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I have a device that needs to use a controllable source of hot air. I want to use a hairdryer as they're available everywhere and very cheap too.

I have a board with these relays on it (and I'd be happy to be able to use it :))

enter image description here

I am in the EU now and the hairdryer would be 220v / 1500w so the specs below should work.

but the main question is:

Should I use a mechanical or solid state relay for this kind of load?

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    \$\begingroup\$ Thats like the following question: Should I use bike or car to get at work? Without further requirements, both should work ok. \$\endgroup\$ – Stefan Wyss Jun 22 '18 at 12:55
  • \$\begingroup\$ The only extra data is that the relay will be closed for roughly 10 seconds and open for 3-4minutes and the cycle repeats. I'm questioning if the solid state relay could heat up more over time, etc; this kind of things \$\endgroup\$ – Thomas Jun 22 '18 at 13:04
  • \$\begingroup\$ No, there’s no difference, because nothing gets heated up if the relay has sufficient power rating. \$\endgroup\$ – Stefan Wyss Jun 22 '18 at 13:30
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    \$\begingroup\$ The \$1500 \:\text{W}\$ argues to avoid the SSR, as the SSR will dissipate about 1% (\$2\:\text{V}\$ out of \$220\:\text{V}\$), or \$15\:\text{W}\$. Which is a heating problem requiring bulk space and cost. But the 3 minute cycling argues for solid state, depending on how many cycles and for how many months or years you want operation. I'd consider a hybrid here. Use the SSR, then fire the relay and hold it, then release the relay, then release the SSR. Should save the relay contacts that way. \$\endgroup\$ – jonk Jun 22 '18 at 13:41
  • \$\begingroup\$ @StefanWyss, hopefully the air will get heated up. :-) \$\endgroup\$ – Cristobol Polychronopolis Jun 22 '18 at 13:45
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The relay will probably give you something like 150,000 operations life under that service. So at 3.5 minutes per cycle = 17 cycles per hour, you would get about 8800 hours operation or about 1 year 24/7 before the relay wears out.

If that's acceptable, the relay is easy and cheap (assuming the safety agency approvals work for your jurisdiction).

If not you can consider an SSR with a suitable heatsink for about 7W. The cyclic heating and cooling of the SSR can cause early failure, and I suspect off-brand types may be more susceptible so between that, the heat sink and the extra cost, they are not a panacea. They also almost invariably fail 'on' unless subjected to massive overcurrent, so that may not be as desirable (you should, however, make sure that the consequences of an 'on' failure are acceptable regardless of which path you choose to follow).

The hybrid approach @jonk suggests can work nicely but it's still good to have enough heatsink available that if the relay fails to close for one reason or another the SSR is not going to fail. One approach is to put a slow-blow fuse of a few A in series with the SSR.

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  • \$\begingroup\$ +1 I custom-build from pairs of SCRs, use "bimetal temperature control switch thermostats" in series with the low-current cycle by cycle activation circuit. Separately monitor both temperature and the thermostat with an MCU. (I stay away from triacs, uncomfortable with the asymmetry relative to the gate and also gate triggering vagaries. And SSRs, since I have a mental problem trusting what I can't see inside, made from suppliers I may not know well. Plus SCRs have better specs, like dV/dt, and they are reliable.) I turn off SCRs after relay is engaged, so if relay fails current just stops. \$\endgroup\$ – jonk Jun 23 '18 at 20:25
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Since hair dryers use a diode for DC motor with resistance wire (nichrome) wire current limit and switches for heater wires, any hiV transistor 10A switch or SCR will work rated for 400V with a line filter for transient protection and EMI suppression with MOV. Even a dimmer Triac will work.

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