Troubleshootng Power Supply

I bought a powered breadboard which is supposed to give me a regulated +5vdc 1Amp output, OR +15vdc variable / -15vdc variable output. The +5 regulated seems to work fine, but the +15/-15 is totally out of wack..

Here are my measurements with a multimeter, no load conditions:

+5 to ground = +4.96 vdc

+15 to ground = (pot turned full left) -0.16 to (pot turned full right) +14.97 vdc

-15 to ground = (pot turned full left) -14.93 to (pot turned full right)-14.93

As I turn the pot from full left toward the center, the voltage decreases until I get to the center, when it will hit -5.87 vdc, then as I continue to turn toward the right past center the voltage increases again until I am back at -14.93 vdc.


-15 to +15 = (both knobs left) 14.7 vdc. Rotatinig +15 knob to the right increases voltage until max out at 29.8 vdc. Leaving the +15 knob at max, I move to the -15 knob and begin rotating it to the right, voltage decreases until I get to the center position, at which point it is at 21.1 vdc, then continuing to the right the voltage increases until max out at 29.8 vdc.


I don't think it is supposed to function this way, though I am known to be wrong about these things I expect the -15 to ground to give me 0 vdc when turned left, then decrease until max at ~ -15vdc. Does anyone have an idea what may be the problem here? Voltage regulator? Possibly something else? Thanks in advance..

may i suggest youto post the diagram of your project, this will help us analyze the problem.

L2jsikan said:

may i suggest youto post the diagram of your project, this will help us analyze the problem.

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Yes, and there might be an internal adjustment to get rid of the -0.16v.

With a diagram it might be obvious that one open connection will cause your symptoms.
There are a lot more possibilities if it is a short from some point to some other point.

When measuring the -15 volts, be sure you observe the correct polarity with the probes of your MM.

Sounds like the pot for the -15V supply is wired wrong. Almost like they mixed up one of the legs with the wiper. The +15 and +5 supplies seem OK to me.

So you have a power supply with two terminals for +5V and 3 terminals for +15/-15V that share a ground. Is this correct?

What do you mean by OR?
"I bought a powered breadboard which is supposed to give me a regulated +5vdc 1Amp output, OR +15vdc variable / -15vdc variable output."
Is there a switch to change between the power supplies?

It doesn't sound like a ground problem. Can you provide a picture for more details?

This is what I have:
https://www.mpja.com/prodinfo.asp?number=15614+TE

Breadboard with built-in multi-output power supply.
Features 4 power supply binding posts, 5 Distribution strips, 3 Breadboard panels (total 2400 points), one +5VDC @ 1A supply, one 0 to +15VDC @ .5A supply, one 0 to-15VDC supply all mounted in a metal case. Includes 6ft. IEC power cord.

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See above quote from the breadboard manufacturer.

They left out the current rating for the -15V supply, for some unknown reason.

If not as above- a manufacturing defect- and it worked at one time, then the first thing I would do is try to clean, with spray, the pot which didn't work well.

It is basically brand new, and has done this ever since I 1st removed it from its box. I had it for several months before I even opened it, so sending it back for replacement is not an option at this point..

0x00x0 said:

+15 to ground = (pot turned full left) -0.16 to (pot turned full right) +14.97 vdc
By left you mean CCW?

-15 to ground = (pot turned full left) -14.93 to (pot turned full right)-14.93

it will hit -5.87 vdc, then as I continue to turn toward the right past center the voltage increases again until I am back at -14.93 vdc.

-15 to +15 = (both knobs left) 14.7 vdc.
You mean -14.7v?

Rotatinig +15 knob to the right increases voltage until max out at 29.8 vdc. Leaving the +15 knob at max, I move to the -15 knob and begin rotating it to the right, voltage decreases until I get to the center position, at which point it is at 21.1 vdc,
I would expect 15v + 5.87v = ~21v; so far so good.

I expect the -15 to ground to give me 0 vdc when turned left, then decrease until max at ~ -15vdc.
So do I

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I assume this breadboad uses two ea. heat-sinked three terminal regulators. Can you copy the numbers off of these chips?

Your measurements were taken at no load?

From the specs, I assume the -15v is also supposed to deliver a half-amp? It's not specifically mentioned.

Please post a photo of the component side of the circuit board, and
please post the voltages [with respect to ground] on all three -15v regulator pins and
all two or three pot terminals with the -15v pot at each extreme, and at the pot setting that gives -6v.

Do you have a scope? If you do, make the measurements with a scope.

OK, then it probably is a manufacturing defect. I would check this first:

kchriste said:

Sounds like the pot for the -15V supply is wired wrong. Almost like they mixed up one of the legs with the wiper. The +15 and +5 supplies seem OK to me.

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Likely, noting the price, it probably uses LM317+LM337 regulators.