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I have seen and dealt with mostly DC relays until now I have a circuit that involves AC relays. In most of the DC relays we use a diode(like in the pic below) to protect the components like transistors, MOSFETs etc from the high voltage back e.m.f spikes.

My question is do the AC relays also need some kind of protection across its relay terminals to prevent arcing when its turned off? The relays I'm working on are 120VAC ones.DC Relay with Diode protection

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It's not unusual to switch one relay with the contacts of another with no "protection". You can add an RC snubber (typically something like 100 ohms in series with a few tenths of a microfarad) across the contact or across the coil to reduce EMI and sparking.

Such a snubber can also be useful across an inductive load. Or across the coil to prevent high dv/dt at turn-off from preventing commutation of a thyristor (if you are using an SSR, it will often be built into the SSR).

Sometimes MOVs (Metal Oxide Varistors) are used, but I would suggest not using them for such repetitive transients as they actually wear out over time and still allow rather high voltages and don't reduce dv/dt signficantly. If you must use them, take into account their eventual failure mode (usually shorted, followed by a certain amount of flames and smoke and then opening up, if they are across the power).

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    \$\begingroup\$ This is consistent with OMRON's application notes. Rahul would be wise to review MTBF reduction of inductive loads from 1 million mechanical ratings to 10k cycles without snubbers. \$\endgroup\$ – Sunnyskyguy EE75 Nov 10 '17 at 21:52
  • \$\begingroup\$ @TonyStewart.EEsince'75 Indeed. It's best to pick a relay that has a rating for the specific type of load (motor, for example) as well as the cos() power factor expected. \$\endgroup\$ – Spehro Pefhany Nov 10 '17 at 22:25
  • \$\begingroup\$ @SpehroPefhanyYes I have plenty of RC snubbers in my component collection when I checked.No, the relays that I'm using are the electro-magnetic ones, though it sounds SSR is better from what you said. I checked and saw Opto22 and OMEGA as Tony mentioned above. Seems to be of good quality (US made) but the price is like $20 more than electro-magnetic ones. \$\endgroup\$ – The_Vintage_Collector Nov 11 '17 at 9:39
  • \$\begingroup\$ Also,is there any way I could safely measure the initial back e.m.f and after using the snubber see how much of the back e.m.f has been suppressed? \$\endgroup\$ – The_Vintage_Collector Nov 11 '17 at 9:40
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You don't need one if you turn off the AC relay with a SCR or triac, because those turn off at current zero automatically. In that case, there isn't any voltage induced at turnoff.

If you turn off the AC relay by another contact, you place an RC network, a snubber, over that contact, so the current isn't turned off immediately, but slowly reduced to the much lower value of the RCL series connection. That way the induced voltage is low and will not arc over the opening contact.

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Instead of using a snubber you can also use a varistor. As long as the relay is energized the varistor has a very high resistance. If you switch of the relay the voltage created by the self-induction of the coil rises to a higher level than the supply voltage and the varistor becomes conductive.

The varistor must be selected by the supply voltage of your coil and the energy stored in your coil.

You can calculate the energy of your coil by: \$ E=\frac{1}{2}LI^2 \$

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  • \$\begingroup\$ But as mentioned above it seems to have a higher failure rate than using a snubber it seems. \$\endgroup\$ – The_Vintage_Collector Nov 11 '17 at 9:54
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    \$\begingroup\$ Have a look to this pdf file from Siemens: cache.industry.siemens.com/dl/files/985/24500985/att_2330/v1/… They tell you some advantages and disadvantages of the types of protection for switching overvoltages. As they do not mention the 'higher failure rate' I think this is an insignificant point. Also they mention that if you are using an RC element you have to carefully select the resistor and capacitor in compliance with the inductance of your coil. If you are using a varistor you just have to make sure that your varistor is like I've written in my answer. \$\endgroup\$ – Michael S. Nov 12 '17 at 12:05
  • \$\begingroup\$ Thanks Michael. I'll do some research and see which is better. As Spehro had mentioned above varistors are more prone to failure, it got me confused. \$\endgroup\$ – The_Vintage_Collector Nov 12 '17 at 15:47

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