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I am not an EE, so please be gentle! I will obviously get a certified EE to review my design before anyone else touches this so please assume that I know enough to at least be safe with 120VAC and am comfortable with frying my own electronics :).

What I'm doing

I'm using one of these ubiquitous Sainsmart 5V relay modules with an onboard SRD-05VDC-SL-C relay.

You can follow that SRD-05VDC link to a datasheet about the relay module itself. While it's rated for 10A @ 125VAC for resistive loads, it's only rated for 3A @ 120VAC for inductive loads.

Since my application is for controlling a standard (US) AC outlet, I don't know what people might plug in (blenders, vacuums, leaf blowers, etc).

My question

My question is about whether it would provide any additional inductive protection if I placed a TVS diode across the relay terminals? If so, what would be an appropriate diode to use for general household 120VAC applications?

If my question is dumb

If that's a bad idea in general, what would by an appropriate relay module to switch a general purpose 120VAC outlet using a 3-5V input signal?

Thanks!

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  • \$\begingroup\$ I'm on my phone, so I'm not going to type a nice answer :) Look up "snubber circuits". An RC circuit across the contacts is common practice. An MOV is another option. \$\endgroup\$ – bitsmack Nov 22 '14 at 2:29
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I think their concern is that with the inductive load, when you open the relay it will force some current across the open contacts and spark, damaging the contacts. A TVS diode should protect you, and you'd need to choose values above the mains voltage (peak voltage, not rms). In this case you'd want >170V. This wouldn't protect it 100%. I think you'd still have some current forced across the gap a tiny fraction of a second after opening it (before the voltage across the relay surpasses 170). So I can't say how much above 3A you'd be gaining.

The other option would be a bigger relay of course.

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  • \$\begingroup\$ Thanks Dave! I looked up snubber circuits from @bitsmack's comment and it seems like that might be a better idea in general for what I'm trying to do. Any idea? \$\endgroup\$ – DougW Nov 22 '14 at 23:06
  • \$\begingroup\$ Yeah that would work. You'll need to tune the RC properly but you'll find many good explanations online. \$\endgroup\$ – Dave Nov 25 '14 at 17:07

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