Chest FREEZER Whirlpool AFG 605 DGT - Identification of faulty PCB transformer

[spec]

As there is no 5 volts the relay could not be energised so I think the contacts may have welded together on the relay. (Unless thy are normaly closed contacts which is umlikely.)

Hi Les,

I suspect that the relay is normally closed and hence the circuit fails safe to powering the fridge on (so the food does not spoil) in the event of a failure on the board.

spec

 

[JohnMcC]

Hi Les,

You have already measured the resisistance (In post #15) of the varistor by measuring the resistance between the tracks that went to the transformer primary. You said that the reading was 300 ohms

Correct. I have removed the varistor from the board.............still 300 ohms.

The board is just the 5 volt power supply and relay with a driver transistor

Yes, that is my opinion as well

In post#12 you say that the compressor still runs. I suspect that the relay on this board controls the power to the compressor.

Yes the relay on the board is for driving the compressor. There is another connector on this board we are looking at which goes to the display board (situated in the freezer handle) where is shows the temperature on a digital display, and allows you to make it colder (or not so cold) It has an alarm and other things on it. None of that currently works because there is no 5 volts supplying it. Another plug (from our current board) goes to the light in the fridge (which works), and the remaining plug is for the Freezer Thermostat.

so I think the contacts may have welded together on the relay

This is a possibility. Or as you point out they are normally closed. Normally closed makes a lot of sense to me, because if the electronics burns out (as they have done), the freezer continues to work even though the compressor runs continuously. I am very grateful it is still running, because otherwise all my food would have been wasted ! From a security point of view I understand why you would argue it should be normally open, and you may well be correct. I currently have put the mains plug on a timer so that the compressor does not run 24 hours per day.

Can you measure the resistance between the square pad under the relay with 4 pins comming through it (Directly below the word plug of power plug.) and the track just to the right of it.

Yes I have done this............there is no resistance. The relay is either normally closed or the contacts have welded together.

Do not test the board out of the freezer unless you feel confident working on a board with open live mains on it.

I have to take this board out every time I wish to look at it because it is under the bottom of the freezer and awkward to get to. I have to put it back when I have finished and connect 2 of the four plugs otherwise the compressor will not run. I have not been bothered to bridge the contacts as the wires (on the plugs) are short and awkward to get to. The plugs on the board I have to connect are the power (from the mains) which is CN3, and CN4 which goes to the compressor.
CN1 I believe goes to the thermostat, and CN2 (to the left of CN1) goes to the display board in the handle.

Thank you again
John

 

[JohnMcC]

Correction............there is no CN2..............it is called CN5

 

[JohnMcC]

Hello Les and Spec,

So in conclusion, I understand that you believe it is the varistor which is faulty and needs changing.
Also that I can remove the varistor, replace the burnt 5W100ΩJ, and it "should" work ? (Even though long term it is better to replace the varistor, it is OK to test the circuit without it) ?
Please confirm this is correct.

Another question please.............with the transformer (230v to 6.6v) ...........can I connect the primary directly to the mains (with a fuse in the plug) and see if it gives an output of 6.6v AC (off the circuit board) ??
I am not sure if you can do this with no load..........that is why I ask.............will it spoil the transformer ?? if the transformer is working correctly, I do not wish to spoil it !

Thanks
John

 

[spec]

Hello John,

I understand that you believe it is the varistor which is faulty and needs changing.

Yes, you can confirm this by removing the varistor and measuring the resistance across it. If the reading is a short the varistor is faulty. In effect the varistor has sacrificed itself to protect the rest of the circuits.

Also, that I can remove the varistor, replace the burnt 5W100ΩJ, and it "should" work ? (Even though long term it is better to replace the varistor, it is OK to test the circuit without it) ?

Yes, that is correct. Just the same as you can drive your automobile without the seat belt on.

with the transformer (230v to 6.6v) ...........can I connect the primary directly to the mains (with a fuse in the plug) and see if it gives an output of 6.6v AC (off the circuit board) ?

Yes, but put a 1A fuse in the mains plug.

I am not sure if you can do this with no load..........that is why I ask.............will it spoil the transformer ?? if the transformer is working correctly, I do not wish to spoil it !

Have no fear. The transformer primary is designed to be connected directly to the mains supply. The 100 Ohm resistor in series with the primary is added protection. The transformer will not be damaged by running with an open circuit secondary- it will think that it is on holiday.

Good luck- let us know how you get on.

After the transformer test we will advise what to do next, if you want.

spec

 

[ronsimpson]

can I connect the primary directly to the mains (with a fuse in the plug) and see if it gives an output of 6.6v AC (off the circuit board) ??

Yes. Be careful.
With no load it might read a small amount high.

I think the point of this board is to make 5V at 0.5A and have a relay. Long before now I would have removed the 7805 and connected a cell phone charger (USB type) to the output of where the 7805 went.

 

[JohnMcC]

OK, thanks Spec and RonSimspson.

I have been reading up on Varistors..............and consider that all circuits should be protected by them !!!!!

"A varistor is an electronic component with an electrical resistance that varies with the applied voltage. Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode."
Overvoltage Circuit Protection
Good youtube videos for anyone who may be interested :

 

[JohnMcC]

Thank you everyone for all your very kind help.
I am glad to inform you that it now all works !

I put everything back on the board except the varistor (which between us we had established was faulty).
Of course I put a new resistance 5W100ΩJ on the board.

I started by only putting the power cable plug into the board...........and to my delight this time the 5W100ΩJ did not blow.
Then I put in the other plugs, and it all works......FANTASTIC !!!

I have since removed the 5W100ΩJ from the board until I get a new varistor.

I have a question please about the varistor.
Are all varistors with the reference 07D431K the same ?
On ebay I have come across some which say 07D431K................... 275Vac/350Vdc
and others
07D431K................... 430V ±10%

Obviously I wish for one that protects at 275Vac, but the 430V may just be a rating, like a maximum allowable rating ??
Thank you

 

[spec]

I am glad to inform you that it now all works!

Great news- well done.

I have since removed the 5W100ΩJ from the board until I get a new varistor.

John, the 100R resistor must be left in circuit; it will not be damaged by the absence of the varistor.

I have a question please about the varistor.
Are all varistors with the reference 07D431K the same ?
On ebay I have come across some which say 07D431K................... 275Vac/350Vdc
and others
07D431K................... 430V ±10%

They are all the same- just sloppy adverts.

spec

 

[JohnMcC]

John, the 100R resistor must be left in circuit; it will not be damaged by the absence of the varistor.

Yes..............for it to work properly I understand that the 5W100ΩJ must remain.
Until I have the varistor, I am leaving the freezer on the timer............with none of the electronics working.
The voltage fluctuations here are enormous...........and I do not wish to burn anything else out by leaving the freezer running "normally" without the varistor protecting it.

They are all the same- just sloppy adverts

The way the adverts are written one has the impression that there are multiple voltages for the same reference 07D431K.
It makes much more sense that they are all identical.

Thank again.

 

[spec]

No probs John.

But advise, in view of what you say about your mains fluctuations, to fit a higher current capacity varistor, but the same voltage. I will have a look.

Also fit a 1A fuse on the board in series with the mains live input. That way a bad mains voltage spike will just blow the fuse.

Do you know, if I were in your position, I would definitely be thinking about a high capacity over-voltage protector for the whole house.

spec

 

[spec]

John, I would be inclined to replace the original varistor (1) with the type at (2) below. Best get a few of (2) varistors for your other equipment too.

Varistors
(1) Bourns MOV07D431K: Vb= 430V+- 10% (387V to 473V), Ipk= 1,200A, energy= 20J: https://www.bourns.com/docs/Product-Datasheets/MOV07D.pdf
(2) Bourns MV20431K: Vb= 430V +- 10%, Ipk = 6,500A, e=190J https://www.farnell.com/datasheets/2026683.pdf?_ga=1.82774486.1144713272.1475169196

spec

 

[JohnMcC]

That way a bad mains voltage spike will just blow the fuse.

Very strangely, when this huge voltage spike happened and blew a lot of my electrical equipment, none of the fuses blew.

My Pioneer HIFI has 2 fuses between the power cable and the transformer (all on the PCB board). Both fuses have not blown...........but the equipment is totally dead.

 

[spec]

Very strangely, when this huge voltage spike happened and blew a lot of my electrical equipment, none of the fuses blew.

My Pioneer HIFI has 2 fuses between the power cable and the transformer (all on the PCB board). Both fuses have not blown...........but the equipment is totally dead.

Hmm, that is odd.

Spec

 

[JohnMcC]

I agree, it is very odd indeed. But fuses protect against current, not voltage.
You will argue, the higher the voltage, the more current that will pass.
However, not a single circuit breaker or fuse blew in the house.

Most of the equipment was on standby at the time of the surge.............and it is the electronic part which no longer works.
This was true of the freezer..........only the electronic part did not work because the varistor burnt out, which caused the resistance to burn out.
On the Daewoo microwave, the light inside still turns on when the door is opened, but none of the electronics works, such as the digital display. The control part does not work either , so the microwave will not cook or turn on.
The Pioneer HIFI display no longer works, and nothing on it works, but as I said the fuses are fine.

Cannot really explain it

 

[Les Jones]

Hi John,
I'm pleased that the only things that are fauly are the 100 ohm resistor and the varistor. I think that this is a particularly good design as most things using varistors as protection just rely on a fuse blowing to limit the surge current which could be many tens of amps. The 100 ohm resistor would have limited the current to about 4 amps (With the mains going up to about 400 volts.) The other products which have failed will each need to be looked at separately to analyse what has failed. The fuses in the microwave probably protect both the high power part and the control part so they would be too large to protect the control part from the surge. For the hi fi the only part that would have been powered up would be the IR receiver (Assuming it has remote control.) and the power switching circuit. It would be worth searching the web to see if you can find a schematic for the hi fi. (I don't think you will find one for the microwave.)

Les.

 

[JohnMcC]

Hi Les,

I'm pleased that the only things that are fauly are the 100 ohm resistor and the varistor. I think that this is a particularly good design as most things using varistors as protection just rely on a fuse blowing to limit the surge current which could be many tens of amps.

Thank you, so am I !! I agree, it is an excellent design, and when you know what you are doing relatively easy to fix. Most people would either have the circuit board replaced at 133 euros plus 80 euros for the technician call out charge, or throw the entire freezer out and buy a new one. Although the freezer is about 10 years old it looks like new because my wife takes very good care of it. I was concerned that the electric motor driving the compressor may have been damaged, but it has been running fine on a timer for over two months now.

Other equipment.
I plan to make new postings on this website for the other equipment, hoping I am not stretching people's kind help too far ?

HIFI:
The fuses on the microwave and HIFI have not blown.
The HIFI does have a remote, and the system was on standby when this huge surge happened. The time on the display went straight out, and there was an awful smell of burning from this equipment. The transformer on the board has a burn mark and looks melted. Pioneer no longer sell this transformer, and I cannot find out much about it despite having done many searches on the internet. I do have a service manual for this equipment, but it is far too complicated for me !!!

Microwave:
I have the service manual, but again it is rather complicated, and I have not managed to gain much from it !

Thank you again

John

 

[spec]

A replacement transformer, if necessary, for your Pioneer HIiFi may not be too much of a problem. You do know that you can start multiple posts on ETO.

I'm dying to see pictures of your HiFi, and if you had a circuit diagram that would be great. Failing that, if we had the model number we may be able to find a circuit diagram for your model or close.

spec

 

[Les Jones]

Hi John,
If you have a Dropbox account (A cloud storage site.) and you have the manual in electronic form (PDF or jpegs) you could put it in your public folder. You can then make it available via a URL. You could then post the URL on the forum (Or send it to spec and myself via a private message on this forum.). The service manual would probably too big to post directly on the forum. You may think of some other way of doing this. It is a pleasure helping people like you that are prepared to answer questions about the problem. Some people seem to think it is there right to be given help by other forum members without putting any effort in themselves.
Les.

 

[JohnMcC]

Thank you very much.
I will do it today.
I did plan to put a link to my ONEDRIVE, because the pdf files are too large for the forum

Kind Regards,
John