Mosfet problem

Hello,

I´ve built a small device to anodize titanium.
There is an normal power supply which provides maximum of 120V, then there is the arduino with an mosfet between in the wires. The other ends go into the acid and to the titanium part.

When you press the start buttom , the mosfet switch for a specific time to get the right titanium color.

Everything just worked fine over 1 year daily use.
Since yesterday there is an weired problem.

When i measure the output voltage direkt vom GND and + after the mosfet , if have the exact voltage for my specific time.
But when the titanium part is in the acid , the mosfet dont switch anymore.
This dont make any sense.

But when i connect measure my voltage withoud the acid I have the correct voltage.
So basically the mosfet just turn off and on the ground wire from the power supply to the kathode in the acid.
Replacing the mosfet hasen´t changed anything.

Do you have an idea ?

regards Tim

Check all the voltages in your system - including between your ground and what you think is ground in other places. I think you are going to find a high resistance connection somewhere (especially with acid around, connections corrode). Same issue you find with car batteries often the positive terminal will get corrosion under it and when you hit the starter, everything goes out. Cleaning the terminal will fix that issue.

mikey

The "arduino stuff" is in a sealed box, everything inside looks perfect.
When i directly connect my power supply to the kathode and the titanium part, everything is fine. I just have to turn off an on by hand.

Define "sealed box" years ago I worked in an anodize shop - we had sulfuric acid, chromic acid and nitric acid among other things (10 foot deep 40 foot long tanks). The computers in the room above the anodize line started failing after time - seems the acid fumes were working their way through a concrete floor overhead to eat the computers. Wire-wrap stuff was bad, circuit boards were corroded - and this was in a computer room with it's own air conditioning system.

The wires can completely corrode to crud inside their insulation in a salt/acid environment unless you use
silicone double-layer insulation (ie. marine rated cabling is what you ideally want). Measure each piece
of wire with an ohm-meter capable of sub-ohm resolution.