I answered a question recently regarding power dissipation in solid state relays. This is the calculation:

Consider this 40 A rated triac - BTA40.

Vto = Threshold voltage = 0.85V = Voltage drop across triac.

Rd = Dynamic resistance = 10 mohm

Assume IT(RMS) = Current through the triac = 25 A

Power dissipation = [0.9Vto X IT(RMS)] + [Rd X {IT(RMS)} X {IT(RMS)}]

Where 0.9 = 2*sqrt(2) / pi

Putting the values, we get P = 25.375 Watts

For 1A current, P = 0.775 watts which seems to be a tolerable value without any heat sink.

Besides this calculation, I tried testing a few triacs rated 4A to 40A. All of them get pretty hot after 2-3 amps of current. Even after putting a heat sink (approx 5cm X 5cm X 2cm) the condition didn't improve much.

As such, I am wondering what might be the intended purpose of these solid state relays? Is it just used for high ampere switching at a very low duty cycle where the triac could get ample time for cooling itself?

Because, for continuous currents above 5 amps, I think I will better off with an electromechanical relay rather than going for solids.

  • 1
    \$\begingroup\$ Typically SSR heatsinks are HUGE, serious heatsinks. \$\endgroup\$
    – Ecnerwal
    Dec 22 '15 at 17:08

Yes, they are used for high power switching, and not necessarily at a low duty cycle. I run hundreds of SSR's anywhere from 5A/240VAC up to 100A/460VAC/3P. Past 100A, it is more common to use back to back SCR's and a small firing circuit (and large heat sink).

The typical application is for resistance heat control. Contactors/Relays are not a serious option, because of contact wear (the typical 'duty cycle' is 2.5 sec on/2.5 sec off).

Basically, a timer is free running for a certain period (I use 5 seconds mostly). The heater is switched on and off for some period out of that 5 seconds to control temperature, based on the output of a PID loop. 10% output is on for 0.5 sec, off for 4.5. 90% output is on for 4.5, off for 0.5. Those duty cycles kill electromechanical devices quickly.

There are also hybrids, that use a relatively small SCR to turn on a load, then close an electromechanical contact around it (to prevent closure arcing), and then the SCR is turned on again when opening the contacts. Those are pretty special purpose though, and expensive.

Mercury Whetted relays used to be used, which didn't have the contact wear problem, but in the past 15 to 20 years, industry has been trying to eliminate them due to the problems of dealing with heavy metal contamination, and disposal.

  • \$\begingroup\$ Thanks for the answer. Can you link me to a typical heat sink or cooling system that are used in SSRs handling approx 100 amps. I am just curious to see. Also, how about the heat wastage? Has the industry considered it as a necessary evil and stopped bothering or are there other solutions which would produce less heat. With my little knowledge, I think all solid state devices will have at least 0.6-0.8V junction voltage which will lead to almost similar power wastage levels. Maybe this wasted power is cheaper than replacing electromechanical counter-parts. Is it so? \$\endgroup\$ Dec 22 '15 at 17:58
  • \$\begingroup\$ @Whiskeyjack A typical 100A SSR I buy is just plain prepackaged. eurotherm.com/products/power-control/rsda-rsaa is an example. The heat dissipation is a necessary evil, but minor. Say 100 to 200W dissipation at the device, but it is controlling a 40KW heater. And a typical extruder here has 5 or 6 of those heaters, plus a motor anywhere from 100 to 500 HP. \$\endgroup\$
    – R Drast
    Dec 23 '15 at 11:17
  • \$\begingroup\$ Earlier I thought if I am dissipating 20-25 watts, it means I am doing something wrong but now I see. 100 watts dissipation in 40kW appliance circuit is just 0.25%. \$\endgroup\$ Dec 23 '15 at 19:04

(from the datasheet)

They can be used as an ON/ OFF function in applications such as static relays, heating regulation, water heaters, induction motor starting circuits, welding equipment... or for phase control operation in high power motor speed controllers, soft start circuits...

You're supposed to fit adequate heatsinking, possibly even active cooling. The device is rated to work happily at 80C.


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