psu tap switching question

[ME]

Aam i rite in thinking i can check if a lab psu tap switching is working by looking at the pass transistors collector voltage when the voltage knob it rotated  from min to max measured against the psu -ve output terminal ?.

[UnijunctionTransistor]

Yes. A collector voltage sudden jump would most likely mean a tap change.

[ME]

well i have done a test , with my red meter probe on the pass transistor collector and the black on the psu negative output terminal and the  psu output set at zero i measure 32.22 volts , on increasing the output voltage up to 12v it stays at 32.22v ,when i hit 12v the collector volts jump to 51.97v , it stays at that all the way up to 31v output on the psu ,on decreasing its output it stays at 51.97 untill it reaches 4.1v output then the collector voltage drops to 32.22v with an accompanying relay click, It has 4x pass transistors and 4 relays,its like 2 groups of two pass transistors running in paralell all pass transistor collectors measure identical, do those numbers look ok , or have i more work to do trying to fix this skytronix 650.682 lab psu?.

[UnijunctionTransistor]

The collector overhead voltage seems high.

[ME]

Would a dead opamp cause this?.

[ME]

It has an lm741 and an ha17741 opamps so i guess it uses them as comparators for tap switching and one is bad as it only changes taps once , i guess is should change/switch taps twice using each opamp to do 2 tap/relay switches at 2 different voltage levels?, ie each opamp controls on tap change?.

[UnijunctionTransistor]

Do you have a schematic?

Without it, we would be only speculating.

[ME]

No i cant find any schematics for this turd.

[Gyro]

You ought to be able to reverse engineer your own schematic, at least for the relevant section. I doesn't sound like a complex PSU. You will have it then for future repairs.

[ME]

At the moment i am in the process of extending all the cables from the main power and control board so i can lay them on the workbench and probe them with it all powered up, At least then i should be able to see what is going on as regards the tap switching, at the moment it only switches taps once, i am sure it should be twice!.Its a pita tho as there over 30 wires.

[floobydust]

Can you post pics of the control board? Schematics I have for various generic Manson, GW Instek etc. can have 1-4 relays per channel. Usually 2. Is that how many there are per channel?
Same basic circuit which actually might be working... the tap-changer relay contacts can arc/weld and get stuck. Some really cheap PSU's don't have any snubber caps for them.

Here is a generic schematic for a 3 relay model Mastech HY-3020 and it is just a window comparator, that I think looks at output voltage only to figure out what tap relay to turn on.
Note these bench PSU's are notoriously hard to understand because control board common is actually output(+)!  The control circuit floats along with output(+) because it has an isolated aux winding.


 

[Zero999]

It has an lm741 and an ha17741 opamps so i guess it uses them as comparators for tap switching and one is bad as it only changes taps once , i guess is should change/switch taps twice using each opamp to do 2 tap/relay switches at 2 different voltage levels?, ie each opamp controls on tap change?.

Here's a crude circuit I designed using transistors awhile ago. It's only good for a few hundred mA, as is, but can be modified for higher currents.

[ME]

Well so far i found a small transistor shorted on all legs a resistor burned open , i replaced those but i now get a tap switch at about 11volts thru to 31v and doesent switch back unil the volts adj is wound down to about 4 v !, I have just found the sole tl431 shorted in every direction ,c ould that cause the tap switching issue ?, It has 4 relays for tap switching.

[dietert1]

Any information on make/model available? Image available? We need two images of the board with the relay (both top and bottom).

[ME]

Its a skytronic 650.682 0-30v 0-10 amp unit, here are some pics not great but the best i get using my phone and site kb limit etc.

[ME]

more pics

[ME]

more pics

[ME]

I s more concerened as to wheather or not a tl-431 shorted in every direction will cause the tap sitching problems i have , i got his psu from ebay and its clear its been repaired a few times and the boards are cheap and nasty!, chinese junk tbh!

[coromonadalix]

It has an lm741 and an ha17741 opamps so i guess it uses them as comparators for tap switching and one is bad as it only changes taps once , i guess is should change/switch taps twice using each opamp to do 2 tap/relay switches at 2 different voltage levels?, ie each opamp controls on tap change?.

Here's a crude circuit I designed using transistors awhile ago. It's only good for a few hundred mA, as is, but can be modified for higher currents.

the tap is supposed to be used as a ground   ... or a middle point in a symetrical supply

[ME]

The shorted tl-431 is marked u-02 on the left by he trimmers on the pic.

[dietert1]

When taking a look at the images, i think the four relays are wired to be 2x two pole two throw relays. As there are two bridge rectifiers i'd guess that supply has adjustable + and - output where the output voltage and so the tap switching is the same on both + and -. Still with two relays we may expect 4 tap switching levels and not two. In order to find the fault, one needs a schematic of the relay drive circuitry.
I found a 2016 thread on that skytronic model:
https://www.eevblog.com/forum/testgear/info-on-skytronic-650-682-0-10a-0-30v-adjustable-dc-power-supply/msg1022160/#msg1022160
Sadly all of the links i found from there are dead. Maybe ask some AI for the schematic?
A manual is here: https://www.farnell.com/datasheets/29426.pdf but that appears to be a single output supply. There was the Skytronic model 650.685 with two output channels.
https://www.avsl.com/assets/manuals/6/5/650685UK.pdf
If it is really about the 30 V 10 A model, they may have doubled up the rectifier circuits in order to achieve the 10 A capability.

Regards, Dieter

[Zero999]

It has an lm741 and an ha17741 opamps so i guess it uses them as comparators for tap switching and one is bad as it only changes taps once , i guess is should change/switch taps twice using each opamp to do 2 tap/relay switches at 2 different voltage levels?, ie each opamp controls on tap change?.

Here's a crude circuit I designed using transistors awhile ago. It's only good for a few hundred mA, as is, but can be modified for higher currents.

the tap is supposed to be used as a ground   ... or a middle point in a symetrical supply

Why?

[ME]

zero99 why what?.

[G0HZU]

I've got one of these Skytronic 650.682 PSUs here. I've had it for many years although I've hardly ever used it. The clicking from the internal relays is quite unsettling (to me) as the voltage controls are changed.

However, I managed to find it and it still seems to work OK. Here are the voltages where there is an audible click from a relay as the voltage control is increased. The numbers in brackets are for the relay unclick as the voltage control is reduced.

Skytronic 650.682         30V 10A PSU   June 2026 tests
Relay click points:


Click  (Unclick)
3.8V (3.6V)
7.6V (7.3V)
11.5V (11.2V)
15.1V (14.8V)
18.9V (18.7V)
22.8V (22.5V)
26.7V (26.4V)

The fan runs constantly  from a cold start. I've used this PSU a few times, mainly to provide power for 28V GaN and SiC RF power amplifier testing. I'm not sure I'd trust it for use on something valuable or important. However, it has always been reliable for me. I don't want to take it apart and fiddle inside, especially in this really hot weather. But maybe someone has a schematic for it?

[MathWizard]

You ought to be able to reverse engineer your own schematic, at least for the relevant section. I doesn't sound like a complex PSU. You will have it then for future repairs.

Yes this goes for me too, I made a full schematic of a 1990's bench PSU Instek PC-3030 with 555-timers involved doing the range-tap selection. But I haven't gone back to figure out what it was doing, since I figured out the basics of the V and I control loops.