I needed a bench supply and not being ready to invest in something serious I decided to go cheap.

I'm sure you've seen the plethora of ~30V @ ~5A supplies based, at least externally, on the venerable Tektronix PS282 lab supply—this is one of those.

Unlike the original, this one is a switcher. I bought it cheap(!) and so had low expectations which this supply actually managed to fail to meet. It "works", but it has a couple of features that I didn't ask for: periodic impulse noise and terrible low voltage instability.

The noise is regular, and consists of ~1.5uS pulses of about 200mV PP, with a following pulse about 1/4 the amplitude 1uS later.

The instability is a positive going, very short voltage about 200mV above the baseline. The supply itself shows this jumpiness on its output display.

I am not familiar enough with switching supplies to know if this is a recognizable fault, or where to start looking for what it wrong. I don't need to repair this thing, and I am replacing it with something much better, but I would like to use it as a bit of a lesson in SMPSs and get the extra bonus of a stable extra supply.

Anyone have any ideas, hints, suggestions, wisecracks, remarks, or sage advice?

I've included a photo of the noise over a few seconds in case that helps.

 

I’d try adding capacitors across the output, both a large electrolytic (>100uF) and a low value ceramic (~10nF).

If that fails, then try an inductor in the output path, 100uH (6A) can be had for not a lot of money.

https://www.ebay.co.uk/itm/100uH-6A...h=item3aed2fd938:g:57QAAOSwTfhZi7tq:rk:7:pf:0

 

I’d try adding capacitors across the output, both a large electrolytic (>100uF) and a low value ceramic (~10nF).

If that fails, then try an inductor in the output path, 100uH (6A) can be had for not a lot of money.

https://www.ebay.co.uk/itm/100uH-6A...h=item3aed2fd938:g:57QAAOSwTfhZi7tq:rk:7:pf:0

A quick lowpass filter knocked down the spikes, but it didn't help the unstable output. I expect that the output shouldn't jumping around.

I am hoping that some improvement on the component selection, rather than the fundamental design, is possible.

 

Take a gander at an old used ATX computer supply. I found an old version that even had -5V. Bought a terminal for it on ebay...a couple of bucks.

 

Take a gander at an old used ATX computer supply. I found an old version that even had -5V. Bought a terminal for it on ebay...a couple of bucks.

I have several junk box ATX supplies, this is really a learning thing for me.

I also want to build a nice, beefy linear supply for a TS-440S, something around 30A, so I am on the lookout for a transformer and suitable pass transistors.

 

I got a 30 amp linear for my TS-570D. I hardly use it. Mostly use a motorcycle battery. I don't run high power. I use the atx supply for breadboarding. It's quick and versatile and the best part....it's cheap.

If I need quiet voltage I use a battery.

 

As mentioned above, stack some capacitance across the outputs. That should absorb the irregularities. If you're adventurous, crack it open and see if you can figure out where the irregularities originate, and put the capacitance there.

Also, being that it's a switcher, you cannot just blindly believe your scope. The scope probe will pick up emf from the inductor in the switching part of the circuit, and you'll see big spikes that are not really there. Especially if your probe has a hook on the end, and via the ground loop on your probe. There are several articles written about how to get good scope readings in this type of situation (high frequency inductor). If it's just for bench use, scope it a foot or two down the wire, where your actual load will be connected.

 

I have one of those similar supplies that had a jumpy output. Just needed to clean the voltage level control.
SG

 

I have one of those similar supplies that had a jumpy output. Just needed to clean the voltage level control.
SG

Did that once, but I will try again since you had that experience. Thanks.

 

I like to use noisy PSs, because they force me to design noise-proof circuits.

 

Did that once, but I will try again since you had that experience. Thanks.

So no joy with contact cleaner but I discovered something sinister. I can stabilize the output by switching the power off and on, or, if I do it enough, disable the output which slowly reduces to zero and sits there until the power is cycled. So, this is something very strange.

I am probing at the end of a 1m lead plugged into the scope, the noise is really in the output. There is a toroid on the output leads coming from the board to the terminal.

 

A lot of switching supplies, especially ones in equipment require minimum loads.

 

A lot of switching supplies, especially ones in equipment require minimum loads.

It behaves the same under load, though at first I thought it didn't more testing showed it did.

 

Take a gander at an old used ATX computer supply.

Exactly what I've done. As you can see, I have negative 12 volts available at 1 amp and 12 volts available at 36 amps. 3.3 volts @ 17 A and 5 volts @ 2 A.

From there I plug into a fuse box and have an assortment of fuses available so that if I'm working with a circuit that needs protection at 12 volts 1 amp I can install a 1 amp fuse into the box and connect from there. Same would be true with all the other outputs.

The indicator light is just a rectangular LED shoved under a stiffener bar (or whatever that hole is there for). As you can see, the LED indicates power on, as is also confirmed from the position of the switch.