Crydom GN series Solid State AC relays block diagram.
Fig. 1

Current Limiter Circuits for Opto-Coupler LEDs

by Lewis Loflin

See Exploring Solid State Relays and Control Circuits

I started of looking into circuits based on Crydom high-current GN series solid state relays. Notable is the use of constant current sources (current limiters) for LED inputs on the associated opto-couplers.

Here I'll look at current limiter circuits using the popular LM334 and the LM317. The idea is construct circuits without the use of exotic custom parts.

For the opto-couplers in themselves see Comparing Photo Triac, Photo SCR Opto-Couplers.

For reference to the rest of the circuits presented here see:

LM334 used a current limiter for optocoupler LED input.
Fig. 2

LM334 used as a current limiter for optocoupler LED input. The formula is simple with the Ref. at 0.064V / Rset.

In this case 0.o64 divided by 3.3 Ohms = ~19mA. 19mA will operate any opto-coupler I have. If more current is needed replace Rset with 2.7 Ohms for 24mA or even 1 Ohm for 64mA. Most LED input are limited to 60mA.

The voltage input range is 3-32 volts DC.

LM317 used as a current limiter for opto-coupler LED input for solid state relay.
Fig. 3

Fig. 3 illustrates an LM317 used as a current limiter for opto-coupler LED input for solid state relay. While simpler than Fig. 2 but the input voltage range won't go below 7-volts. This is due to the LM317.

If there is any question of enough current drive for the LED change the 100 Ohm resistor to 47 Ohms.
24-volt AC-DC opto-coupler input circuit using an LM317.
Fig. 4

Fig 4 is a 24-volt AC-DC opto-coupler input circuit using an LM317. If DC only input is required leave out the diode bridge, Zener diode, 22uF cap, and 27 Ohm resistor.





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