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I'm assuming the control circuit is an inductive load.
Its a PWM driver that uses a 0-5vdc (not 0-20ma) input for 0-100% duty cycle. I was afraid a transistor would distort the signal a bit. If not I suppose a 10K at the base and I could replace the relay.
What is the contact rating on the relay?
What voltage is connect to the inductive load?
Is one end of the load connected to the supply voltage, or to ground?
How many times per second is the load switched on/off?