Thread: One for the fridgies

Yeah generally MOSFETs just short. A good one should have no conductivity between gate and anything else. In circuit generally the gate is held off by passives, so you should only see the body diode between drain and source. Most shorted MOSFETs I've seen have had a hole blown in them, so I don't have a lot of in-circuit data to go on

If you are going to have a crack at repair, I've used 5-21W bulbs as dummy loads in place of the motor windings, and I always power up with a current limited supply. I've only done one Danfoss BLDC compressor, but I've done a few Engels and other miscellaneous low voltage multi-phase drives.

Sometimes if the drive is clever enough it'll error out not seeing any back-emf from the motor, but it'll always try and start up.

Not sure if he was a fridgie, and couldn't get a pic as I was driving, but I saw a guy using the coil he'd cut of the back of a fridge as a cargo barrier on the back window of his single cab ute.

They're welded steel, so depending on the threat mass probably "fit for purpose".

Originally Posted by p38arover

So, I guess my question is - is the 0.1 ohm resistance OK? If the fridge needs a new compressor, I doubt it's worth repairing unless one is a fridgie.

I did some further checks with some other test equipment and each winding is 0.6 ohms resistance and 0.06mH inductance (both including the test leads). I can't find anything obviously wrong with the MOSFETs.

Perhaps there is a TVS or similar protection on the DC input that is shorted then. When faced with a similar issue and as a last resort I'll often power up with a current limited supply and put a watt or two into the device, then use my calibrated finger to find the warm bit.

Once or twice when faced with a bit of disposable tech I have just hooked it up to a battery to see where the smoke comes out but I assume you want to fix it

TVS?

Originally Posted by p38arover

TVS?

Sorry. Transient Voltage Suppressor. Common in automotive stuff. Like a zener diode on steroids and they have a nasty habit of failing short .

They are generally found across the DC input and serve 2 purposes. The first is being a diode, reverse polarity connection will blow the input fuse before the diode.

The second is to absorb input spikes. They are *very* fast, so will nicely clamp transients. Sometimes however when they get tired of being hammered by transients and want a lie down they just short and blow the input fuse.

Fixed! But not as you'd think.

I bought a replacement controller via AliExpress taking a punt that the compressor was OK.

I fitted it, checked for a short circuit, and plugged it in - and blew the fuse. In fact, smoke emanated from my battery box. Gaaah!

Unplugged it and checked for a short, all OK. While I had the meter leads on the Merit plug, it suddenly showed a short circuit. I opened it up and found nothing wrong. Reassembled it and again had the short. GAAAHH (again). Opened it up again and found the problem.

When the plug was inserted into a socket, the negative parts would be squeezed into contact with the centre conductor and short circuit the supply. I bent the pieces outward to prevent it and also put a sleeve of heatshrink insulation over the centre pole.

Reassembled it all and tested it and it works. Hmm, thinks I. Is the original controller OK? I removed the new one and refitted the old one. It works. GAAAHH (again). I've wasted $50 on the new controller.

What is odd is that this fridge had been in use in my car for a while. I guess the plug didn't short out initially but did when after I unplugged it and replugged it. I wonder if I was given the fridge because the donor had problems with it but knew I had been a tech (I worked with her late husband) and could probably "fix" the fridge. I've never had a faulty Merit plug before. Chinese crap again.

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