Electronic – Is a 25 amp SSR safe to use in a circuit that draws 25 amps

pwmsafetysolid-state-relay

Specifically, my circuit consists of a 5,500 watt hot-water heating element on a 220 volt circuit. I am controlling the heat output using a PWM signal – the heating element is turned on/off with a solid state relay. The PWM signal runs at a 0.5 Hz frequency with a duty cycle typically ranging between 80% and 100%

I have found a relay that is rated for 240V, 25 AMP on the control side. Is it safe to use this relay in my circuit? By 'safe' I mean do I risk a fire hazard or other life-threatening condition by using this relay in this manner?

Here is a link to the Data Sheet Link.

Best Answer

Use a couple sizes up. 40A SSRs are easily available. You must consider the 100% duty cycle case when the heater is operating for a long period of high demand.

Here is a typical derating curve for a 40A SSR based on the ambient temperature maximum and your heatsink performance.

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You should stay well below the curve. A 1.5°C/W (natural convection) heatsink is quite large (maybe 5" square x 1.5" tall).

Do note that SSRs will typically fail 'on' and you must ensure that no safety hazards arise when (not if) this eventually happens.