Solid State Relay Solutions

Use SSRs when: (1) High switching frequency (>1 operation per minute) - SSRs have unlimited life

(2) Silent operation required - no mechanical clicking

(3) Fast switching needed - <1ms vs 5-15ms for mechanical

(4) Explosive or dusty environments - no arcing

(5) Vibration-prone applications - no moving parts

(6) Precise control needed - compatible with PWM. Use mechanical relays when: (1) Very low on-resistance required (<10mΩ)

(2) Complete galvanic isolation needed (no leakage current)

(3) Very high current switching (>100A)

(4) Cost is primary concern for low-frequency switching. Cosmo SSRs are ideal for heating, lighting, and motor control applications.

Use SSRs for high-frequency, silent, or fast switching applications.

To size an SSR heat sink: (1) Calculate power dissipation: P = I² × Rds(on) for DC, P = Vdrop × I for AC

(2) Determine maximum allowable junction temperature (typically 100-125°C)

(3) Calculate required thermal resistance: Rth = (Tj_max - Ta) / P - Rth_jc - Rth_cs

(4) Select heat sink with thermal resistance less than calculated value. Example: 10A AC SSR with 1.2V drop, Ta = 50°C, Tj_max = 110°C: P = 1.2V × 10A = 12W, Rth = (110-50)/12 - 1.5 - 0.5 = 3°C/W. Select heat sink with Rth < 3°C/W. Always include safety margin and verify with thermal measurements.

Calculate based on power dissipation and maximum junction temperature.

Inductive loads (motors, solenoids, transformers) require protection against voltage transients when switched off: (1) For DC inductive loads: Use freewheel diode across load (rated for load current), or TVS diode for faster switching

(2) For AC inductive loads: Use metal oxide varistor (MOV) across load (voltage rating 1.5-2× line voltage), or RC snubber network

(3) For motors: Consider soft-start circuit to reduce inrush current

(4) For all loads: Ensure SSR voltage rating has margin above peak transient voltage. Without protection, inductive kickback can exceed SSR voltage rating causing failure. Cosmo SSRs include internal protection but external protection is recommended for large inductive loads.

Use freewheel diodes for DC, MOVs or snubbers for AC inductive loads.

Zero-cross switching means the SSR turns on only when the AC voltage crosses zero, minimizing EMI and inrush current. Benefits: (1) Reduced EMI - switching at zero voltage minimizes conducted and radiated emissions

(2) Reduced inrush current - no voltage step when switching resistive loads

(3) Extended lamp life - eliminates filament shock for incandescent lamps. Use zero-cross for: resistive loads (heaters, lamps), applications with strict EMI requirements, and when minimizing inrush is important. Use random (instant-on) switching for: inductive loads where fast response is needed, phase-control applications (dimming), and when immediate turn-on is required. Cosmo offers both zero-cross (KMOC3041) and random (KMOC3051) triac photocouplers for SSR designs.

Use zero-cross for resistive loads and EMI-sensitive applications.

SSRs can be paralleled for higher current with precautions: (1) Use SSRs with similar characteristics from same production batch for current sharing

(2) Implement individual current limiting or balancing resistors

(3) Ensure adequate heat sinking for combined power dissipation

(4) Monitor temperatures to detect imbalance

(5) Consider using one larger SSR instead - often more reliable and cost-effective. Challenges with paralleling: (1) Current imbalance due to Rds(on) variations

(2) Thermal runaway if one SSR carries more current and heats up

(3) Complex protection and monitoring. For currents above single SSR rating, consider: (1) Using higher current SSR model

(2) Contactors for very high current

(3) Multiple independent circuits. Cosmo offers SSRs up to 10A

contact us for higher current solutions.

Use single higher-rated SSR when possible; parallel only with current balancing.