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.
