I am planning on building my own smart home socket and I want to mainly control LED lamps. But of course I can't make sure that someone is not plugging in let's say a toaster into the socket. (Yes I know that I could warn everyone since it is only for private use but this seems to be not the right approach).

Of course I know that dimming capabilities using phase cutting is not suitable for most LED bulbs but it suffices to just turn them on and off at this point.

Now my problem is that by using a snubber circuit I can protect the Triac from voltage spikes caused by the cut-off of an inductive load but this is also mostly a problem when switching LED Lamps since the capacitor charge is often enough to let the lamp flicker once in a while (depending on the lamp circuit).

Therefore I would like to know if it possible to use a bidirectional TVS Diode instead of a snubber and if it still offers enough protection.

Here are some of my circuit parameters:

Voltage (Load) 230 VAC Triac BT136 600E MOC 3021 TVS Diode 1,5KE250CA

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  • \$\begingroup\$ "I can protect the Triac from voltage spikes caused by the cut-off of an inductive load " , that's not true, since the triac is ZCS. The current is zero, so it can't kick back. \$\endgroup\$ Mar 29, 2021 at 10:05
  • \$\begingroup\$ Snubber circuit not for overvoltage protection. It is for compensating phase shift of current in case of inductive load. Triac will turn of only then current less then minimal. And triac is tolerable to higher voltage, it is only turn on then voltage is over some value. \$\endgroup\$
    – user263983
    Mar 29, 2021 at 11:13
  • 1
    \$\begingroup\$ Hi thanks for pointing this out. However according to this app. note from ST the main function of the snubber is to limit dV/dt in order for the gate capacitance not to fire the triac again. But I could not find any information regarding a phase shift compensation effect. \$\endgroup\$
    – B. Ueno
    Mar 29, 2021 at 12:29
  • \$\begingroup\$ it's fine for resistive load but doesn't protect against inductive turnon \$\endgroup\$ Mar 29, 2021 at 13:01

1 Answer 1


TVS Diode 1,5KE250CA

The TVS Diode type 1,5KE250CA has a breakdown voltage of between 237 volts and 263 volts. This isn't an RMS rating; it's a peak voltage rating - take note.

Voltage (Load) 230 VAC

220 volts (diagram) or maybe you mean 230 volts as per the above statement has a peak voltage of \$\sqrt2\times 230\$ = 325 volts and will instantly fry the TVS you chose. The TVS will clamp somewhere between a peak voltage of 263 volts and 344 volts and you cannot determine where this will happen so, no that TVS isn't suitable.

And, unfortunately, none of the TVS diodes in the data sheet will really be any good for you because most countries will have specifications on indirect lightning surge protection for households of around the 1500 peak volt limit so, it's unlikely that any TVS is going to be able to be used at all effectively.

Therefore I would like to know if it possible to use a bidirectional TVS Diode instead of a snubber and if it still offers enough protection.

No, it won't be at all suitable - stick with snubbers - they don't hard clamp and therefore they won't try to take a massive current peak should indirect lightning surges affect your neighbourhood.

  • \$\begingroup\$ Thanks for this clarification, you are absolutely right and I should've noticed that it is a peak voltage. However for my project I am modifying a socket strip that has already a lightning surge protection inbuild. This should make the use of a 400v TVS diode possible, right? On the other side, when using a snubber how do I make this work with LED bulbs since the Snubber capacitance will suffice in turning them on continously or from time to time (dependent on the circuitry). \$\endgroup\$
    – B. Ueno
    Mar 29, 2021 at 12:35
  • \$\begingroup\$ More than likely, if you looked into the details, the lightning surge suppressor will limit any "spike" to around 1500 volts peak. Nobody, uses TVS diodes on mains AC lines (a bold statement of course). \$\endgroup\$
    – Andy aka
    Mar 29, 2021 at 12:38
  • \$\begingroup\$ But is there no known use case where someone used a triac with snubber to switch an led lamp and an inductive load? \$\endgroup\$
    – B. Ueno
    Mar 29, 2021 at 13:14
  • \$\begingroup\$ Are you saying that your load is an LED lamp in parallel with and inductive load? Anyway, there is no use case for TVS diodes to protect a load or triac in these circumstances. The TVS diodes will fry. \$\endgroup\$
    – Andy aka
    Mar 29, 2021 at 14:54
  • \$\begingroup\$ Hi, yes I understand the TVS diode problem now ;) Not in parallel but one or the other. The problem with the snubber is that the charge of the capacitor will suffice to turn on the LED Lamp. \$\endgroup\$
    – B. Ueno
    Mar 30, 2021 at 5:56

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