| Relay module | Solid-state relay (SSR) | MOSFET | |
|---|---|---|---|
| Switches | AC or DC | AC only for the common "DA" type (DC types exist) | DC only |
| Isolation from your board | Yes — separate contacts (plus an optocoupler on many modules) | Yes — optical | No — shares ground with the board |
| Switching speed | About 5–15 ms; a few switches per second at most | AC types switch at the next zero-crossing | Microseconds — fine for PWM dimming and speed control |
| Noise | Clicks | Silent | Silent |
| Wears out | Yes — contacts are rated for a limited number of operations | No moving parts | No moving parts |
| Off-state leakage | None | A few mA — LED bulbs may glow faintly | Negligible |
| Heat | Coil only | About 1–1.5 V drop: needs a heatsink above a few amps | Very low with a logic-level MOSFET |
| Typical price in India | ₹60–120 (1-channel module) | ₹150–450 (fakes common) | ₹30–150 |
All three let a 3.3 V or 5 V pin control something much bigger. Picking the wrong one leads to burnt contacts, glowing bulbs, hot heatsinks — or worse. Behaviour described here is typical of the parts sold in India; always check your part’s own ratings.
Mains safety first
If the load runs on 230 V AC:
- Mains wiring is electrician-only work, inside a proper enclosure, with an MCB/RCCB and earth. Never on a breadboard.
- A relay module or SSR drives a contactor coil or a small lamp within its rating — never a motor or pump directly. Motors draw several times their running current at start-up and weld relay contacts. The contactor switches the motor. Our water tank alarm and smart pump controller show this arrangement.
- The ratings printed on a cheap relay (10 A 250 VAC) are optimistic for inductive loads.
Relay module: the default
- Switches AC or DC with a real air gap — when it’s off, it’s off.
- Works for lamps, contactor coils, solenoid valves, 12 V pumps.
- Most Indian modules are active-LOW and have the flyback diode and driver transistor built in. Day 25 shows safe wiring with a 12 V lamp.
- Too slow for PWM; contacts wear out over time. Clicks but nothing switches? Fix.
Component: relay module.
SSR: silent AC switching
- No click and no wear — good for heaters, incubators and lights switched often.
- Leakage: even “off”, a few mA flows. A low-wattage LED bulb may glow or flicker. Not a safe isolation point for working on the circuit.
- Heat: about 1–1.5 V is lost inside, so a 10 A load makes 10–15 W of heat. It needs a real heatsink.
- Fake ratings are common. Many cheap 25 A or 40 A SSRs can’t carry anywhere near that. Derate heavily, or use a small 2 A SSR module for light loads.
MOSFET: for DC loads and PWM
- The right choice for 12 V LED strips, DC fans, DC pumps and motor speed control — it switches thousands of times a second.
- Pick a logic-level MOSFET (fully on at 3.3–5 V gate voltage, e.g. IRLZ44N, or an AO3400 for small loads). The popular IRF520 module is not logic-level; from a 3.3 V pin it barely turns on and runs hot.
- Use a gate resistor (100–220 Ω) and a pull-down (10 kΩ) so the load stays off at boot.
- Inductive loads (motors, solenoids) need a flyback diode across them.
Verdict
| Load | Pick |
|---|---|
| 230 V motor or pump | Relay or SSR → contactor → motor (electrician wires it) |
| 230 V lamp or contactor coil, switched occasionally | Relay module |
| Heater or light switched often, silent | SSR with a heatsink, derated |
| 12 V LED strip, DC fan, DC pump, PWM speed or dimming | Logic-level MOSFET |