Bench power supply question

[smallwaffles]

Hello, I'm very new to electronics and recently got a bench power supply (an Eventek kps305d) for a metal etching project. It worked great for the first few tests, but now I've run into a problem: when I hook up my positive and negative wires and turn on the bench power supply, it will only go up to around 3 volts. At first I thought maybe I blew a fuse or something, but when checking the power supply with a multimeter it performs very well, easily going up to 30 volts.

Any ideas as to where I can start troubleshooting my problem?

Many thanks!

[bob91343]

How much current is the supply giving?

[smallwaffles]

It was going up to around 5-5.5 amps

[smallwaffles]

I have used a few different wires with the power supply, although I'm not sure if that would cause this to happen. I'm running a test now, and it was maxing out at 2.5 V/5.5 amps, and after two hours it suddenly changed on its own to 0 amps and 30 volts. I'm not sure why this would happen as I hadn't touched the knobs, but now I've got it down to 4 Volts / 0.01 amps.

Completely new at this so just looking to understand the machine and why these limits or changes would occur.

Thanks!

[ledtester]

Does the "CC" (constant current) LED turn on when you get those lower than expected voltages?

[bob91343]

I'd guess you overloaded the unit.  If it doesn't work with the load disconnected you probably had a failure.  Most likely a transistor or a resistor.  Just hope not the transformer.

[smallwaffles]

Yes, the CC LED turns on during those lower than expected voltages.

I hope I didn't overload the unit - are there any precautions I can take to make sure I don't do that? I only need the unit to give 4 Volts constantly for around 2 hours.

[MosherIV]

If the 'cc' indicator came on, the psu has switched to current limit mode (constant current is confusing).
It does this because it is set to limit the maximum current, it has to lower the voltage to maintain max current.

Good bench psu can supply cv or cc indefinitely.

[Ian.M]

Its down to exposed electrode surface area, electrode  separation and etchant concentration.    Reduce area and concentration and increase separation to reduce the current.  The current may increase as etching progresses due to metal ions building up in the etchant.

Although a *GOOD* PSU can deliver any voltage or current within its ratings indefinitely, I really wouldn't be happy running a cheap 5A PSU at over 4A long-term.  Set its current limit at 4A, and wire up an alarm, consisting of a pizeo beeper, a phototransistor and a 9V battery wired in series.  Bluetack the phototransistor over the PSU's CC LED and check the alarm sounds when the PSU is in CC mode.

[ledtester]

If this is the first time you've use a current limiting supply, you might want to watch this section of a power supply tutorial starting around the 2 minute mark:

https://youtu.be/FfI3GRQbV0s?t=2m04s

I would do this sanity check:

- Connect a pair of test leads to the supply (like alligator clips or some other set of clips).
- Set the supply to a low voltage like 5V
- Turn the current limit knobs all the way to the left.
- Short the test leads by clipping them together.
- Increase the amps limit by rotate the amps knobs to the right. Amps display should also increase. You should be able to go up to the max current of your supply (5A).

Throughout this test the volts meter will be 0 or close to 0.