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We are doing home automation project. In that we are using triac for ac switching and snubber circuit (R=100ohm, c=10nf) for its protection. Here we have inductive and resistive load, for inductive load there is no problem but in resistive load when we connect less than 5W means it is glowing continuously without switching on. Suppose we remove the snubber for resistive load it's not glowing. so how to reduce that small leakage via snubber. Please suggest me your solution.

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  • \$\begingroup\$ Try using a flyback diode \$\endgroup\$ – David Norman Nov 6 '13 at 6:02
  • \$\begingroup\$ What do you mean by 'blowing'? \$\endgroup\$ – user36129 Nov 6 '13 at 6:14
  • \$\begingroup\$ sorry sir its glowing \$\endgroup\$ – kalis Nov 6 '13 at 6:32
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    \$\begingroup\$ @DavidNorman Sir, But We are using AC supply how its possible \$\endgroup\$ – kalis Nov 6 '13 at 6:45
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    \$\begingroup\$ BTW ideally a snubber would be across the inductive load, not across the switch. You want the loop (inductor-snubber) as small as possible. That would also solve the leakage problem when switched off, but it would leak when switched on. Disadvantage would be that you have to modify the load. \$\endgroup\$ – jippie Nov 6 '13 at 7:09
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Simply set smaller enough capacitance in the snubber or remove it at all.

Notice that the internal capacitance of the triac, together with the high resistance of the load will act as a snubber by itself, preventing high dV/dt on the triac.

The snubber actually does not "protects" the triac, but prevents it by incidentally switched ON because of high dV/dt - mainly when you switch the power ON or on high surges in the power network.

But such wrong switching is a big problem only in a DC powered thyristor circuits. Working in AC, even incidentally opened, the triac will close by itself on the next half period of the power voltage - i.e. after max 10ms. I don't think it is a big problem for the most loads. At least not for resistive, low power loads.

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  • \$\begingroup\$ Apart from the high dV/dt issue across the triac, there is the EMI issue that may interfere on other devices. For example an unsnubbered triac based dimmer circuit can be heard on a radio set, or an unsnubbered switched motor can interfere the image on your monitor for a brief moment. \$\endgroup\$ – jippie Nov 6 '13 at 7:31
  • \$\begingroup\$ The EMI have to be suppressed by other methods IMHO. And there is no universal method of "EMI suppression" or "universal triac snubber". But for lower load currents, the EMI will be also lower. \$\endgroup\$ – johnfound Nov 6 '13 at 7:38

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