Easy lab PSU

[Nigel Goodwin]

As I mentioned in the OP, I simply added an extra resistor in the feedback network - and I posted the details on Banggood - however, I can't find them now. But it was obvious and trivial to do.

Whilst looking I found this post:

Good product, works fine, I have removed resistor R19 2,32kE and repaced it with potentiometer 1kE and resistor 2kE in series so the output voltage is adjustable between 19,5V and 27,5V becouse I need 21V output.

The PSU uses a common TL431 voltage reference chip, with a two resistor divider from the output - the top resistor (R18) is 203 (20K) and the lower one (R19) is 2321 (2.32K). The top end of R18 connects across to R20, and from there to the output - so I cut the track between R18 and R20, and added an extra resistor in there - according to ohms law an extra 5.6K should give you a reasonable 30V. There was enough room to add a standard through hole resistor between R18 and R20.

 

[Pommie]

Ahh, that's what the 3 pin chip is, no chance to read it under artificial light. I'll have a play tomorrow as I appear to have different value resistors (or they may be the same in sunlight tomorrow!!).

Thanks,

Mike.

 

[Nigel Goodwin]

Ahh, that's what the 3 pin chip is, no chance to read it under artificial light. I'll have a play tomorrow as I appear to have different value resistors (or they may be the same in sunlight tomorrow!!).

LED torch and a 'Emmett Doc Brown' magnifying headset

But regardless, you need to make the top one larger, or the bottom one smaller. Bear in mind you can't measure the resistors, as there's a dummy load (large SM) directly across the output.

 

[Pommie]

LED torch and a 'Emmett Doc Brown' magnifying headset

Got both but old eyes. I'll wait for sunlight and all will be good again.

Mike.

 

[Pommie]

Interestingly, my R19 is 2.26k which makes my board need a 4.7k resistor. Could they be calibrating at the factory by fitting different value R19s?

There was enough room to add a standard through hole resistor between R18 and R20.

I'm assuming this was a typo and you meant R18 and R19.

Mike.
Edit, forgot the Vbe drop. Now calcs show 5.5k so 5.6k it is.
Edit2, now realize Vbe has nothing to do with it.

 

[Nigel Goodwin]

Interestingly, my R19 is 2.26k which makes my board need a 4.7k resistor. Could they be calibrating at the factory by fitting different value R19s?

I suspect they are simply different boards made in different factories?, so there are a few variations. What value is your R18?

I'm assuming this was a typo and you meant R18 and R19.

No, R20 is a resistor from the output feeding the opto-coupler - and the track between R18 and R20 is a convenient place to cut, and insert a resistor. So I cut at the top end of R18, whereas cutting between R19 and R18 would be at the bottom end - and (in my board at least) that was far less convenient. Obviously, if you have a good range of SM resistors, you could simply pop R18 off and replace it with a higher value.

 

[Pommie]

On my board cutting the track between R19 and R20 would be above where the output gets fed back and so wouldn't be in the divider at all. Or I'm completely misunderstanding how this works.

My R18 is also 20k and I've got a 0805 27k resistor which should give me ~32V. Think I'll give this a try and if there's any problem I'll parallel a high value on top of the 27k.

Mike.

 

[Nigel Goodwin]

On my board cutting the track between R19 and R20 would be above where the output gets fed back and so wouldn't be in the divider at all. Or I'm completely misunderstanding how this works.

Possibly the PCB layouts are slightly different? - I've got another PSU at home (that I've been getting the component numbers off) I'll take a picture when I get home.

My R18 is also 20k and I've got a 0805 27k resistor which should give me ~32V. Think I'll give this a try and if there's any problem I'll parallel a high value on top of the 27k.

Mike.

Stick the 27K in, and see what you get

There's plenty of 'slack' as the PSU works on mains inputs down to 110V, so at 240V there's plenty of capability for increasing the output voltage (depending on capacitor voltages etc.) I also scoped the pulse width before and after, to make 110% sure