Broken Cheap 400W Power Inverter

Do not waste any more time (unless you enjoy troubleshooting). Tell him to replace it and select one with better specs (if the size and cost are not a problem).

Most likely some other parts that you did not change are also defective. These are not worth servicing. I would suggest to buy a much better quality inverter if you want it to last. You will get what you pay for.

--

JANA _____

Unfortunately it failed again with same symtoms when installed in his motor home and connected to a 13 inch TV set. As it failed, the fault light and beeper came on indicating a fault and the input voltage was only 10 volts. I assume the fault was due to too much drop in the long wires. But it was too late to shut it off. The unit continued to indicate a fault after the load was removed.

Is this one of the usual failure modes where the input is too low to support the output? I would think the unit should be protected from overloads and low voltage, but apperently it fails easily when the battery voltage is too low? Also I don't know if a battery charger was attached to the battery or what condition the battery was in. The connection at the unit read 12.3 volts with no load and 10 volts after it failed. i don't know how much current was being drawn after it failed.

This thing is only worth about $40 and I see them on sale at Harbor Freight for $19.99 so it's probably not worth spending too much time on. If he buys another unit, what brand would you recommend for long life and ruggedness? Something more expensive perhaps?

-Bill

While I would agree about buying a better one, that doesn't mean these aren't worth servicing if you can do it yourself at home. They're not very complicated and parts are not expensive. It can be worthwhile to fix to keep it out of the landfill.

Well, I'd like to fix it, but I don't have the schematic diagram, and there are 14 small transistors in there and a whole bunch of small diodes. The 140 volt DC supply appears to be working but the modified sine waveforms on the gates of the FETs are not correct. One is correct, and another looks like a square wave with a very slight drop that should be zero, and another two are low amplitude. I ohmed out a few transistors but they all look good. Hard to figure out without a schematic.

I went shopping at Harbor Freight and found a 1000 watt unit for $100 and a 700 watt for $50 and a 400 watt for $19.95.

-Bill

It shouldnt fail IMMEDIATELY with a low input voltage-- low input voltage is going to lead to higher than usual currents, and if the design is any good... well, maybe that's the problem.

Also a TV is not a happy load-- lots of peak current draw at the top of each cycle.

Probably should shop for a higher-quality inverter, say one with a one-year guarantee.

Harbor Freight stuff is cheap, which often translates to "marginal quality".

Well, I think the Harbor Freight design is good (except for the circuit board), it's just the workmanship that needs improvement. Looking at the circuit board, the traces are only separated by less than 1/64 inch and there are many little solder drops laying in the groves between the traces. I tried to pick them all out but missed a few. Also the component leads are bent over so they almost short to the adjacient traces. But I bet the design is good if properly built. They must have tested this in the lab and subjected it to all the possible problems, input voltage low, high, shorted output, over temperature, etc.

You can probably buy 5 units, and one will keep working for a long time.

-Bill