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I am currently designing a safety-related device that has to open a relay when a certain event occurs. During most of its life, this relay will be closed, and only open if the safety event occurs, power goes out (the device will be installed in the US so unlikely) or the device needs servicing.

I am designing the "2.0" version and some of the circuits I am using have been taken from the previous version of the product.

One of those circuits is a MOC3083 zero crossing opto-isolated TRIAC drivers which drive a Z0103MN TRIAC in order to provide 110VAC to the relay coil, therefore turning this open/close.

The previously mentioned circuit is as follows:

enter image description here

Where R and Rg are both 330ohm.

From what I have read in the AN-3004 app note from On-Semi some sort of protection or damping is required in parallel with the TRIAC in the form of an RC snubber or MOV to avoid voltage spikes which can potentially cause the relay to close back or even destroy the TRIAC.

However, the previous design has already passed certifications and being sold in thousands without any RC snubber or MOV included.

Therefore my dilemma is the following: should I add an overvoltage protection element or shall I just follow the "if it's not broken don't fix it principle"?

In case I do have to place a protection device, will the MOV do the job given the fact that the relay will be most of the time closed? How do I calculate the energy dissipated on the MOV?

Consider that the relay coil has an inductance of 80H and it stays energized for most of its useful life.

Thanks!

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    \$\begingroup\$ Any reason not to use a 5 or 12VDC relay you can directly control ? You need to consider, it will probably be easier to pass certification if you can demonstrate the previous - same - design already has passed certification and might save you time and money. \$\endgroup\$ – Damien Oct 8 '18 at 3:36
  • \$\begingroup\$ The power supply that powers the electronics does not have enough juice to power the coil of such a relay. Changing the PSU to one that can source more amps will result in higher costs overall. Yes the previous design has already passed certifications, but I am still not convinced if that was correctly designed and I need to add an extra component \$\endgroup\$ – RWeiser Oct 8 '18 at 3:43
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    \$\begingroup\$ You have relay that needs only about 3mA such as this one: V23026F1052B201 \$\endgroup\$ – Damien Oct 8 '18 at 4:20
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    \$\begingroup\$ Have to agree with the lower voltage option... \$\endgroup\$ – Solar Mike Oct 8 '18 at 5:03
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    \$\begingroup\$ If for any reason the triac gets damaged, then it will most probably stay in conduction mode. Triacs are not suited to be used as safety switch. Don't know what they were looking at, but they may change the opinion when revisiting. \$\endgroup\$ – Marko Buršič Oct 8 '18 at 6:28
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If you put a 20mm to 40mm from MT1 to MT2 that protects your triac and driver IC. Another good target is the load itself, basically with the same protection.

MOV's have a soft low voltage clamp and a hard high voltage clamp. For 110VAC to 125 VAC you would use MOV's with a 150 volt rating stamped on them. I like them because they have no effect on power, just voltage spikes due to current surges due to switching off inductive loads. An MOV works best if placed at the point where the surge originates.

For 220VAC to 240VAC use MOV's marked 320 volts. For 277VAC use 420 volts MOV's. With a surge being clamped at the source, very little of it will make it to your Triac Driver.

Sidacs have a very tight clamp voltage but are very expensive.

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    \$\begingroup\$ What do you mean with 20mm to 40mm? \$\endgroup\$ – RWeiser Oct 8 '18 at 4:26
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    \$\begingroup\$ Those are diameter values. Sorry if I did not clarify that. I gave you an option on size because you know what level of surge may be generated. The 40mm MOV's are designed to take 15 hits at 20KA and still survive. \$\endgroup\$ – Sparky256 Oct 8 '18 at 4:30

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