*Request for Help* Fixing a 300W invertor (12VDC -> 230VAC) - Page 1
Forum section: electronics / projects

author: CpHarding
author_rank: Contributor
timestamp: April 03, 2013, 06:03:24 pm
content: Hi Guys,I've had a car inverter kicking around for a while broken, and for a learning project I've decided to get it back up and working. I don't care too much if I fail, as it was free in the first place, and I don't have a specific job for it if I do fix it, but thought it could be a fun learning project to get more experience at trouble shooting.The Inverter:It is a 300W inverter from Maplinhttp://www.maplin.co.uk/300w-12v-inverter-with-usb-charge-socket-518798The Problem:When you power it on, my PSU driving it cuts out, which normally happens when it gets shorted , (PSU is a modified ATX computer PSU, very capable of running this in terms of power)Current Progress:I've stripped the Inverter out of its casing, and started having a look at the board, I can't see any obvious shorts or blown components. I've replaced an electrolitic cap that looked a little dodgy, but can't see anything obviously wrong.http://www.flickr.com/photos/68811524@N04/sets/72157633159475892/Here is a flickr set of images of the inverter.My understanding of the circuit, using the board overview as a diagram.12V comes in far left (large red Wire, black is the just visible. The control circuitry (top right) uses the mosfets (top left) to generate AC and step up the voltage using the transformer. This is then rectified to a higher DC voltage (mid bottom) and using the sleeved mosfets generates the 230V AC, looks like the control circuitry at the top also drives this as you can see the optocouplers. The Live and neutral can be seen coming off the board around the second set of mosfets as well. The USB power is coming from the 7805 top right.The resistance across the inputs is a pain to get an accurate reading on, as the resistance keeps changing due to caps charging, but initially is around 1 ohm. Seems very low.Where should I start looking? Is there anything obvious from the board that i've missed? I can rig up a battery supply to prove its not my PSU thats the problem.Am I correct in how I think this works? I can provide more pictures if people think it might helpThanks in advanceChris.

author: SeanB
author_rank: Super Contributor
timestamp: April 03, 2013, 07:00:23 pm
content: Measure across the input side and output MOSFETS. Any reading under 10 ohms between drain and source ( middle lead and the one to the right of it means a dead unit. Do the same with the gate and source ( left lead to right, should be either open circuit or over 1k). Check the diodes are not shorted, and the big capacitor on the secondary side is not shorted.Then connect to a battery and try it, with a 10A fuse on the input lead.I bought a similar one, and aside from replacing a broken fan and some really junk input capacitors, along with wiring them in correctly so the input actually went to a switch, along with wiring the output to have a connected neon indicator in the switch, it works fine.

author: Alana
author_rank: Frequent Contributor
timestamp: April 03, 2013, 07:05:13 pm
content: Connecting to a car battery without checking few things up will cause more damage.First thing to do is to check components for shorts with DMM - mainly power mosfets and diodes.I'd also try to power it up via series resistance - here car main light bulb may be really useful.Incandescent lights are PTC and when cold have very little resistance but when allowed to get hot [for example because DUT if faulty] they will heat up and limit current.And i have a hunch that HV side is faulty - shorted mosfet or diode - but thats just a hunch.