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:
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:
- Should I be getting 0V across the motor when the opto triac is off?
- If yes, would a gate resistor be a good idea and if so, should I connect that to neutral?
- 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?
Best Answer
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.
Figure 1. A typical opto-isolated circuit. Image unattributed on Triac Switching circuit with Optocoupler
With such a small load the snubber will pass enough current to give that voltage across the load.
I don't think it will make any difference.
The leakage current is likely to be the cause of the hum and otherwise shouldn't cause any problems.
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