Help determining condition of this triac

[fastline]

https://www.mouser.com/catalog/specsheets/Q5015L6.pdf

I have the fastpak style of this device in the front end of an HD battery charger. Appears 120VAC comes into it and is further fed to a PCB. I am showing a short between G and MT1 but nothing to any other connections. I am honestly not real up on understanding triacs and probably need to understand what this is doing in the circuit. I am hoping if it is shorted it did not damage the PCB with it.


I appears the output to the gate from a microcontroller has a series resistor that is good, then goes through an optocoupler before going to the gate. Is it a fair bet that the optocoupler is damaged as well?

 

[KeepItSimpleStupid]

I do believe you should see a diode drop between the gate and MT1 using the diode test of a multimeter.

See: https://en.wikipedia.org/wiki/TRIAC

They are quite difficult to test because of max and minimum gate current and the quadrants of operation and the minimum holding current. It usually requires two power supplies to test. One to supply the minimum gate current and another to supply a minimum load.

Fortunately, they usually fail shorted.

To give you an idea of how a triac works, you should first understand an SCR. In an SCR, the currents can only go one way. They turn on with a momentary or continuous pulse of current on the gate and they will stay turned on )until the voltage across the device goes to zero or the device is short circuited) with the gate drive removed.

Triacs are bi-directional and have all sorts of minimums/maximums that have to be satisfied. Triggering is usually specified by quadrants. The same x-y graphs you might be used to in algebra. The gate can be positive or negative relative to MT1 being positive or negative and the triggering thresholds usually vary.

 

[fastline]

It sounds like for sure the triac is toast. I seem to remember that being a common failure mode, shorting the gate.

Any thoughts on other component failures when this occurs? I am debating if I should pretty much consider the optocoupler toast a well? The series resistor is fine to the uC and the series resistor that sends power to the gate. I am not sure if this short would guarantee fail the coupler or not? I would think something would have been hurt when this occurred?

 

[KeepItSimpleStupid]

I would think the optocoupler and/or the series resistor is toast as well.

 

[fastline]

It appears that the source for the gate power comes from T2 on the the triac. Someone will have to better explain how this works but possibly like the SCR in which the uC will send power to the gate which will latch up the triac to conduct from T1 to T2 until it is at zero potential, then drop out? I am a little confused here why power out of T2 loops back around in the PCB and goes to the gate?

In any case, since it seems that T1 is shorted to the gate, and the opto seems to send power from T2 to the gate, that possibly everything is at the same potential so no issue for the coupler?

https://www.fairchildsemi.com/ds/MO/MOC3041M.pdf

This is the opto I have. T2 is driven by the uC. T4 goes to the gate of the triac, and T6 is sourced from T2 on the triac. They clearly show operating a triac with this opto but I guess I am too uneducated to grasp the concept yet.

 

[KeepItSimpleStupid]

Look at figure 12 of the datasheet. The 180 ohm resistor limits the gate current. The 1K resistor is a place for leakage current to go. The RC combination is a "snubber" and limits dv/dt. Triacs can turn on if the change in voltage vs time is higher than a specific value. The little triac in the opto turns on the big triac with the added feature of turning on the big triac at zero voltage. Turning a triac on at zero voltage reduces RFI.

Note that in fig 12, there is a LOAD between hot and neutral.

 

[fastline]

I was reading up on them a bit and it seems the design I have (alternistor) is specific in that it will only operate in 1-3 quadrants so it has improved turn off ability. I guess false triggering is a common issue for these.

I will have to do more reading.

However, I believe I have a Q4025P, rated at 400V. I was going to just replace with a higher voltage rating like I might do with a diode or mosfet but in my reading, it almost seems that a higher voltage could cause a change in turn on/off characteristics and may need retuned for the circuit?

 

[KeepItSimpleStupid]

I think you would be better off putting a ZNR across the AC line. If it has a grounded plug, then you need 3.
Two suitable for 120 VAC and the other for 5V (neutral to ground) assuming your in the US.