A solid state relay circuit that in common use design to be controlled by the input voltage from 3V to 24V.
However, in actual usage. We found that at low input voltage. Performance of the circuit is decreased.
Because the most input circuits will be using the resistor to control the current flows through the LED transmitter.
On as low-voltage doing the current is very small, so that the output circuits to drive the triac gate curren reduced, the current flowing through the load decreases. If the lamp is a load, it is obvious that The lamp does not light or reduced light.
Modifications may be made by reducing the resistors. When used high input voltage. The current that flows through the LED transmitter will too much, until to may be made it damaged.
To resolve this issue. We designed a new inputs circuits, so supply to the LED transmitter is the current constant at all level voltage input.
The operation of the circuits.
From the circuits can be seen that the transistor Q1 acts as a constant current to the LED transmitter within IC1.
When we attach the R1 to the LED, the base voltage of Q1 will be a constant about 2 volts and at emitter pin of Q1 also as about 1.5 volts.
From value of resistor R2 in this circuit, we will have a constant current flowing through the transistor Q1 and the LED about 30 mA to the output also constant current at pin 4 of IC1 to drive the gate of the triac.
Therefore, the current flowing through the load is a definite quantity, required at all times.