Halfords Inverter Failrepair
By alecjw on Sunday 23 September 2012, 04:18 - Permalink
Just a quick one for this morning... My experiences with an in-car inverter, along with a bit of a rant. (edit: having finished writing this it's turned out not to be quite as short as I expected. Sorry!)
I bought one of these broken off ebay a while back for 99p - I figured I'd replace the output transistors on the H-bridge and everything would be fine. It arrived and I opened it up - what I saw was utterly horrific. The inverter circuitry was just a 556 circuit and was about 15Hz (30%!) out of callibration. The DC-DC converter is completely open loop: at 15V input (within spec!) it will put out around 400V. There is an optoisolator but it doesn't seem to do anything most of the time, although shorting the output switched the thing off so I think it has something to do with the thermal cutout. The inverter circuitry has a few more ICs alongside the 556 timer which I'm guessing provide some sort of crude RMS voltage regulation by varying the pulse width but quite frankly I can't be fucked to try and work it out because clearly someone couldnt be fucked to design this thing properly and I'm just plain fed up of it by this point. On top of this, the mechanical design was shoddy - the PCB was held in place by the transistors. I cannot stress strongly enough - do not buy one of these. I'm not sure if these design flaws are common to all in-car inverters but they're certainly widespread. If you do need an in-car inverter, I don't know what to suggest. Build your own, I suppose... sorry. Or if you must buy a cheap one, be aware of their limitations.
Anyway, here's what I did with mine: after I was unsucessful in fixing the problem by replacing the H-bridge transistors, I couldn't be bothered to do any more faultfinding and decided to demote it from inverter to DC-DC converter. It would quite happily put out ~300VDC, so I figured this would be fine for anything with a switch mode power supply. I removed the H-bridge transistors and soldered the live and neutral lines to the +ve and -ve rails. Here you can see the top of the PCB where i've removed the transistors and the underside of the board where I've soldered the cables to the rails:
I plugged my laptop charger in and hoped for the best, being ready to catch the magic smoke if need be. All went well and I'm now writing this with my laptop charging from this inverter. It won't be good for much else since DC is no good for line frequency transformers or voltage doubler circuits, but I'm happy that I have something I can use my laptop with in the car. Yeah, a bit of a cop out option - I could probably have fixed it if I could be bothered but I deemed it not to be worth the effort. It works with my laptop charger anyway so that's all that's I care about.