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I've bought an off-the-shelf optotraic/triac board for a little one-off project. It needs to drive a 0.42W syncronous motor from mains, once a minute for a few hundred milliseconds.

I have traced out the schematic and it's as follows:

enter image description here

I'm in the UK, so mains is 230V. I have mains connected to one side of the motor and the other side is connected to the LOAD terminal on the circuit above. (As per the instructions with the triac board.)

When I switch the GPIO low it turns the motor on which drives what it needs to perfectly well.

However, once the opto triac is turned off, I sometimes, but not always, get the motor gently humming away. I always have about 7Vac across the motor when the triac is turned off.

The board was very odd on the input side and I've modded it to make it work and so the input is galvanically isolated form the HV side, so it's possible there something equally as odd with the HV side. It does look slightly different to other example circuits I've seen both here and on Google searches/opto traic datasheets.

So, I have the following questions:

  1. Should I be getting 0V across the motor when the opto triac is off?
  2. If yes, would a gate resistor be a good idea and if so, should I connect that to neutral?
  3. Is it likely that the 7V on the motor is causing that hum? If not, is it possible to say what might be causing it?
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    \$\begingroup\$ 0.42 W is seriosly low for a TRIAC. What's the holding current of yours? Your 1. is almost impossible without a relay. \$\endgroup\$
    – winny
    Oct 27, 2020 at 14:02
  • \$\begingroup\$ @winny sorry, I missed your reply. Holding current for the triac is 10mA max. I think transistor is right in that the issue is leakage through the snubber. I need to find a way of reducing that so the motor doesn't hum. \$\endgroup\$
    – DiBosco
    Oct 27, 2020 at 19:35
  • \$\begingroup\$ Seems like you have several problems at once with this setup. But low current SSRs are available, as well as regular relays and hybrid relays. Have you considered one of those? \$\endgroup\$
    – winny
    Oct 27, 2020 at 20:11
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    \$\begingroup\$ Well, I hadn't, but will see if there's anything suitable. Will I not have the same sort of snubber issue with an SSR? That's seemingly the big issue here, the snubber making the motor hum when switched off. \$\endgroup\$
    – DiBosco
    Oct 28, 2020 at 9:37
  • \$\begingroup\$ If it's MOSFET based, should be no problem, but read the datasheet since the leakage current chould be specified. \$\endgroup\$
    – winny
    Oct 28, 2020 at 10:33

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I think your reverse engineering is in error. C101 and R103 would be a snubber directly across MT1 and MT2 and usually shown to the right of the triac. R102 would usually be connected directly to MT1.

enter image description here

Figure 1. A typical opto-isolated circuit. Image unattributed on Triac Switching circuit with Optocoupler

Should I be getting 0V across the motor when the opto triac is off?

With such a small load the snubber will pass enough current to give that voltage across the load.

If yes, would a gate resistor be a good idea and if so, should I connect that to neutral?

I don't think it will make any difference.

Is it likely that the 7V on the motor is causing that hum? If not, is it possible to say what might be causing it?

The leakage current is likely to be the cause of the hum and otherwise shouldn't cause any problems.

Links

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  • \$\begingroup\$ Many thanks for the reply. I have been back and doubled checked my reverse engineering and it looks right (ie the schematic I posted is correct). Looking at the LV side which was an absolute dog's dinner, it does not surprise me that they have got the traic end wrong. The snubber supplying that low voltage makes sense. Do you think I should mod the circuit to reflect yours? (Maybe without the gate resistor if you think that's not necessary.) \$\endgroup\$
    – DiBosco
    Oct 27, 2020 at 16:23
  • \$\begingroup\$ I've added some links. The Littlefuse reference is a good place to start. Just a thought, but your motor current is so small that you might be able to drive it directly from the opto-triac. Check the specs. \$\endgroup\$
    – Transistor
    Oct 27, 2020 at 17:29
  • \$\begingroup\$ I'm clearly no expert on optotriacs, but the datasheet for these MOC devices states: This optoisolator should not be used to drive a load directly, it is intended to be a trigger device only. I have ploughed through a lot of info on triacs and had I designed a PCB rather than taken an off-the-shelf board I'd've done it with what's in the MOC datasheet. I foolishly assumed this board would have been done in that way! \$\endgroup\$
    – DiBosco
    Oct 27, 2020 at 17:53
  • \$\begingroup\$ That settles that idea then. I hadn't checked. \$\endgroup\$
    – Transistor
    Oct 27, 2020 at 18:24

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