EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: JoeN on June 21, 2017, 12:07:46 am
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Fridge stopped. Looked at the parts and this one was bad, the descriptions online say it should have some continuity but didn't. Replaced it. Fridge works again. So I took it apart. It seems damaged. It's quite simple. What does it do? Is this just an AC capacitor? It has two terminals and only one of them is connected to AC power. It feeds through to two terminals on the compressor, which has three terminals. The other terminal is connected to AC power via a PTC device which was clicking on and off while the compressor was not running right because the compressor was getting hot.
(https://www.eevblog.com/forum/chat/fridge-question-'overload-or-relay-start-capacitor'-what-does-this-device-do/?action=dlattach;attach=325567;image)
(https://www.eevblog.com/forum/chat/fridge-question-'overload-or-relay-start-capacitor'-what-does-this-device-do/?action=dlattach;attach=325569;image)
(https://www.eevblog.com/forum/chat/fridge-question-'overload-or-relay-start-capacitor'-what-does-this-device-do/?action=dlattach;attach=325571;image)
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I think you're right, it's simply an AC capacitor used to increase the phase lag of the compressor motor. When the compressor is stalled (overload) it needs more lag to be able to turn. A relay inside the compressor housing bypasses that capacitor when it is running.
This is one of the components that would be offered as a "hard start kit" for aging refrigerators in difficult climates. A larger capacitance would make more phase lag, helping the compressor if it is running poorly, but at the cost of poorer efficiency.
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I don't think this is the capacitor. I think its some type of thermal fuse which plugs into the compressor to stop the microcontroller in the control unit inside the refrigerator from switching the compressor on if it is over heated protecting the compressor motor.
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Just get a "3 in 1" at your local HVAC parts store. Bring the start and run capacitors (if present) so you'll be able to get a compatible replacement. The run capacitor, if there is one, is typically not built into the 3 in 1 and they're cheap enough that you might as well buy a new one.
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Pretty sure it is a thermal switch which energizes the motor starter winding until motor current warms it up.
Probably some bi-metallic clicky disc thing. if you warm it up you might get it to click open (unless being stuck open is what is wrong with it).
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Pretty sure it is a thermal switch which energizes the motor starter winding until motor current warms it up.
Probably some bi-metallic clicky disc thing. if you warm it up you might get it to click open (unless being stuck open is what is wrong with it).
I think you're on the right track.
A quick Google search finds this description on the part number: P6R8MD PTC Start Relay ... and I came across this diagram....
(https://sc01.alicdn.com/kf/UT8t6MxXElXXXagOFbXo/31884/UT8t6MxXElXXXagOFbXo.jpg)
They have taken quite a bit of poetic licence in the use of the term "relay".
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I had one of those go just the other day. Mine seems to have failed in a similar manner -- it looks like there was arcing on the surface of the "coin."
It's not a capacitor; it is actually a PTC resistor (often called a "PTC relay" as a misnomer?). It energizes a start winding of the motor. By the time the motor gets started it has heated up enough that the resistance increases to where it is no longer providing any significant power to the start winding.
See an example vendor document: http://www.sensata.com/download/8ea.pdf (http://www.sensata.com/download/8ea.pdf)
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There was no continuity between the sides so I think it was "stuck open".
So, all I had to do to get the compressor to run was put a momentary switch between those terminals (AC rated, of course) and push to start. That would have been a fun experiment. Ultimately useless, of course, the compressor motor would go back into seizures as soon as the fridge got cold enough and the thermostat stopped the compressor for the first time. But it might be enough to do that once every 4-6 hours while you are waiting for the part to keep the fridge cold if you know this in advance. I ordered that part on Sunday and got it today, so it wasn't too long anyway.
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But it might be enough to do that once every 4-6 hours while you are waiting for the part to keep the fridge cold if you know this in advance.
No.
The thermostat control circuit would apply power to the motor circuit when the temperature rose above the set point. Unless you were there to press that button within a few seconds, power would be flowing through the motor windings in a stall situation. The motor would soon heat up and you would be in a much worse position. If you were really lucky, the motor might start turning - but I wouldn't make any bets.
These mechanisms are in place to provide the automatic operation in an orderly manner. If you're not prepared to sit around, waiting to press the button when the thermostat signals the motor start, then don't try it.
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There was no continuity between the sides so I think it was "stuck open".
So, all I had to do to get the compressor to run was put a momentary switch between those terminals (AC rated, of course) and push to start. That would have been a fun experiment. Ultimately useless, of course, the compressor motor would go back into seizures as soon as the fridge got cold enough and the thermostat stopped the compressor for the first time. But it might be enough to do that once every 4-6 hours while you are waiting for the part to keep the fridge cold if you know this in advance. I ordered that part on Sunday and got it today, so it wasn't too long anyway.
Yes, that's a PTC termistor. Very similar devices were to be used to energize the degausing coils on TV CRTs at turn-on. Similar devices (not sure it they're NTC or PTC) are also used for the door release catch delay on washing machines.
The reason is is open circut is because the silver electrode plating on the surface of the thermistor disk has burned away where it presses against the contacts. If it works with a momentary switch then it's probably just degredation - it's a common failure mode in the washing machine ones.
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Its the device, which is designed to die first in the fridge. Just so you need to buy a new fridge it you are not competent in repairing. Like 99% of people. No judgement.
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My expierence with these things is that they have different amp ratings and are designed so you can't install one with a different rating from your unit. This is done to prevent you from inserting one from a higher amperage unit in a lower amp rated refigerator. Its a "safety" feature. After discovering this "safety" feature I bought an extra one from a junk yard just in case the fridge stopped working with a freezer full of of stuff. The chances are good that the compressor is probabily OK and if you fix the relay problem the fridge will continue to work well without buying a whole new appliance for many many years to come. Of course if the compressor is shot an you like the refigerator have a service man replace the compressor or do it yourself. Don't rush out an buy a capacitor, its the least likely thing to break.
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Don't rush out an buy a capacitor, its the least likely thing to break.
Those things actually fail pretty often. Particularly when the manufacturer cheaps out and uses one rated 250V instead of the more common 370V or 440V. Just get a 440V cap (doesn't cost much extra) and it will last much longer.
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Don't rush out an buy a capacitor, its the least likely thing to break.
Those things actually fail pretty often. Particularly when the manufacturer cheaps out and uses one rated 250V instead of the more common 370V or 440V. Just get a 440V cap (doesn't cost much extra) and it will last much longer.
Except its not a capacitor its a PTC resistor like the others have already posted
It shows how long since I've worked on domestic fridges, they used to use little magnetic start relays to engage the connect / disconnect the start winding which were sized to the compressor and very much not mix and matchable.
On the other hand I always used to keep a solid state start relay handy in my toolbox as they were reasonably generic and could fit a range of models. Actually it looks like you can still get the same ones now, one of these would work fine to replace it:
http://www.supco.com/web/supco_live/products/ICG-220.html (http://www.supco.com/web/supco_live/products/ICG-220.html)
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But it might be enough to do that once every 4-6 hours while you are waiting for the part to keep the fridge cold if you know this in advance.
No.
The thermostat control circuit would apply power to the motor circuit when the temperature rose above the set point. Unless you were there to press that button within a few seconds, power would be flowing through the motor windings in a stall situation. The motor would soon heat up and you would be in a much worse position. If you were really lucky, the motor might start turning - but I wouldn't make any bets.
These mechanisms are in place to provide the automatic operation in an orderly manner. If you're not prepared to sit around, waiting to press the button when the thermostat signals the motor start, then don't try it.
There is a temperature sensing device that clicks off when this happens. That's why the fridge didn't self destruct the first time because I most certainly wasn't there when this device first failed me.
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This recent fire in the UK, which killed a still unknown but likely quite large number of people including entire families, began when a malfunctioning refrigerator exploded.
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This recent fire in the UK, which killed a still unknown but likely quite large number of people including entire families, began when a malfunctioning refrigerator exploded.
I believe it. Many types of appliances can catch fire and it is a major problem. Newer appliances have more protection devices in them. My fridge is only about 8 years old, so it makes me unhappy a necessary part already failed, but part of the protection is an additional device called an overload that I mentioned in my original post which senses that the compressor motor is overheating and cuts out. This is the part:
https://www.midwestapplianceparts.com/index.php?main_page=product_info&products_id=12574 (https://www.midwestapplianceparts.com/index.php?main_page=product_info&products_id=12574)
This part is working and it would switch off the compressor after it about 30 seconds of failing to start based on sensing the temperature of the compressor. It saved the compressor and maybe prevented a fire. It still works, so I didn't replace it. Hopefully it fails to a NO switch.
And the part I had to replace:
https://www.midwestapplianceparts.com/warehouse-parts-2803/lg-thermistor-start-relay-6748c-0004d-12375.html (https://www.midwestapplianceparts.com/warehouse-parts-2803/lg-thermistor-start-relay-6748c-0004d-12375.html)