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At board power-on, the fan output connector has mains (230V AC) voltage without the fan connected, that drops to ~2V AC with the fan connected.

At power-on, on the attached display, the board shows a message error code that means "insufficient fan speed". From this, I deduce that the fan is supposed to be starting immediately but it doesn't.

As far as I can understand it, the output is controlled by an ATMEGA micro-controller, through an ULN2003 Darlington array, through an MOC3063 opto-coupled, zero-cross TRIAC driver and finally a BT136 TRIAC. It includes what I believe is a snubber circuit comprising 3 resistors and a capacitor and also a motor (start or run) capacitor.

I have determined the motor (and its external capacitor) to work by connecting it directly to mains. It rotates vigorously.

The motor also has cables (that go to another part of the control board) for an internal speed sensing mechanism that takes +5VDC (and GND) and outputs ~4.3VDC at certain rotor angles that I have also determined to function correctly.

I have changed the TRIAC and the TRIAC driver on the board with new parts to exactly no change in behavior.

What information should I add to this question?

What further steps can I take to try and fix this issue?

the control board

the control board

optocoupler input

optocoupler input

enter image description here

Darlington input (green) and output (blue)

EDIT: I've shorted terminals 1&2 of the TRIAC and checked that the motor spins OK. Given the third picture, showing scoped input/output of the Darlington, can it be concluded that it is the cause of my problem, seeing how its output seems insufficient to trigger the optocoupler?

enter image description here

enter image description here Darlington input (blue) and output (green), referenced to the common cathode.

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  • \$\begingroup\$ Can you see the activation voltage of 1-1.5 V on the LED side of the coupler? \$\endgroup\$
    – Jens
    Jan 6 at 14:29
  • \$\begingroup\$ @jens The measurements I can make on the LED side of the coupler with a voltmeter appear to vary between 0.2VDC and 0.6VDC. I'll try to hook the scope and see if it is pulsed. \$\endgroup\$
    – gdee
    Jan 6 at 15:03
  • \$\begingroup\$ I have added a board image and the scope view of the optocoupler input \$\endgroup\$
    – gdee
    Jan 6 at 16:02
  • \$\begingroup\$ Check the resistor between ULN and the coupler. A failure there would explain both screen shots. If the ULN has a load at the output, the signal looks inverted to the input. \$\endgroup\$
    – Jens
    Jan 6 at 18:00
  • \$\begingroup\$ It has "361" written on it and I measure it at 360Ω. I suspect the measurements in the first scope screenshot to have been badly influenced by the measurement procedure. Now that I've soldered "tails" on the measurement point, I can't distinguish the scope plot of the Darlinkton output from the optocoupler input (across that resistor). \$\endgroup\$
    – gdee
    Jan 6 at 18:11

1 Answer 1

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There's no convincing drive signal there- it should be about 1.2V when on (-1.2V the way you have it connected).

I hope you are being careful with grounding, it would be pretty easy to seriously damage something if your scope has an earth connection, and pretty easy to get a shock if it does not.

Furnaces have a sequence they go through and if something fails they may retry a few times then give up. If it's a gas furnace there may be multiple fans. I would look first at those row of relays- relays wear out. Also at the power supply electrolytic capacitors- they also wear out/dry out.

The focus is poor and I can't read any date codes, but I would guess it's quite old, so such problems are to be expected.

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  • \$\begingroup\$ Thank you for the warnings. I'm being careful. I know about the sequence. I'm doing my measurements inside the "trying" part. The relays are not, AFAIKS, in the control/supply path of the fan motor. Not sure about the cap. \$\endgroup\$
    – gdee
    Jan 6 at 17:56
  • \$\begingroup\$ Look for a largish aluminum capacitor, probably near the power transformer. \$\endgroup\$ Jan 6 at 18:00
  • \$\begingroup\$ I saw at what capacitor you were referring, I've meant that I'm not sure about it being in said path. Could it influence the voltage level at the Darlington's ouputs? \$\endgroup\$
    – gdee
    Jan 6 at 18:13
  • \$\begingroup\$ Yes, indirectly. A bad power supply can cause systemic problems that manifest in all manner of ways. \$\endgroup\$ Jan 6 at 18:16
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    \$\begingroup\$ I've changed all caps and the motor starting working. Thank you. \$\endgroup\$
    – gdee
    Jan 8 at 19:10

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