Help with component identification

Thread Starter

LesB

Joined Oct 19, 2022
6
Hi. I'm a mechanical engineer (sorry lol) and need a bit of help.
I have a board that has a failed transistor of some sort. The control board is in a fridge freezer and the component is related to the auto defrost where a mains voltage heater element is switched. I have changed out the heater as I think it may have short circuited. The transistor, type unknown to me, has clearly popped.

I'd like to try replacing it as the board is not available and the freezer being only 4 years old is working, albeit I have to manually defrost it twice a week.

Can anyone identify the part? Its made by STMicroelectronics but I can't match it. Or having done so, even an equivilent would do.

Any help would be much appreciated.2022-10-15 11.57.01.jpgFreezer 1.png
 
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Thread Starter

LesB

Joined Oct 19, 2022
6
I can't believe this; Despite taking it apart a week ago I just managed to find the missing part in the dust and bits under the freezer! I can't help thinking it may help. I've laid the photos alongside each other.

Anyone able to assist with a part ident so I can source a replacement?
 

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Thread Starter

LesB

Joined Oct 19, 2022
6
Thanks guys. Now I have to try getting one. Not so easy as like hen's teeth; Digikey and Farnell either don't have them or the min qty is mad. Or it comes from the states and I have to buy a part of an airplane to get it here. It is possible to get from an appliance spares supplier though but not exactly cheap!
 

Thread Starter

LesB

Joined Oct 19, 2022
6
Hi. I had thought that but as a mechanical guy myself confidence in finding a safe alternative is practically zero lol. To me it would look the same but detail criteria is black magic to me. In any case, I think I can get hold of one for around £15 including shipping. Seems a lot considering the part itself normally costs around a pound but that's what you get by buying from some appliance spares places.
 

MrChips

Joined Oct 2, 2009
27,679
Let's look at it pragmatically.
It is a triac whose function is to act as an AC switch. What can go wrong if you used a substitute?
1) It might not work at all.
2) It might be underated and blow up, just like what already happened.

There may be some critical component parameters that must be satisfied. Let's look at some of them.
Max operating voltage
Max operating current
Gate turn on voltage
Gate turn on current
On resistance
Switching characteristics
Package
Pinout

The beauty here is that you have a reference point to work with, the original ACST610-8T data sheet:
https://media.digikey.com/pdf/Data Sheets/ST Microelectronics PDFS/ACST6.pdf

We know the following:

Max operating voltage 800V
Max operating current 6A
Gate turn on voltage 0.9V
Gate turn on current 10mA
On resistance 80mΩ
Switching characteristics
Package TO-220AB
Pinout COM OUT GATE

https://uk.farnell.com/stmicroelectronics/btb08-800twrg/triac-800v-8a-to-220ab/dp/3366967

In stock £1.61 each. Now you still have to add shipping and VAT.
 

Thread Starter

LesB

Joined Oct 19, 2022
6
Hi. Good point of course. When the issue first occurred I checked the condenser defrost heater and found it to have the same resistance as a new one so I assumed ok. At the same time I checked the thermal fuse and found it had gone open circuit. I bought a new heater/fuse assembly but only fitted the fuse as the heater is a much larger task. I will be checking the fuse again to make sure it's still ok but see no reason why not as the heater has not been working so hasn't got hot. Wiring from the board at the top of the freezer goes directly to the heater element so I'll stick a meter on that and double check but again can't see why that would be an issue. I can't see water damage or anything. They do seem to sell a lot of heater assemblies so hopefully the issue is with the heater element and I will now be fitting the new one I already have.

The whole excercise may be doomed to failure but it's a good fridge (when it works ok) and was quite expensive. It's only 3 years or so old. I can but try.

More important though is the support and information received here. Many thanks.
 

MrChips

Joined Oct 2, 2009
27,679
Besides resistance measurements, conduct visual checks of the wiring harness and heater element. Look for any signs of corrosion, exposed wiring, and any possible contact with the chassis.

A bad thermal fuse would cause the heater to stop working, not blow the triac. Perhaps it would if there were a short to chassis depending on where in the circuit the fuse is located.
 
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