up the wattages on a number of resistors and I have not installed the fan regulator, I am running that from a completely separate supply.

So my questions:
1) What could prevent a TL081 from nulling?
2) Given the 741 works, could it be a suitable replacement?

This is the circuit:


I am sure you are all fed up with seeing this again, but if you can help I would be really grateful!

73s Ted

 

hi aztec,
How far is U2 off being nulled, does it prevent the Vout setting of the PSU.?
E

 

hi aztec,
How far is U2 off being nulled, does it prevent the Vout setting of the PSU.?
E

Hi Eric
Thanks for getting back.

With the TL081 fitted, the DC output is almost zero with the voltage control fully anticlockwise.
But I would still expect the RV1 to work, as various instructions and many youtube builds demonstrate.

I have seen this fault reported elsewhere but without a solution. I think most must give up on it as it is so close.

I would just like to understand why adjusting RV1 has no effect on the TL081.

73s Ted

 

hi aztec.
Which pin numbers on the U2 do you have offset pot.?
E

 

Thanks Eric
1 & 5
It is curious that with a 741 fitted, RV1 works perfectly.
73s Ted

 

From the TL081 datasheet, R10 should be 1.5K.

 

Thanks Albert
I would be very interested to see that datasheet, that example is not included on the one I have.
The R10 270k resistor is connected to the 0v rail from the circuit.
73s Ted

 

hi a.
Clip.
E

 

Thanks Eric
73s Ted

 

Thank you both

This is my current thinking:
1) The Null works for many builders reported on the net with the circuit "as is"
2) When substituting U2 for a 741, the Null works on my build

I am thinking this might be a supply voltage issue, all the youtube vids of the null working are on 24v AC, mine is 18v AC.
If correct, I wonder what other parts of this circuit might have issues at 18v AC?

Luckily I have a 24v transformer, so at the risk of frying the TL081s I will test my theory tonight.

73s Ted

 

With the TL081 and R10 at 270k the null adjust pot will have very little effect. It will be OK with the correct resistor value fitted.

 

even to 0v Albert? Or should I drop it down to the -5v pump rail

 

even to 0v Albert? Or should I drop it down to the -5v pump rail

Good point. It should go to pin 4 of TL081 which, on the schematic, is connected to the -5V.

 

Hi Albert
I tried a 1.5k to both -5 and 0v, sadly neither worked.
I know (but can't explain why) that this circuit works for most, I have seen the null working in many YouTube videos
I will try my 24v test later, it is the only thing I can think of right now.
Many thanks for helping me with this.
73s Ted

 

Nope.
24v test was the same, no null offset with RV1
Swapped back to 18v and popped a 741 back in for U2, and of course RV1 nulls as it should.

The PSU does work, but I am really using this build as a learning exercise and I will figure this out.

The one thing that has just occurred to me is maybe these TL081 op amps are Chinese copies and are manufactured without null circuitry inside. I'll have a look around for a breadboard test of the null as I need to rule that out.

73s Ted
2E0THH

 

With the power off, and the null pot in the middle, measure the resistance between the null pins. It should be 2.16k (ish).

 

Well that is interesting Albert
741 measures about 2.9k
I then swapped in the TL081 and it measures 100k !
Looks like these chips have been the problem all along.
I've only got 741s here, so I will have to get them from RS or Farnell to be sure, their postage is annoying for such a small order.
However, I don't think ebay would be a good bet.
Really, many thanks for all your help, very much appreciated.
73s Ted
2E0THH

 

it measures 100k

And that 100k will be pot itself, so nothing inside the chip at all.

 

Buy cheap, get cheap.
Unmarked opamps??

 

Thanks Audioguru

I know you have vast experience of this beast.
I bought it to understand how PSUs work but looks like I learned about fake chips into the bargain!

Certainly I will continue to try to improve it with a view to building it up to be a low power bench supply (1.5A max)
I would be most interested to see where you finally ended up on your modifications, is there anywhere you could point me to?

73s Ted
2E0THH