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I have a problem in trying to make a TRIAC switch circuit using an MOC3021 and a BT136 600E.

When I control the contactor without a snubber network, everything works.

When en I try to control the contactor and the snubber network is connected, the contactor turns on, but when I turn it off, the contactor keeps load. Any ideas?

circuit diagram MOC3021

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    \$\begingroup\$ Kind of weird schematics, please redraw it. What's the purpose of R4=330 ohm? Are you sure you connected it that way, in my opinion the schematics is wrong. \$\endgroup\$ Commented Sep 12 at 19:12
  • \$\begingroup\$ But AN 958, which does discuss snubbing but doesn't use a MOC30xx device, and AN 3003 and AN 3006 that do use the MOC30xx devices may help with your design process and testing. \$\endgroup\$ Commented Sep 12 at 19:55
  • \$\begingroup\$ I note the deletion of a prior comment to my above comment. Interesting. (I added the "but" as an addition to the prior comment.) \$\endgroup\$ Commented Sep 12 at 20:23

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But when I try to control the contactor and snubber network is connected, the contactor turn on, but when I turn it off, the contactor keeps load. Any ideas?

I suspect that your snubber on its own can supply a small amount of power to the load irrespective of whether the triac is connected or not. So, establish what you mean by "load" and determine what voltage the snubber forms with the "load" and, ask yourself if that is enough to activate the "load". I suspect it is.

It looks to me like the LC1D09P5 contactor coil only needs 7 VA at 50 Hz to hold the contacts close so, if this is your load then, quite possibly, the snubber will keep it powered.

You have the additional effect of the contactor coil inductance and snubber capacitor being in series and potentially drawing significant current when the triac is unpowered. This is due to series resonance so, you might consider using a lower value capacitor in your snubber to partially avoid this effect.

But, the problem only really occurs when the triac turns off and, this only happens when the contactor coil current is quite low in the AC cycle - this is how a triac works - so, you should benefit from this fact and use either no snubber at all or, a snubber with a higher impedance (lower capacitance). Try a 10 nF snubber.

Without knowing the contactor coil's inductance you are in the dark so, to design the snubber correctly you should contact the supplier/manufacturer.

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  • \$\begingroup\$ U have any idea to fix it? \$\endgroup\$ Commented Sep 13 at 10:39
  • \$\begingroup\$ @Matew find out what the coil impedance is (not specified properly in the data sheet I linked) by talking to the supplier. This might be my first port of call however, I would ask if you really need a snubber given that the triac will turn off when the contactor coil current is close to zero amps. This means the back-emf will not be excessive so, maybe a higher impedance value snubber might be better i.e. the 100 nF could reduce to 10 nF thus reducing the leakage current quite possibly below the contactor limit. \$\endgroup\$ Commented Sep 13 at 11:07
  • \$\begingroup\$ Additionally, if you don't know the inductance of the contactor, you cannot really design a snubber that uses a series capacitor. This is because the capacitor and contactor coil inductance forms a partially resonant circuit and, takes a current that can be quite a large fraction of the current taken when the contactor coil is directly connected to an AC supply. I would definitely try a smaller snubber capacitor in order to reduce this problem. \$\endgroup\$ Commented Sep 13 at 11:19
  • \$\begingroup\$ @Matew noting that you are a new contributor, if we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. \$\endgroup\$ Commented Sep 13 at 11:26

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