Norcold Needs a Little Electonics Help

[TomH]

I decided to dig out the Norcold I picked up a few years ago and see if I could get it cleaned up. I ran it overnight on 110 and it works great. Decided to try 12V and immediately got that smell of burning electrical so I shut it down and took it apart.

I shut it down right away so there wasn't much damage on the circuit card. One burned component and the trace lifted a little - no big deal I hope.

I need some help identifying the specifics of the component. Seems to be a common part but my electronics knowledge is very low so I'm hoping someone can help me pick the right part.

It's marked:

IRFZ44
I<diode symbol>R8919
5H 1H

Can I just grab any IRFZ44 or do I need something specific?

Thanks

 

[TomH]

Some pictures for future reference.

 

[TomH]

More pictures.

 

[greentruck]

Diodes are often used as polarity protection devices, IIRC. My guess is that you got the 12 volt polarity backwards? If so, it did what it was supposed to do -- hopefully -- and protected the rest of the circuit board.

Try looking it up at Mouser.com. They carry just about any common component, but be sure it's rated the same. Like a fuse, you don't want to replace it with something different or you'll lose your protection factor.

 

[george_tlc]

IRFZ44 is an n-channel FET. Quite a common part.

It is **NOT** a Diode.

IR is the manufacturer = International Rectifier, though is also made by other companies.

cheers,
george.

 

[Bogo]

Digikey: Digi-Key - IRFZ44VPBF-ND (Manufacturer - IRFZ44VPBF) looks like it. Just a couple bucks each.

Important thing is to figure out why it fried in the first place.

BTW, big HEXFETS like it have an intrinsic diode in them. It is common to use that diode in power switching circuits. It reduces the need for another part, and often the diodes built into them are faster acting and higher current handling than the dedicated diodes.

 

[TomH]

Thanks for the help. I had looked up the part and datasheet, but like I said, my electronics knowledge is limited. The funny thing is I spent 15 years working in electronics and microelectronics manufacturing so I know most of the processes for assembly but have no clue how things work.

Important thing is to figure out why it fried in the first place.

This is the unknown. I have no idea why it failed. Couple thoughts

1. There was some corrosion on the 12 volt pins so when I plugged in the cord, I had to jiggle it a little before it started. The on/off switch was working the same way so it required a little jiggling too. I cleaned up both of those parts.
2. There was some corrosion or something under the five diodes in the upper right corner of the first picture I posted. There may have been some shorting. It all cleaned up easily so I'm hoping if that was the problem, it has been resolved.

Any other thoughts?

My plan was to source a new component, solder it in, and see what happens.

One other question - you can see the plastic that covered the FET. I assume this is to insulate from the metal. Well the plastic burned through. Any idea what I should use to replace it?

Thanks again.
Tom

 

[Bogo]

Was the plastic between the plastic and the metal I see behind it? Kind of looks like that to me. If so you may need to insulate the HEXFET from the metal.

There is a stud post that the cover plate of metal goes over. Did it have a nut on it? I bet it is supposed to have one. If it fell off, the cover plate could have shifted and shorted the drain and source pins. My guess is that cover place clamps the FETs to the metal behind them for heat dissipation. I bet that plastic piece is supposed to provide electrical insulation between the FETS and case. Edit: Note that the middle drain leg of the FETS is stuck out. If they had used inline pins I bet it wouldn't have shorted.

 

[TomH]

Was the plastic between the [STRIKE]plastic[/STRIKE] FET and the metal I see behind it? Kind of looks like that to me. If so you may need to insulate the HEXFET from the metal.

Yes - the plastic is kind of matchbook shape and was between the FETs and the metal case. On the backside it went a good half inch below the board.

There is a stud post that the cover plate of metal goes over. Did it have a nut on it? I bet it is supposed to have one. If it fell off, the cover plate could have shifted and shorted the drain and source pins. My guess is that cover place clamps the FETs to the metal behind them for heat dissipation. I bet that plastic piece is supposed to provide electrical insulation between the FETS and case. Edit: Note that the middle drain leg of the FETS is stuck out. If they had used inline pins I bet it wouldn't have shorted.

No nut on the plate - the plate is threaded and was securely fastened when I took it apart. Unlikely it shifted. The plate comes up about 1/3 to 1/2 way up the FET so there is a fair amount of clearance between the plate and the leads. There could have been something there that cause it to short and there is a pretty good burn mark on the plate so I wouldn't rule that out as a possibility.

Back to the markings on the FET, I found a little info on the datasheet.
IRFZ44 - Part Number
I<diode symbol>R - Manufacturer's symbol/trademark. International Rectifier as George pointed out.
8919 - Date code. Put the fridge vintage at about 1989, which seems about right.
5H 1H - probably a lot number.

Tom

 

[greentruck]

If you need insulation, use heat shrink. In most cases it's fine, only where it might get hot in normal operation would it require something better.

There are nylon washers that can also be used if the studd needs to be insulated, but I don't think that's the case here?

Now, burning, nothing but asbestos is going to help with that. You don't wanna go there.

 

[Bogo]

May have been a washer, nut or other bit of metal that shorted the lead.

Search for thermal pad at Electronic Components Distributor | DigiKey Corp. | US Home Page
Some choices will come up.

 

[george_tlc]

From the picture it appears that the FET is just sandwiched between the case and the clamp (gold coloured metal). The plastic is likely there to protect the bottom pins of the FETs from shorting to the clamp section of metal. It isn't needed on the body of the FET itself since that is plastic (insulator) anyway.

Given I don't see any thermal paste or thermal pad where the FET was pushed against the case, it would appear the FET isn't "expected" to be running that hot - or the person that assembled your unit skipped a step....

Anyhow, I'd just find a sheet of thin mylar/plastic and cut it to shape and use that as the replacement - it doesn't appear to serve any seriously important function.

Without a schematic of the board it's hard to guess what the FET is actually being used for. Your plan to replace the part and see if it is then fixed is likely your best option right now.

Oh, FET's are static sensitive, so take some basic antistatic precautions while you handle the part and solder it into the board.

cheers,
george.

 

[rusty_tlc]

I'd get at least a couple of extra FET's, @ $2 each it would be safe insurance in case you blow another when you power up again.

It might be easier to just replace the whole PCB assembly.

 

[TomH]

May have been a washer, nut or other bit of metal that shorted the lead.

Based on the way the rest of the fridge looks, I'd say a piece of rust. I think you're probably right though and something shorted there, the orientation of the board makes this likely.

From the picture it appears that the FET is just sandwiched between the case and the clamp (gold coloured metal). The plastic is likely there to protect the bottom pins of the FETs from shorting to the clamp section of metal. It isn't needed on the body of the FET itself since that is plastic (insulator) anyway.

Given I don't see any thermal paste or thermal pad where the FET was pushed against the case, it would appear the FET isn't "expected" to be running that hot - or the person that assembled your unit skipped a step....

I doubt any thermal paste was used. The board slides into the housing so it would make it difficult to apply paste once everything was together.

Anyhow, I'd just find a sheet of thin mylar/plastic and cut it to shape and use that as the replacement - it doesn't appear to serve any seriously important function.

Mylar sounds like a good choice - I'll grab a piece.

Without a schematic of the board it's hard to guess what the FET is actually being used for. Your plan to replace the part and see if it is then fixed is likely your best option right now.

Oh, FET's are static sensitive, so take some basic antistatic precautions while you handle the part and solder it into the board.

cheers,
george.

Now where was my wrist strap? I think it will be easier to just buy a new one than to find the one I have. I've been slowly working towards getting an electronics 'workstation' set up. Probably a good time to buy a mat and wrist strap.

I'd get at least a couple of extra FET's, @ $2 each it would be safe insurance in case you blow another when you power up again.

It might be easier to just replace the whole PCB assembly.

Yep, extra FETs were in the plan. Replacing the whole board isn't really an option The whole case and all the parts in it are sold as a power supply - for the low price of about $300. Since I got the fridge for free, I'm willing to put a few dollars and some time into it, but not $300.

Some more pictures.

1. The Norcold schematic showing the orientation of the power supply in the fridge. I'll get some actual pictures later.
2. The burn mark on the FET cover.
3. The plastic that surrounded the two FETs.

Thanks again for all the help. I'm going to get the FET ordered and go from there.

 

[TomH]

New parts have arrived from Digikey. I built a new Mylar cover and I'm pretty much ready to go. One more question though. Remember I mentioned a lifted trace? Think I should try to bond it back down to the board or just go with it? If I were to bond it, what adhesive should I use?

Some more pictures:
1. Lifted trace
2. New part
3. New Mylar cover.

 

[george_tlc]

You won't really be able to glue it down, since the heat of the solder will cause whatever you use to fail.

The two remain good holes/traces should hold the FET, just be gentle in your soldering to not damage the 2 remaining pad/traces.

It is a ***VERY*** cheap PCB, since it only has copper on the bottom side and no plating in the thru holes. Basically a cheap & nasty board.

cheers,
george.

 

[Bogo]

If you want, take a short bit of wire and jumper it from the lifted trace pin to the next part along the trace.

When you bend the leads, be gentle. It helps to use a small pliers. Remember to dissipate static first.

 

[TomH]

I hadn't considered the heat from soldering. When I worked in microeletronics assembly, we had conductive and non-conductive epoxies that were used for die bonding, but they cured at 150C. I doubt these would handle the heat of soldering, plus I wouldn't want to throw the board in a 150C oven for an hour.

I was a little surprised at the quality of the board - just didn't seem like something you would find in an expensive refrigerator. Lends some credence to these lower cost models like Edgestar and Waeco.

A jumper wire may be a good solution for the trace. I think George is probably right that two good pads should work but I'm still a little concerned about vibration. I'll have to give it a little more thought.

I picked up a wrist strap from Digikey and wired in a gound point last night so I should be good there. I was planning on using some pliers to form the leads.

Hopefully I'll get this soldered up tonight but not sure I'll have time this weekend to put everything back together for testing.

Thanks again for all the input.

Tom

 

[Bogo]

On that third leg, cut away the lifted trace. When you insert the part, bend the end if the leg over to meet that close by solder joint on the same trace and solder it to it. Use a short jumper wire if it doesn't reach. After it is soldered up and cleaning, then use a non conductive epoxy to glue the leg in place. Do the gluing after you verify it works.

 

[TomH]

Success.

Found a little time today to work on this. Got the new component soldered in and then put everything back together. Plugged it in and ran it a couple minutes on 110V and it worked fine. So I hooked up the 12V and watched for smoke but didn't see any. Ran it for about 15 minutes with no problems but will have to wait before I can do anymore testing because...

... now the rest of the story.

The reason I got this unit out was because I wanted to take a look at the thermostat. See, there is a Fisheries Supply just down the street from me and every spring they host a swap meet. I love a good swap meet and even though I don't have a boat I can usually find something I "need".

This time I found a Norcold DE251E which just happens to be the right size for my tent trailer. The guy wanted $10 for it so I asked him if it worked. He said something about using it for parts and never actually running it. I figured for $10 it was worth a chance.

I get it home, plug it in and go to turn it on, but there is no on switch. In fact, the thermostat assembly is missing - guess that was the part that was needed. Well, Norcold doesn't support these older units so no thermostats available. But wait, I have this other unit - wonder if the thermostat is the same. Sure enough - exactly the same except for the sticker that goes on the front.

Well, I hate to rob parts from something that is working so I'm having a hard time taking the thermostat from the MRFT60, that is until I see smoke coming from it. So, the thermostat is now in the DE251E and the MRFT60 is without.

I jumped it today for testing (you can see the blue splice in the last picture of the next post) but now will have to keep my eyes open for Norcold parts. I did find a used one locally, but the guy wanted $70 for it which is a little out of my price range for a $10 fridge.

So this story will continue in the future.

Thanks again to all that offered help.

Now for some pictures. I really wanted to document some of this because there isn't a lot on the net about repairing these things. Maybe this will help someone else.