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I have an apparently bizarre situation with a production pcb out of a Sprada coffee machine I am repairing. A BT136-600E TO-220 triac (NXP brand) is mounted the wrong way around on the pcb - pin1 (MT1)is swapped with pin3 (gate) ie. the triac has been rotated thru 180 degrees. Pin2 (MT2) is obviously as it should be. I deduce this because the datasheet show the pins as above but pin3 is directly connected to mains neutral on the pcb and pin1 is connected to a processor pin via a 270 ohm resistor. Pin2 is connected to the load, the load's other side to mains live.

The bizarre part is that the pcb silkscreen shows that the triac should be mounted this way, and this board has been manufactured and presumably tested this way. A second board I looked at is exactly the same so pcbs of this version and revision were presumably manufactured in bulk. The machine model is circa 2015 so the pcbs are not new. There is no evidence that the soldering has been disturbed ie. the triac has not been removed since manufacture. Since the triac drives an AC water pump there is no way that this could have been missed if it didnt work from day one. The fault on this board is that the triac has shorted pin1 to pin3. The water pump motor is rated at about 60W @ 230Vac (261mA) and the BT136 is a sensitive gate device which triggers at approximately 5mA.

So my only conclusions are that

  • a triac can operate when reversed, gate <-> MT1
  • there are versions of the BT136 triac that have pins 1 and 3 reversed
  • I've missed something obvious

I have not found any NXP/Philips/WeEn datasheets that point to the second possibility, so at the risk of making a clot of myself (3rd point) and going with the first one, my question is whether this is possible. How can the gate be triggered to conduct to MT2, and if so how much load current can it handle.

For this issue, I have done a bunch of research on the operation of triacs without finding a possible clue how triacs could operate in this configuration - perhaps I dont fully grasp the implications of gate vs MT1/MT2 currents and on/off states in the various quadrants. I have also used triacs in projects myself, but in a "conventional" configuration using MOCxxxx drivers.

If anyone could shed light on what could be happening I would be very grateful.

Edit: here are some pics Triac BT136 Topside

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    \$\begingroup\$ Some high quality pictures would help in ruling out the "missing something obvious" option. \$\endgroup\$ Jul 19, 2021 at 20:50
  • \$\begingroup\$ are there any active components between the gate and terminal? \$\endgroup\$
    – dandavis
    Jul 20, 2021 at 0:32
  • \$\begingroup\$ I have added 2 pics, there are no active components connected to the triac pins. One 270ohm resistor (R62) from triac pin1 to the CPU and two series 470k resistors from pin2 to the CPU. One of these is R60 in the close up photo. I presume R62 is the gate drive resistor and R60 + ?? are sense resistors for the cpu to detect voltage on triac pin2 when the triac is conducting. No bottom-side pics unfortunately but there is a fat trace from triac pin3 to X6 connector with blue wire to the right and behind the triac. This is the line neutral wire. \$\endgroup\$ Jul 20, 2021 at 15:30

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I think it's possible the triac would work, sort of, with MT1 and gate swapped.

All the load current would go through the gate, which would be hard on the triac, but if the load current is relatively small (like a couple hundred mA) it could work, and thanks to the series gate resistor the driving MCU could survive.

A short from gate to MT1 is what one might expect from gate abuse.. so..

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    \$\begingroup\$ Thanks @ Spehro, accepted as an answer. Just amazing that this has found its way through the design, testing and production phases of a relatively modern high-end consumer product. I still suspect there may have been some design intent, probably misguided tho. \$\endgroup\$ Jul 20, 2021 at 17:43

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