Benchtop Power Supply troubleshooting

[bw1]

First, I should mention that I don't (yet) even have the unit that this powers, so I can't do much testing at this point. I'm told it won't power their testing unit.

Unit is rated at 12V/6A output.

By the looks of it, and my limited knowledge of power supplies, the transformer feeds a full-wave rectifier. Measurements at rectifier are the following:

AC-IN: 15vAC (approx)
DC-OUT: 19vDC (approx)

Does this look correct so far?

The only measureable output at the front is two pins that read around 3vDC, but for all I know the 12v output needs a trigger to turn it on.

Any suggestions before I can get the actual unit that this powers?

The actual circuitry on the board looks very simple. I'm just not familiar with the functions of each component at this point. The mess at the back AC-IN is a switchable 120/240v, set correctly.

Thoughts?

[sjwilson122]

Re: Benchtop Power Supply troubleshooting

With no load those voltages look ok. The bridge could be failing under load, but doubtfull. You could check by pulling the red and black lead from the bridge and use clip leads to connect a heavy load (maybe car headlight) and check voltages. Low beam headlight is about 35 watts, high beam about 55 watts and with 12v/6a you should be able to do 72 watts. You are probably correct about the trigger. In picture two, the pot on the left named "shut down" is probably the adjustment for some type of comparator circuit that shuts the supply down if the expected voltage is out of spec.

[Toasty]

Re: Benchtop Power Supply troubleshooting

http://cgi.ebay.com/ebaymotors/ws/eB...m=230641739186

Is the TO-3 cased item a transistor or voltage regulator? Also, a better shot of the (cleaned up) board to see if there are other components would be helpful. Can you remove it from the case and give a top-down and solder side pictures?

[bw1]

Re: Benchtop Power Supply troubleshooting

Removing the board revealed more. (I know, I know. Should have seen it. It was hidden under the one big cap and not really visible.)

Looks to be a fried 5v regulator? So, is that likely the cause of the problem, or just a symptom?

[Toasty]

Re: Benchtop Power Supply troubleshooting

mmmmmm......... Toasty!


It's an adjustable voltage regulator, LM2931CT by Motorola.

2.7v to 29.5v

"The LM2931 series regulators are designed with many protection features making them essentially blow–out proof."
OOPS!!

Also, on the solder side there's a 78L05, SO-8 package, 5v regulator.

.

[bw1]

Re: Benchtop Power Supply troubleshooting

Replace it with identical chip and try it out? Or should I be testing/checking anything else before doing that?

[Toasty]

Re: Benchtop Power Supply troubleshooting

Prior post updated.

Depending on how it's in the circuit, I would start by checking the power transistor. It's probably okay because the (2) 0.22Ω resistors look undamaged. It appears to use the 2N5884 to boost the current, as the LM2931 is only good for 100mA or so.

Check the other transistors as well. Q2 and the 1P's on solder side.

Also, if the filter caps have gone weak, they could be allowing a lot of ripple into the circuit.

.

[bw1]

Re: Benchtop Power Supply troubleshooting

This has been waaaaaay on the back burner for a while now. I haven't had time to troubleshoot any more with the circuits themselves, but since the adjustable regulator was obviously fried, and very cheap, I do have a replacement part here for that.

If there are other transistors that are bad, or other components, is the probability that it just wouldn't work, or is there a chance that it would output too high and damage equipment hooked up to it?

If it just will work / not work, I'm just going to try replacing the obviously bad part first. If it could output high because of something else, I'd take more time on it and see. It's not a very expensive power supply, and I don't have a lot of time for it right now.

Any thoughts?

(added: about the caps, they all look fine visually. further than that, i don't have the equipment to test/verify them.)