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I need to switch a 220VAC/40A resistive load. I only need on/off switching, no phase control. I would like to avoid mechanical (moving parts) components if at all possible because of reliability issues.

Digi-Key doesn't stock any TRIACs capable of switching 40A. I've thought about paralleling 2 or more TRIACs rated at a lower current, but if I understand this correctly, it may not be a wise thing to do.

I understand that two of these SCRs connected in anti-parallel should do the job. Although they are only capable of 40 A RMS, which appears to imply a safety margin is not included, that's not the case: according to this application note, when two SCRs are connected in anti-parallel they can handle \$\sqrt{2}\$ times as many current as each individual SCR.

I'm looking for either:

  • a lower cost solution, or
  • a solution of similar cost, but which dissipates less power (I estimate the SCRs would dissipate on the neighborhood of 30 W each, thus 60 W combined).

Any suggestions?

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swineone
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  • FOTEK do some cheap 40A SSRs - about $2 on eBay. SSR-40DA. Or, spend $8 and get a whopping 75A one - SSR-75DA – Majenko Jul 03 '14 at 21:42

1 Answers1

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You can buy a "50A" zero-crossing SSR for about $50 and a similar "75A" SSR for $77 in singles, plus maybe another $30-40 for a suitable heat sink. Isolation built in, so much nicer to work with.

Don't assume reliability will be better with semiconductors, however lifetime may be better if the number of switching operations is large. Semiconductors can fail at any time due to surges (and will typically fail on) relays wear out and tend to fail off. Thermal cycling due to self-heating can also shorten the life of power semiconductors.

Edit: There's also the non-UL-approved Chinese specials on eBay for much less if you're into that sort of thing.

Spehro Pefhany
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    +1 for "if you're into that sort of thing" – Phil Frost Jul 03 '14 at 22:24
  • Unfortunately that exceeds the budget of this circuit. The SCRs I indicated cost only $2.28 each in small quantities according to Digi-Key, plus $0.74 for a MOC3063 and a few cents for resistors. – swineone Jul 03 '14 at 23:01
  • The parts you chose are not bad- and (in theory) good for the current with sufficient heat sinks, and if one fails on you'll only have 50% power. However, normal fuses can't protect them. You can't have too high a rating or too much heatsink. Paralleling triacs is not generally a good idea. – Spehro Pefhany Jul 04 '14 at 00:10
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    It's hard to believe that you involved with a device that uses over 8 kilowatts but can't afford the cost of an SSR to control it. By the way, the SSR includes isolation, drive circuitry,doesn't require other components, and is a tested proven design. Designing, breadboarding, and testing a similar device on your own with a number of components will easily exceed the cost of the SSR. You should re-think your budget. – Barry Jul 04 '14 at 00:14
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    @Barry Yes, and the cost of field failures can be crippling- cost and reputation. – Spehro Pefhany Jul 04 '14 at 00:25