Outputs and switching

SSR-40DA relay with ESP32

An SSR-40DA switches AC only. The DA means DC control and AC load, and it will not switch a 12 V or 24 V DC load: it may turn on and never turn off. If that surprises you, choose a MOSFET or an SSR rated for DC output.

This is mains equipment, so turn off the supply before you wire it, use proper terminals and an enclosure, and add a fuse. Have an electrician do the job if you are not qualified. On the control side, watch the 3.3 V margin. Listings give 3 to 32 V, with on above 2.4 V, but one vendor notes that a newer version needs 4 to 32 V. A transistor switching 5 V to the input removes the doubt, and one published build used an optocoupler for the same reason.

The '40 A' on the label is not a figure you can use. About 1.6 V is lost across the relay, which is about 64 W of heat at 40 A. One seller recommends a heatsink above 4 A and running at no more than half the rating. Without a heatsink it will fail at a far smaller load. Off-state leakage is 2 to 3 mA in listings, enough for a small LED lamp to glow and for a load to read as live. It is not an isolator, so treat the output as live even when off. Spec sheets vary between clones, and there is no easy way to confirm a genuine part. Switching is silent, and suits heaters with slow on and off cycles. For fast PWM it does not work, as these parts usually turn on near zero crossing. LCD Ultra Pro can set a pin on or off from a rule or switch without code.

Power and logic levels

3.3 V: check your module first

Supply	Input 3–32 V or 4–32 V
Signal level	3.3 V may not be enough
On an ESP32	Drive it with a transistor if yours needs 4 V.

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

const int SSR_PIN = 5;
const unsigned long WINDOW_MS = 2000;
int powerPct = 40;                    // heater power, 0 to 100

void setup() {
  pinMode(SSR_PIN, OUTPUT);
}

void loop() {                         // slow on/off cycle, not PWM
  unsigned long t = millis() % WINDOW_MS;
  digitalWrite(SSR_PIN, t < WINDOW_MS * powerPct / 100);
}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.