Will this power a bench supply kit?

[Sidecar Bob]

My ancient Heathkit bench supply died a while ago so I bought one of the 30V 3A variable power supply kits like the link below that are all over eBay to replace/upgrade the insides of it with.
https://www.ebay.ca/itm/373735998370

The kit doesn't include a transformer. I was planning on using the Heathkit's original transformer to power it; The voltage is OK but I just realized that the Heathkit's output was only 750 mA and I'd like more.

So I went shopping online again. Prices for actual transformers are outrageous (as expected with the high price of copper). I was thinking about getting a 24V 3A "brick" type power supply and powering it from that when I came across these
https://www.ebay.ca/itm/353653795351?var=622990551326

And these (it will be mounted inside a steel case)
https://www.ebay.ca/itm/254898301469?hash=item3b591f6a1d:g:5mkAAOSwYvNfuSpS

But I don't know much about switching power supplies (other than that's probably what's in the brick anyway). Would one of those be OK to power the bench supply kit?

 

[Harald Kapp]

The AC/DC converters look o.k. Personally I might prefer the ones with the metal case for added safety, even when used within another case for the power supply. The variable output supply (your kit) on the other hand does look (visually only) to be a bit low in the filtering side, so I'd expect quite some noise from this. You may want to look for one that is explicitly labeled as "low noise".
You'd have to see whether the quality of the output of your kit is o.k. for you.

You may also have a look at this kind of module (just an example) which comes with a display for voltage, current and power and can be set to constant voltage/constant current mode.
Or consider buying a complete bench power supply at a moderately higher prize from China.

 

[Sidecar Bob]

I think I prefer the one in the case too, especially since they are the same price and there's lots of room inside the case I'll put it in so I've ordered that one.
Besides safety, the metal housing might help reduce stray hum.

I bought the kit last spring so I'm committed to using it. I also have a combination voltmeter & ammeter I bought for a project a few years ago and ended up not using; It isn't as sophisticated as the one you linked to but I already had it and the Heathkit didn't have anything so it will be a big improvement over that.

I was gathering info on the kit a few days ago and I came across the page linked below. As it happens he used the same meter I have. I hadn't thought about a fan but I think that's a good idea and I also have a fan similar to his on hand (also bought for a project that didn't need it)
https://leap.tardate.com/equipment/adjustablepowersupplykit/

I guess I'll have to do a project log for this when I finally put it together ;-)

 

[Audioguru]

When you buy cheap junk from "over there" then you get cheap junk.
That power supply is a Chinese copy of an old kit from Europe that powers its old opamps with a voltage higher than their max allowed voltage, has many parts overloaded and they get way too hot. It does not produce regulated 30V output at 3A.

 

[Sidecar Bob]

The reviews I've seen online didn't indicate any problems.

Anyway, the kit depends on rectifying 24VAC to produce the 30VDC output. With the 24VDC supply the kit's output will be lower so the components will be stressed much less. And I probably won't run it anywhere near 3A very often.
It almost has to be better than the dead Heathkit.....

 

[Audioguru]

There are many fake products and fake reviews "over there".

Without a load, most 24VAC transformers produce 25VAC or more. Rectified it produces 34VDC. It also has a negative 5.1V for the opamps then they get 34V + 5.1V= 39.1V or more but their absolute maximum allowed supply is only 36V.
The circuit has so many voltage losses that its maximum output was only 25V at 2A, not 30V at 3A. Years ago when I fixed the project at electronics-lab.com I increased the transformer to 28VAC and added a second output transistor with a more modern driver transistor.

I used TLE2141 opamps that are rated at 44V and they needed only a 1.4V negative supply. I increased the power ratings of many resistors then they could survive. The new Chinese kits did not use my improvements.

With a supply of only 24VDC the circuit will not work without an additional 5.1V supply. With the negative supply your max output will be about 19VDC at 2A.

 

[Sidecar Bob]

I'll try to start the log for this project in the next few days so we can discuss this in the discussion thread for it. It may be delayed because I'm installing some drawers under one of the other workbenches in the shop so things are pulled apart and there are carpentry tools on the electronics bench while I work.

 

[Sidecar Bob]

Project log started. Discussion thread here:
https://www.electronicspoint.com/forums/threads/bench-power-supply-upgrade.296479/