Does it make sense that a fridge relay start winding would get only 20 volts from the thermostat?

Submitted ⁨⁨3⁩ ⁨days⁩ ago⁩ by ⁨diyrebel@lemmy.dbzer0.com⁩ to ⁨askelectronics@discuss.tchncs.de⁩

cross-posted from: slrpnk.net/post/26897688

Either the relay is broken or the thermostat. I know this because I could force the fridge to run by hotwiring 220V across the common and run pins then briefly jumping the start pin to the run pin (thus sending 220V through the start winding for a split section).

The thermostat assembly has too many wires to know what to measure resistance on. But I figure that the thermostat can be black-box tested simply by checking whether it sends 220v to the relay switch. There are THREE wires involved (in addition to ground). With the relay removed (thus compressor disconnected), I plugged the fridge into mains. Got these measurements:

common pin to run: 220v
common pin to start pin: 20v

I’m tempted to conclude that the start winding /can/ accept 220v (proven by hotwiring), but 20v is also sufficient to cause enough motor movement to get it started. Does that sound reasonable? Can I conclude that the thermostat works and the relay is toast?

It’s hard to be confident without a service manual, particularly when the relay¹ has continuity. The common pin goes through a cylinder marked “Klixon MRP342LZ-32Q1”, which as 0.5 Ω. The start and run pins go through what seems to be a ceramic disc having 45 Ω. Is that reasonable?

I am ½ tempted to attach the relay, bypass the thermostat, and wire the relay to mains with start-run jumped persistently. Is that crazy? I assume the relay would internally disconnect the start winding. The fridge has no fans AFAICT, so no need for anything else to have power in this test.

¹ The relay I have has part № “A 3B7 8100” printed on it, which I cannot find online. But it looks exactly like this one.

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lemming741@lemmy.world ⁨2⁩ ⁨days⁩ ago
Those PTC start relays die all the time. These are rated a 200k starts, approx 5 years at 4.5 starts an hour.

sensata.com/…/sensata-8ea-series-ptc-motor-starte…

A proper application datasheet- everything you could ever want to know about them

www.vishay.com/docs/23086/ptc305c.pdf

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Onomatopoeia@lemmy.cafe ⁨3⁩ ⁨days⁩ ago
Is there a step-down transformer anywhere? That relay isn’t big enough to have such a transformer or switching power supply (I don’t think).

It would be strange (to me) to build everything for 220, including the t-stat, but not the start winding. That would then require either a transformer or switching power supply for that one thing.

The thermostat should be easy enough to test - you know where the supply is, and which wires energize this relay. Test voltage at the relay when the thermostat closes.

If something is dropping 220 to 20v, surely it’s got to be heating up?

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- diyrebel@lemmy.dbzer0.com ⁨3⁩ ⁨days⁩ ago
  
  Is there a step-down transformer anywhere?
  
  I doubt it… unless that’s something we would expect to find in a fridge? I see no big coils. There are also no fans on this fridge. No start capacitor either. The YT videos I saw do not correspond well with this. There is no freezer→fridge damper either. There are coils in both.
  
  The thermostat should be easy enough to test - you know where the supply is, and which wires energize this relay. Test voltage at the relay when the thermostat closes.
  
  I cannot easily remove the thermostat… hard-wired in, otherwise I would try putting it in a working freezer then check that it closes. But OTOH that test is perhaps not useful since the problem is not continuous running. The problem is not running at all.
  
  I can’t access that wiringwork page, but I would expect current and voltage to both be zero when the thermostat is happy. In any case, I appreciate the feedback.
  
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  - Onomatopoeia@lemmy.cafe ⁨3⁩ ⁨days⁩ ago
    I wouldn’t expect a transformer anywhere, I was just shooting in the dark. That would make no sense to me, but I’ve seen crazy stuff in appliances.
    
    A better test may be to forcibly energize the relay so it closes. If it closes and the motor starts, the problem is in the thermostat. If it closes but the motor doesn’t start, the relay is the problem.
    
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