Triac keeps "failing"

[JacquesNel]

Good evening all

I looked through the forum and couldn't find an answer to a problem I'm encountering. If I missed something, I apologize.

I am an electrician, but I work 240V-11kV, and my electronics experience and knowledge are only college-based.

I'm trying to repair a washing machine control board that has previous repair attempts, I dislike guys who insist on replacing the whole board instead of fixing the problem.

The solenoid for the cold water wasn't getting any voltage(required 220-240V) and I tested a bit here and there, from what I can find online the triac supplying the solenoid (4.2Kohm) was faulty, and I couldn't get a like for like replacement, so I replaced it with a BT136 triac, which worked once, the cycle started and I assumed that the problem was solved.

I closed the machine up and then heard the solenoid energizing and de-energizing rapidly without any cycle being started. I tested the voltage on the solenoid I'm getting 160-190v.
I figured the triac was a dud and replaced it again, the same thing happened. but I also noted a small ark on the pins supplying the one solenoid and the drain pump.

So I figure I'll order the correct part(https://www.mantech.co.za/Stock.aspx?Query=TRIAC+SOT223+600V+1A+5MA+Z3M+iGT=5mAand) and when it comes replace the BT138(https://www.mantech.co.za/ProductInfo.aspx?Item=35M0680) with it and see what happens, the same problem has occurred pulse the drain motor runs as soon as I start a cycle. so the machine will fill and empty itself at the same time.

No error codes from the machine either.

What I'm trying to figure out is what's causing the triacs to pop. I'm a bit lost here, any advice would be much appreciated. please let me know if you need more info

Below are some pictures below for more info.

Thanks in advance guys.

 

[barry]

There is probably a snubber (resistor and capacitor) across the triac, for suppression of high voltage spikes. If the snubber is defective you might be getting triac-killing spikes.

 

[JacquesNel]

Hi Barry, thanks for your advise. When I get back from work I'll see what I find and test it. I report back with more info.

 

[KlausST]

Hi,

generally semiconductors get killed mainly be these reasons:
* overvoltage, even very short spike in the microseconds
* overcurrent, even short term in the milliseconds
* overtemperature. Ambient temperature plus temperature rise due to power dissipation

***
Now you say "sarks". This means you get very high rise in current ... resulting in very high voltage spikes due to stray inductance.
--> at first you need to ensure that the spark never happens. Proper isolation, potting, molding...

Then you need to check for the snubber, as Barry says. Check it´s health.

***
Usually triacs are rather robust. Can withstand a lot of AC voltage (not spikes, though), can witdthstand a lot of RMS current (even rather high and short peaks).
Due to their "switch OFF close to zero current" they don´t suffer from high energy switch OFF spikes. But if there are other switches, contacts, sparks ... involved this may cause harm.

For sure ... if they get hot you need to use a heatsink.

***
I appreciate you´re looking for the root cause.

Klaus

 

[FvM]

If a low power triac fails after longer time of regular operation, the primary fault is most likely no the triac itself. Can be e.g. a (temporarily) shorted solenoid.

 

[JacquesNel]

Apologies for the silence. I was away for work and only had a chance to return to my watching machine yesterday.

There is probably a snubber (resistor and capacitor) across the triac, for suppression of high voltage spikes. If the snubber is defective you might be getting triac-killing spikes.

After removing them from the PCB, I tested every component I could with my multimeters, and they all tested okay, there was one Cap that had a good reading and then no readings, etc. so wasn't sure if it was me, the meter, or the cap so I replaced it in case.

the only resistors and caps I couldn't test were the SMD ones, I could be wrong, but is it safe to assume they ok as the control system seems to be working correctly?

Hi,

generally semiconductors get killed mainly be these reasons:
* overvoltage, even very short spike in the microseconds
* overcurrent, even short term in the milliseconds
* overtemperature. Ambient temperature plus temperature rise due to power dissipation

***
Now you say "sarks". This means you get very high rise in current ... resulting in very high voltage spikes due to stray inductance.
--> at first you need to ensure that the spark never happens. Proper isolation, potting, molding...

Then you need to check for the snubber, as Barry says. Check it´s health.

***
Usually triacs are rather robust. Can withstand a lot of AC voltage (not spikes, though), can witdthstand a lot of RMS current (even rather high and short peaks).
Due to their "switch OFF close to zero current" they don´t suffer from high energy switch OFF spikes. But if there are other switches, contacts, sparks ... involved this may cause harm.

For sure ... if they get hot you need to use a heatsink.

The sparks I was referring to were two of the pins between the back of the PC board, the two pins (220v supply) are for the water inlet solenoid and the drain pump. the spark does stop but not sure if the "arc" would damage the triac(ACS108) for the water inlet solenoid.
The Triac for the drain pump(AC120) does seem to be giving strange readings if I compare the diagrams of the ACS108 and ACS120. they should test identically, right? Could a faulty triac cause a high-voltage arc/surge?
Both are SMD components, so I can't put a heatsink on them. but they don't get hot/warm to the touch. There is only one relay on the board that seems to be working, it only switches when you turn the machine on.

If a low power triac fails after longer time of regular operation, the primary fault is most likely no the triac itself. Can be e.g. a (temporarily) shorted solenoid.

The original triac I replaced was an AC108, I replaced it initially with a BT136 which worked once, then let through a 170V all the time which it wasn't supposed to do as the machine wasn't "running" any cycle.


I thought it might have been because I used a BT136, I ordered the ACS108s and thought that would solve the problem. but it also stops working after the first use.

Thank you for the advice so far.

I'm going to replace both the ACS120 and ACS108 and cross my fingers.

If you guys have any more pointers please let me know. but I'll let you know if this solves the problem.

 

[JacquesNel]

If a low power triac fails after longer time of regular operation, the primary fault is most likely no the triac itself. Can be e.g. a (temporarily) shorted solenoid

I tested the solenoid, which gave me a constant 4.2Kohm. Is there another way to test one that might show me if something is wrong with it?

 

[barry]

I tested the solenoid, which gave me a constant 4.2Kohm. Is there another way to test one that might show me if something is wrong with it?

You mean, like apply a voltage to it, and see if it actuates?

 

[Easy peasy]

Sometimes if the triac fails it takes out the gate drive from the sending source - this can be very problematic

also if the wiring loom chafes and shorts it can takes out the triac - this is more common than one might think

lastly - if water gets into the solenoid - you can get random failures at higher voltages that do not show up for very low volt testing ( DVM )
- this is also often the case too !

 

[JacquesNel]

You mean, like apply a voltage to it, and see if it actuates?

Ok, maybe I misunderstood. Without the actual circuit, I'll apply 220V directly to it and see what happens. It does work when I replace the triac though, but only once.

 

[JacquesNel]

Sometimes if the triac fails it takes out the gate drive from the sending source - this can be very problematic

also if the wiring loom chafes and shorts it can takes out the triac - this is more common than one might think

lastly - if water gets into the solenoid - you can get random failures at higher voltages that do not show up for very low volt testing ( DVM )
- this is also often the case too !D

I have checked the look for any shorts or loss in insulation to the ground and between cables. but I could have missed something, so I'll double-check.

Can I ask what the gate drive is or what it looks like? please forgive my incompetence with electronics.

The solenoid is hard to check as it's a sealed unit. but I could do an insulation resistance test on it—testing between a terminal and the body.

 

[JacquesNel]

A video of what is happening I'm a bit lost now, so here's a video of what's going on. it's my G-Drive.

I also meant to say I did an insulation resistance test between the terminal nad body of the solenoid and "8.9Gohm...not Gigawatts lol.

 

[BradtheRad]

The sparks I was referring to were two of the pins between the back of the PC board, the two pins (220v supply) are for the water inlet solenoid and the drain pump

Arcing tells you that a thousand or more volts is traveling between wires and/or components. A chief cause is high-voltage kick when current is abruptly stopped through an inductor. A solenoid contains an inductor. The high-voltage kick can produce a severe spike (and even arcing) to neighboring circuitry.

I can't be certain the solenoid you're working on is faulty or whether another component is the cause. Once a thyristor turns on it's supposed to let current flow as long as it can until it drops below a tiny threshold. Ordinarily the triac by itself would not produce high voltage. Wherever the spark comes from I believe you should solve it.

 

[Easy peasy]

You don't need a lot of moisture laden dust, dirt or simply sudsy residue on a pcb ( or connector ) for it to track electricity and cause things to go awry

gate drive is the low level signal originating from the uP ( 5V or 3v3 ) that is buffered to drive the Triac - often through an opto-coupler - but may be via a simple transistor ( and resistors ) - if this drive circuit has been affected by the back feed of energy from the triac blow up - it will be a lot harder to remedy.

--- Updated Sep 28, 2024 ---

cleaning the whole pcb with methylated spirits and drying with a hair dryer - is a good start to any repair.