Inverter repair

This is just a quick post on repairing an inverter. I have a Studer Joker 200 inverter. This is a device which converts DC electricity into AC electricity – converting 12V DC from a battery into 240V AC which we get from our plug sockets (in Europe). I have had this inverter for quite a long time (approx 12 years) and its been used at a load of events and projects. It is a very well built unit and is a pure sine wave type inverter, which means the output is at least as good as that which the grid provides. It is rated at 200W, with a max of 350W for 5 seconds, I think.

I have had it in a box for the past 5 years or so, but am now using it for my re-vamped solar trailer (more on that in another post). When I tested it, the unit was not working. It would power up but when I applied a load it would shut down within a few seconds. With very low loads (10W or so) it continued to work, but with anything higher it would switch off. This is a bit about testing and fixing it.

The first thing was to take the unit apart. It is a really nice unit with a heavy transformer (always the sign of a good bit of kit). Its pretty simple inside, with a PIC microcontroller and 4 MOSFETs.

With this kind of electronic circuit there are two main faults:


These can be damaged by overloading (although there should be protection), heat from high currents or bad heatsinking.

  • Capacitors

Electrolytic capacitors can die just from old age. They are sometimes liquid filled, which can evaporate or the plates (which are very close together) get damaged somehow and ruin the capacitance.

I opened up the unit and tested continuity on a few bits (such the transformer) but it all seemed fine. There was no sign of any over-heating or popped components.

I was thinking of replacing the MOSFETs, but then just thought to check the large electrolytic capacitor on the device. I took this off the circuit board to measure its capacitance (you can get false readings if you do this in-circuit). The value on the component was 4700uF with a 35V rating. It was only measuring 1.5uF. Hmmm. I found a new capacitor of 4700uF also with a 35V rating and tested it, just to make sure it was not my capacitance tester (it read 4770uF).

I replaced the capacitor and tested the inverter again – success! Jolly nice to have fixed something, kept it out of landfill and saved me money. If you are trying to fix something like this I would suggest looking at the fuses, MOSFETs and the capacitors.

Oh yeah – Please be careful if opening up something like this – there are very high voltages which are potentially lethal. Make sure you know what you are doing.

The lid off, along with the dodgy capacitor.

Close up of the insides – 4 x MOSFETs to the left. The capacitor is on the other side of the board.

Fixed inverter, put back together.

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