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Old 16th Apr 2015, 2:21 pm   #1
Silvered_Mica
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Default PTC relay? manual start up for fridge

Hello, yesterday I went into the kitchen only to find a pool of water in front of my fridge freezer so I thought something had leaked in the fridge or it was defrosting, I opened the door and it was not very cold so I put my hand round the back and the big heat sink thing was cold but the compressor motor was quite hot and humming but not running.

This is the first time I have tinkered with a fridge but I had to do something quick as my frozen stuff was defrosting.
The first thing I did was to remove the housing that holds the thermostat and lamp switch to see what was in there, it has a simple two pin stat that is controlled by a thermocouple and it showed good continuity so I pulled the fridge out away from the wall so I could investigate what was going on down at the compressor end.
To the right side of the compressor there was a plastic cover so I removed it to find a few spade connectors and a plastic block on the side of the compressor that was held in place by a strip of spring steel after removing the steel I was able to slide it off the pins on the compressor. The compressor seemed ok as it showed a few tens of ohms between each pin, no shorts or open and no leakage to earth.

I could see that the live side of the mains came down from the thermostat to a thermal cut out that was fitted into the compressor connection block and then went to a pin on the compressor so that was pretty straight forward. The neutral side of the mains permanently went to another spade on the block and I could see that went to the other pin on the compressor but I could also see what looked like shattered metal inside a cage and this went to the third pin on the compressor.
I was unsure what this was and what the third pin was on the compressor so I found a circuit diagram for a fridge on the net and could see by this it was a start up thermistor that went into a high resistance state after start up.
So a temporary quick fix solution was needed and I did that by pulling out a plate switch, some wire and a 33ohm 15 watt wire wound resistor. I connected the switch and resistor in series with the permanent neutral and the start-up winding I then powered up the fridge with the temporary switch on and it burst into life I then shut off the temporary switch and it continued to run

Fortunately it holds its cold for about 8 hours before it starts defrosting but I really need to go back to automatic start-up. Unfortunately it's a really old fridge and I cant find an exact replacement for the PTC relay, I was toying with the idea of using a triac and a momentary timer circuit but I'm not sure a triac could cope with lots of inductive surge current over time and may end up going short.
Looking on ebay I see that there is a lot of PTC relays and there seems to be a standard type in one pin, two pin, three pin and four pin but I cant find any pin out diagrams for them as I need one that has spade connections to both sides of the thermistor.

Thanks for reading this
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Old 16th Apr 2015, 4:23 pm   #2
ex 2 Base
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Default Re: PTC relay? manual start up for fridge

We had a similar problem a few years ago and as a temporary fix I wired up one of those time with a multitude of segments which you press down with a ball point pen to select the time duration. I made time duration some thing like, for example 30 mins on and 50 off until I located a replacement. Lots of fridges have thermistors now, I fixed a neighbour by giving the thermister a clean up where the contact "springs" bear down on the thermistor block making contact, cheaper than a relay.Ted
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Old 16th Apr 2015, 8:18 pm   #3
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Default Re: PTC relay? manual start up for fridge

You'll need to check why the thermistor failed.
It's possible that the compressor is drawing excess current through its start windings, for any of a few possible reasons. If so, a replacement won't last long, if it manages to start the motor at all.
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Old 19th Apr 2015, 1:32 am   #4
Maarten
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Default Re: PTC relay? manual start up for fridge

A thermistor will fail on its own by (what else ) thermal stress. I wouldn't worry about trying to find a cause.

Searching on brand and model of fridge, brand and model of compressor, or brand and model of ptc (or just comparing pictures of the ptc unit) should turn up something for sure.
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Old 19th Apr 2015, 7:20 am   #5
dseymo1
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Default Re: PTC relay? manual start up for fridge

Agreed - but fridge compressors also seize, develop shorted turns, have problems with moisture in the refrigerant etc. Of course, it's worth replacing the thermistor, but I'd also monitor the compressor carefully for a while afterwards
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Old 19th Apr 2015, 7:38 am   #6
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Default Re: PTC relay? manual start up for fridge

Is the refrigerant not completley isolated from the compressor windings? I thought the motor part sat in a bath of oil too so should be pretty well insulated.

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Old 19th Apr 2015, 11:07 am   #7
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Default Re: PTC relay? manual start up for fridge

I ran one for a year using a flourescent light starter, the thyrister type.
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Old 19th Apr 2015, 3:25 pm   #8
Silvered_Mica
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Default Re: PTC relay? manual start up for fridge

Thanks for the posts. I did measure the current consumption and the run winding drinks about 800 milliamps and the start winding drinks about 1.8 amps, I don't think that is excessive as it takes considerable more torque to start a motor than run it.

I think the old thermistor died because it has been in constant use for the past 35 years.
I have spent many hours searching the net for an exact replacement unfortunately I could not find any for a LEC T201 but I have found one on ebay with no pin holes just two spade connectors (stand alone) that has a cold resistance of 40 ohms and rated at 12 amps so might be OK. Hopefully it will reach a suitable equilibrium resistance, obviously I will take a few tests and measurements before putting it into service.

Unfortunately it is coming from Hong Kong so looks like I'm going to have to keep starting it up manually for a bit longer.
I was thinking about using a relay and making a delay timer circuit but in the event of fault it could end up going short and I'm not a big fan of moving parts for long term reliability. I think the best thing is a thermistor as they tend to go open not short so less chance of destroying the start up winding.
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Old 19th Apr 2015, 4:26 pm   #9
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Default Re: PTC relay? manual start up for fridge

I wonder if you could use a motor run capacitor instead, a small one, it's only job is to get it turning round after all.
 
Old 19th Apr 2015, 11:12 pm   #10
McMurdo
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Default Re: PTC relay? manual start up for fridge

It will just be the thermistor. Like a degauss thermistor in a tv, they just fall apart with age. You can get 'universal' start thermistors from White Goods spares wholesalers on flying leads so you dont have the problem of fitting it back in the terminal box.
They are commonly known as 'start relays' as the previous generation actually used a current-operated relay in the same position. They're also used in tumble dryers.

http://www.ebay.co.uk/itm/Start-Rela...item35dc943391
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Last edited by McMurdo; 19th Apr 2015 at 11:18 pm.
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