Solid-State Relays

Opto 22 • 43044 Business Park Drive • Temecula, CA 92590-3614 • www.opto22.com

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17

Transformer Loads

Careful consideration should be given to the selection of the

proper SSR for driving a given transformer. Transformers are

driven from positive saturation of the iron core to negative

saturation of the core each half cycle of the alternating

voltage. Large inrush currents can occur during the first half

cycle of line voltage if a zero-voltage SSR happens to turn on

during the positive half cycle of voltage when the core is

already in positive saturation. Inrush currents greater than 10

times rated transformer current can easily occur. The following

table provides a guide for selecting the proper SSR for a given

transformer rating.

Solenoid Valve and Contactor Loads

All Opto 22 SSRs are designed to drive inductive loads such as

solenoid valves and electromechanical contactors. The built-in

snubber in each SSR assures proper operation into inductive

loads. The following table is a guide in selecting an SSR to

drive a solenoid or contactor.

Control Current Calculation

All Opto 22 DC-controlled SSRs have a control circuit

consisting of 1000 ohms in series with an Optocoupler LED.

The LED will drop 1 volt, so the voltage across the internal

resistor will be 1 volt less than the control voltage.

The control current (I

C ) can be calculated from the control

voltage (V

C ) as follows:

I

C = (VC - 1)/1000

Examples:

3 VDC control voltage:

I

C = (3 - 1)/1000 = 0.002 A (2 mA)

32 VDC control voltage:

I

C = (32 - 1)/1000 = 0.031 A (31 mA)

For control voltages above 32 VDC, an external resistor can be

added in series with the SSR to limit the control current. Also, if

the device driving the control current to the SSR is limited, you

can limit the control current by using an external resistor (Re).

I

C = (VC - 1)/ (Re + 1000)

Re = [(VC - 1)/(IC)] -1000

To limit the control current to 2 mA, this simplifies to:

Re = 500 (VC - 3)

120-Volt Coils

SSR CURRENT

RATING

SOLENOID

CONTACTOR

2-Amp

1-Amp

NEMA Size 4

4-Amp

3-Amp

NEMA Size 7

240-Volt Coils

SSR CURRENT

RATING

SOLENOID

CONTACTOR

2-Amp

1-Amp

NEMA Size 7

4-Amp

3-Amp

NEMA Size 7