SSR-25DA Solid State Relay: Electrical Data, Sizing, Heat Sink and Wiring

The SSR-25DA is a DC-controlled, AC-switching solid state relay used to drive resistive heating loads from a PID temperature controller SSR output or a PLC digital output. Full ratings across the 10A–100A family are on the SSR-DA product page.

1. Electrical specification

Parameter	Specification
Control voltage	3–32VDC
Control current	5–25mA
Load voltage	24–480VAC
Rated current	25A (series covers 10A to 100A)
Trigger type	Zero-cross
On-state voltage drop	≤1.5VAC
Off-state leakage current	≤2mA
Isolation voltage	≥2500VAC
Dielectric strength	2.5kV RMS
Operating temperature	-30°C to +75°C
Dimensions	57.4 × 44.8 × 28mm, bolt mount
Indicator	LED status display

2. Sizing the relay

Rated current is a heat-sinked figure at a stated ambient. Size on the real duty, not on the nameplate:

• Calculate steady-state load current: for a single-phase resistive heater, I = P / V. A 3 kW heater on 230 VAC draws about 13 A.
• Select a relay rated at roughly twice that current. The 13 A heater above suits a 25 A relay.
• Derate further for ambient above 40°C inside the cabinet, for continuous duty, and for poor ventilation.
• Inductive or lamp loads have high inrush; increase the margin or use a relay rated for the surge.

3. Heat sink and thermal design

Required cooling depends on the actual load current, ambient temperature, duty cycle, enclosure ventilation and heat sink thermal resistance. A properly sized SSR heat sink with thermal compound is mandatory for continuous heating duty.

• Apply a thin, even film of thermal paste across the whole base — dry mounting is the most common cause of premature failure.
• Mount the heat sink with fins vertical so convection air rises freely; leave clear space above and below.
• Add forced-air cooling for high-current or continuous-duty operation, or where several relays share one enclosure.
• Keep the relay away from the hottest part of the panel and never mount it directly above a contactor bank.

4. Wiring practice

• DC control terminals go to the controller SSR drive output, observing polarity; the LED lights when the relay is commanded on.
• AC terminals carry live and the load — install upstream short-circuit protection sized for the relay.
• Route the DC control wires separately from the AC load wires to avoid induced noise and false triggering.
• Torque the terminal screws to specification and re-check after the first heat cycle; loose terminals overheat.

5. Fault finding

Parameter	Specification
Load runs continuously and cannot be switched off	The output SCR has failed short, usually from overheating or surge. Replace the relay and correct the thermal design; add a series contactor as a safety isolation point.
Load never energises, control LED on	Load-side wiring, blown fuse, or the relay is open circuit.
Load never energises, control LED off	No control signal: check the controller output type parameter, control polarity and control voltage at the terminals.
Small load stays faintly on	Off-state leakage of up to 2 mA can hold a very small load or a neon indicator. Fit a bleed resistor.
Relay hot to touch at moderate load	Heat sink undersized, dry mount, or blocked airflow. Recalculate the thermal path.

6. FAQ

Does the SSR-25DA need a heat sink?

Yes for any continuous heating duty. A single-phase SSR dissipates roughly 1 to 1.5 W per ampere, so 13 A of load produces 13–20 W that must leave the device.

Can it switch a motor?

It is intended for resistive loads. Inductive loads need a much larger margin and, for zero-cross switching, careful review — contact us with the load data first.

Zero-cross or random turn-on?

Zero-cross for heating; random turn-on only where phase-angle style control is required. See zero-crossing versus random turn-on.

How does it compare with a mechanical relay?

No contacts to wear and silent, fast switching, but it dissipates heat and fails short rather than open. Full comparison: SSR vs mechanical relay.

Sizing help and quotations: sales@strong-ele.com.