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Need help troubleshooting pcb relay

Macknumber9

Jun 7, 2023
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I am troubleshooting a relay that i think is bad on a circuit board for a wine cooler....

I am getting 120 vac to one leg of the load but not getting 120 vac to other leg which gives power to the compressor...

I am trying to diagnose if the relay itself is getting the correct voltage on the coil side or not before I determine that the relay is at fault.

I am reading 12vdc on one side and 6vdc on the other side. Are both legs of the coil supposed to read same voltage or should one side read ground voltage?
 

Minder

Apr 24, 2015
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Depends on the circuit, but normally you should have the rated voltage of the coil read across it, IOW a 12vdc coil should measure exactly that across the coil.
What is the coil voltage rating stated on the relay?
The contacts would read their supply voltage when open and zero volts when closed.
 

Macknumber9

Jun 7, 2023
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Depends on the circuit, but normally you should have the rated voltage of the coil read across it, IOW a 12vdc coil should measure exactly that across the coil.
What is the coil voltage rating stated on the relay?
The contacts would read their supply voltage when open and zero volts when closed.
Yeah it is a SMIh-s-112Lm

Not sure on the specs....

So both pins of coil side should be close to the same vdc? If I am seeing 6vdc on one end and 12vdc what does that typically mean? I always thought one side of coil is ground and other is the power...
 

Macknumber9

Jun 7, 2023
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Depends on the circuit, but normally you should have the rated voltage of the coil read across it, IOW a 12vdc coil should measure exactly that across the coil.
What is the coil voltage rating stated on the relay?
The contacts would read their supply voltage when open and zero volts when closed.
If it doesn't read 12vdc on both pins like in my case (12vdc one pin and 6vdc other pin) does that mean I need to track down the other side maybe bad transistor or something? Or does it mean the coil itself has too much resistance and replacing the relay should solve this?
 

Minder

Apr 24, 2015
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You should have 12vdc across the coil when powered, first suspect would be whatever device is switching the coil.
 

Macknumber9

Jun 7, 2023
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You should have 12vdc across the coil when powered, first suspect would be whatever device is switching the coil.
Ok..by the way when I am testing I am putting probe on a known good ground and then other pin one one side of coil...then I record my 12vdc...then I do the same thing with the other side and get 6vdc...just making sure I am doing that right and I shouldn't be probing both sides of coil at the same time.
 

Bluejets

Oct 5, 2014
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.just making sure I am doing that right and I shouldn't be probing both sides of coil at the same time
Yes you should.......that's what "across the coil" means. (as Minder explained)
Picking a return from who-knows-where will simply confuse matters.
 

73's de Edd

Aug 21, 2015
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Sir Macknumber9 . . . . .​



I am reading 12vdc on one side and 6vdc on the other side. Are both legs of the coil supposed to read same voltage or should one side read ground voltage?


Soooooooooooooo . . . . with you finding a full 12VDC one one terminal of the relay you now know WHERE the power is coming in from.
One should suspect that full 12 VDC level to ZIP right thru that mere 200 ohm coil resistance and read basically the same on the other terminal side.
HOWEVER with you only reading 6 V there, means something is NOT right.
Consult my relay spec and the micro schema I made for you at the bottom.
If that driver transistor is not in conduction . . .~12VDC would be expected on its collector . . . . your place of contention.
However if that driver transistor has failed and is going into avalanche, a zener diode assimilation can result.
Thus it is bleeding down to 6VDC.
Now to simply find if that is being the case.
Monitor its collector or that "6VDC" relay terminal.
Place a temporary jumper wire connection from driver transistor Base to Emitter / ground.
If there was a developed /unwanted partial quiescent voltage coming into that base and causing only a partial activation of that relay, the monitored voltage should shoot up, towards the full 12 V supply. . . . . on base short out.
If nothing changes that monitored 6VDC, then lift the collector from the relay driver transistor and expect the relay 6 VDC should change to ~12 VDC then.
THUS . . . . . . your relay driver transistor has now taken on the unwanted characteristics of a 6 VDC zener diode to ground.
Replace it.

TECHNOINFO . . . . .

1686218613116.png


Got a pic of your PCB . . . . . so we can see all ?
Particularly to see if it is using a switch mode power supply.

73's de Edd . . . . .

I recently decided to sell my vacuum cleaner . . . . . since all it was doing was gathering dust.




.
 

Macknumber9

Jun 7, 2023
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Thank you for the detailed response. Still over my head a little so bare with me but I want to understand this. The transistor base is the middle pin right? If I short that to ground (any ground at all) and then run the machine and receive 12 vdc at the side of the relay that is normally giving me 6 vdc then my transistor is ok but it is receiving an unwanted voltage and not working properly? This would require me to find the problem with whatever is "feeding" the transistor?

But if I do that test and nothing changes and I still receive 6vdc....I should then lift the collector from the transistor and should now receive 12vdc....if lifting the collector causes me to receive 12vdc then replace transistor? What do you mean by lift collector? Desolder it and remove it from playing any role in the transistor?

If you need better pics I can take them. The blue is the relay but I'm sure you already knew...thanks so much for your help!
 

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73's de Edd

Aug 21, 2015
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Sir Mack . . . .
What do you mean by lift collector?
You need to disconnect . . . . the other end of the coil relay terminal's solder blob connection . . . . or the solder blob connection to the blue relays driver transistors collector connection.
Do this either by solder sucking or rosin flux + de soldering braid to wick off all of the solder at the connection.
Then in a NO power connected condition, use an ohmmeter in lowest ohms function to confirm that no trace of a solder bridge is present.
When confirmed, as so, then go for power up and do the voltage tests.
There are multiple DUPLICATE foil interconnects spaced apart from the transistors main CBE's such that no tedious, close in . . touchee-touchee . . . work / probing should be needed.
PLUS . . . at no additional extra charge . . . . .
This would require me to find the problem with whatever is "feeding" the transistor?
It will probably be that driver transistor that is avalanching C to E and acting as a 6V zener diode . . . .thus . . . just its replacement.
That will readily be apparent just as soon as the connection from other relay coils connection to this transistors collector is opened up

I see the SMPS just over to the side, so with having that 12VDC presence, it will be working !

I makee Mo' PHOTOGRAPHEE . . . . . .

1686232394501.png

Still over my head a little so bare with me

Can't do that, as it would be a bit embarrassing . . . . .best I can do, is just to bear with you.



Thaaaaaaaaaaassssit . . . . .



73's de Edd . . . . .
EatingPopcorn.jpg


I have 13 kids and no money, now if I had a chance to go back in time , I'd have no kids and 13 money, instead !
 
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Macknumber9

Jun 7, 2023
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Thank you SO MUCH...trying to mix in my full time job with getting back to you and the fact that I'm brand new to electronics...so please forgive my lack of knowledge...I am essentially taking the collector of the transistor out of the equation. I disconnect where it connects to the relays pin (where I'm getting 6vdc) when I confirm no continuity I run the machine and check for voltage there and I should then get 12vdc? If I do get 12vdc does that mean that the transistor is bad or does it mean that whatever is powering the transistor is bad? Transistor not getting efficient power?

I saw where you labeled the blue relays base drive for me....VERY HELPFUL to say the least...the blue relays base drive I tracked back to the little wire harness and it is labeled compressor...so is that the brains behind it all? And it sends power to the transistor to basically switch on and off the relay?
 
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Macknumber9

Jun 7, 2023
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Sir Macknumber9 . . . . .​



I am reading 12vdc on one side and 6vdc on the other side. Are both legs of the coil supposed to read same voltage or should one side read ground voltage?


Soooooooooooooo . . . . with you finding a full 12VDC one one terminal of the relay you now know WHERE the power is coming in from.
One should suspect that full 12 VDC level to ZIP right thru that mere 200 ohm coil resistance and read basically the same on the other terminal side.
HOWEVER with you only reading 6 V there, means something is NOT right.
Consult my relay spec and the micro schema I made for you at the bottom.
If that driver transistor is not in conduction . . .~12VDC would be expected on its collector . . . . your place of contention.
However if that driver transistor has failed and is going into avalanche, a zener diode assimilation can result.
Thus it is bleeding down to 6VDC.
Now to simply find if that is being the case.
Monitor its collector or that "6VDC" relay terminal.
Place a temporary jumper wire connection from driver transistor Base to Emitter / ground.
If there was a developed /unwanted partial quiescent voltage coming into that base and causing only a partial activation of that relay, the monitored voltage should shoot up, towards the full 12 V supply. . . . . on base short out.
If nothing changes that monitored 6VDC, then lift the collector from the relay driver transistor and expect the relay 6 VDC should change to ~12 VDC then.
THUS . . . . . . your relay driver transistor has now taken on the unwanted characteristics of a 6 VDC zener diode to ground.
Replace it.

TECHNOINFO . . . . .

View attachment 59400


Got a pic of your PCB . . . . . so we can see all ?
Particularly to see if it is using a switch mode power supply.

73's de Edd . . . . .

I recently decided to sell my vacuum cleaner . . . . . since all it was doing was gathering dust.




.
Ok so first test is to jump the transistor base to emitter/ground while checking voltage at the coil (side that is reading 6v)...if voltage jumps up to 12v where it should be then that confirms that there is quiescent unwanted voltage coming in not completely grounding out the circuit and giving full power.....now if this ends up being the case wouldn't the problem be with whatever is feeding the voltage to the transistor and the transistor itself would not be at fault?
 

73's de Edd

Aug 21, 2015
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Sir @Macknumber9 . . . . .​

O.K . . . . .one mo'time . . . . . reduced to simplified semantics of using my very best Go Dog Go . . . . . or . . . .See Dick and Jane Play . . . play play play . . . see them play.
You have the compressor switch on relay there and it has its 12 VDC coil connected to a 12 VDC power source on one end of its winding and the other end is then dependent upon a connection to - /or/ ground to activate the relay coil and in turn, its armature and power contacts, then closing.
We see that they have chosen a NPN transistor to do that task.
They have hooked the relay coil connection that needs to be grounded to the collector of the driver transistor and its emitter has been connected to ground, with its base connected to two resistors, one goes to ground and another to a series wired resistor to where and activation voltage would be coming in.
EXPECTATIONS would be for there to be no conduction between collector to emitter, therefore ~ full 12V should be read on the collector.
If a voltage comes in at the third terminal down from the six connections seen , (BLUE referenced) it passes down the BLUE dot line and ends up at the two resistor voltage divider pair and on into the base of the driver transistor, if that applied voltage is on up and above ~> .7v, that transistor should conduct collector to emitter and complete your relay coils grounding with a coincident THUMP of the relay armatures mechanics of a set set of POWER contacts to activate the compressor. (And believe me . . . they have chosen design values such that transistor is driven well into full conduction.)

WHAT TO DO . . . . . WHAT TO DO . . . . . WHAT TO DO . . . . . what to do ?

Now what you could do after initially confirming the collector voltage is still only being 6VDC, is to swing your meters voltage probe over to the base of the transistor to see of there might be a "creep" voltage coming into that base from its BLUE dot supply line . . . . not expecting any, but that low level could be causing only a a partial degree of conduction of the transistor.
BUT . . . . . . . that would be answered by the other suggested short cut test of grounding out the base of the transistor. The full ~12VDC should be at the collector of a NON conducting transistor.
If that voltage is still at 6 VDC only the coll-emitter conduction of that then . . . . UNDRIVEN . . . . driver transistor by avalanching would be at fault.
Then . . . . . . . you open the collector to relay coil connection and expect the relay terminal end to go to 12 VDC.
If that happens . . . . . . with there then being no drive voltage on the base . . . . . and even no connection made to the transistor collector, expect that transistor is at fault.

73's de Edd . . . . . .

I have a dog to provide me with unconditional love but I also have a cat to remind me that I don’t deserve it . . . . . . . . it’s just being all about balance.


.
 

H2814D

Nov 4, 2017
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One thing I think I am seeing on the soldered side of the board are what appear to be cold solder joints, especially in Edd's modified picture in the upper left corner. It may just be the way the photograph was taken, but they can cause all kinds of issues, including problems like you are having. Do you know what they look like, or what they are, and how to fix them?
 

Macknumber9

Jun 7, 2023
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One thing I think I am seeing on the soldered side of the board are what appear to be cold solder joints, especially in Edd's modified picture in the upper left corner. It may just be the way the photograph was taken, but they can cause all kinds of issues, including problems like you are having. Do you know what they look like, or what they are, and how to fix them?
No I don't know what they are or how to fix them. Where do you see them? I'm staring at my board right now lol.
 

H2814D

Nov 4, 2017
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So take a look at where Edd has blown up your photo. On your original photo the area is near the bottom right half of your picture. In Edd's photo, the top left. Look very closely at the solder joint where he has marked "Blk Rly's 12v coil" and the pin to the left. Look very closely at that solder and see if it appears to be cracked around the pin that comes through the board. You may need a magnifying glass to do that. You should also check the rest of that side of the board for more of them. They usually show up where the connection may warm up when power is applied and then cool down when the power is off. The expansion and contractions of the metals causes cracks. Fixing them is simple and usually just needs to have the solder re-melted (reflowed) using a little bit of new solder.

I'm attaching some pictures of what they look like. These aren't yours, though. They are hard to see unless you know what you are looking for, but these should help.
 

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Macknumber9

Jun 7, 2023
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I performed this test. I couldn't do the tests Edd mentioned because I have the board and machine in different locations at the moment....

I passed tests 1 and 2. The other tests failed 3, 4 and 5....on npn transistor (which mine is) it states that you should read OL and I did not get OL...I actually got a number....

I did this on both the transistors...one that feeds the blue relay and black....they both were identical in test results...is this test accurate? Or does not show the whole picture
 

73's de Edd

Aug 21, 2015
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Your test results are additionally confirming what I am definitely wanting to believe. THAT there is something rotten from Collector to Emitter and/or Base of that relay driver transistor.
BUT there is also the uncertainity / validity of your testing, with the transistor still soldered into circuitry.
( If this is true ? )
With that reflecting upon the fact that the other transistor is also reading as a fail . . . . . Probably NOT true.
On that “ Itinerant Wine-o Cooler “ I suspicion that other . . . .black . . . .relay might be functionally related to cycling in a defrost heater .

My suggested test analysis procedure gets the most definitive results with the absolute least desoldering.
Shorting the base to ground confirms that there will be no voltage coming into the base, creating no transistor functionality whatsoever.
THEN if the transistor connection to the relay is still permitting a power flow thru from the collector connection, something is BAD in the transistor junction, either leakage or zener action avalanching . . . . .Collector to Emitter.
Unsoldering at either the Collector or at its other ends relay coil connection will provide the final answer . . . . . .and you KNOW that the floating relay end connection will have a full 12 volts on it then, thus a defunct driver transistors innards will be at fault.

Thaaaaaaaaaaaaaaaassit ! ! ! . . . . . . . . Being “ simewhat somilar “ to Whaaaaaaaaaaaaaaaaaaaaaaasssssup ! ! !


Edd . . . . . . . .
 
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Macknumber9

Jun 7, 2023
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Your test results are additionally confirming what I am definitely wanting to believe. THAT there is something rotten from Collector to Emitter and/or Base of that relay driver transistor.
BUT there is also the uncertainity / validity of your testing, with the transistor still soldered into circuitry.
( If this is true ? )
With that reflecting upon the fact that the other transistor is also reading as a fail . . . . . Probably NOT true.
On that “ Itinerant Wine-o Cooler “ I suspicion that other . . . .black . . . .relay might be functionally related to cycling in a defrost heater .

My suggested test analysis procedure gets the most definitive results with the absolute least desoldering.
Shorting the base to ground confirms that there will be no voltage coming into the base, creating no transistor functionality whatsoever.
THEN if the transistor connection to the relay is still permitting a power flow thru from the collector connection, something is BAD in the transistor junction, either leakage or zener action avalanching . . . . .Collector to Emitter.
Unsoldering at either the Collector or at its other ends relay coil connection will provide the final answer . . . . . .and you KNOW that the floating relay end connection will have a full 12 volts on it then, thus a defunct driver transistors innards will be at fault.

Thaaaaaaaaaaaaaaaassit ! ! ! . . . . . . . . Being “ simewhat somular “ to Whaaaaaaaaaaaaaaaaaaaaaaasssssup ! ! !


Edd . . . . . . . .
Ok thanks for all your help...starting to understand it better now....

Simply out if curiosity...how does the base gets its power? I guess the 12 vdc on the other side is constant and always on but the transistor acts as the switch to turn on/off the relay...but how does the base receive its .7v and 0v? I see it goes back to that pin labeled compressor on the board....does that go back up to some sort of thermostat in the cooler to determine when it needs to turn on/off?
 

Macknumber9

Jun 7, 2023
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Your test results are additionally confirming what I am definitely wanting to believe. THAT there is something rotten from Collector to Emitter and/or Base of that relay driver transistor.
BUT there is also the uncertainity / validity of your testing, with the transistor still soldered into circuitry.
( If this is true ? )
With that reflecting upon the fact that the other transistor is also reading as a fail . . . . . Probably NOT true.
On that “ Itinerant Wine-o Cooler “ I suspicion that other . . . .black . . . .relay might be functionally related to cycling in a defrost heater .

My suggested test analysis procedure gets the most definitive results with the absolute least desoldering.
Shorting the base to ground confirms that there will be no voltage coming into the base, creating no transistor functionality whatsoever.
THEN if the transistor connection to the relay is still permitting a power flow thru from the collector connection, something is BAD in the transistor junction, either leakage or zener action avalanching . . . . .Collector to Emitter.
Unsoldering at either the Collector or at its other ends relay coil connection will provide the final answer . . . . . .and you KNOW that the floating relay end connection will have a full 12 volts on it then, thus a defunct driver transistors innards will be at fault.

Thaaaaaaaaaaaaaaaassit ! ! ! . . . . . . . . Being “ simewhat somilar “ to Whaaaaaaaaaaaaaaaaaaaaaaasssssup ! ! !


Edd . . . . . . . .
So the easiest way to test this without desoldering anything is to check for 12v at the collector end of the relay while I am shorting the base to ground....by shorting the base to ground I am shutting off transistor and there should be NO CONDUCTION from collector to emitter....this should result in 12 volts seen at the collector....if 12 volts is seen at the collector then the transistor isn't bad yet but the followup test would need to take place...

If I still see 6v at the collector then I know the transistor is bad and no further testing is needed...

This sound right?
 
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