Controlling a Relay or Solenoid From a Digital Out

Most digital IO of MCC products are control signals meaning they are TTL levels, but very low current.

For example, the USB-1608G series and USB-200 series each have 8, individually programmable digital IO.

Per the published specification for either device, maximum current sink or source per bit is ±2.5mA.

Most 5V relays and solenoids require much more current than 2.5mA. Or, you may have a relay, solenoid or contactor requiring 12, 24 or 48VDC @ more than 1Amp.

So how do you use the digital IO from a USB-1608G to control these devices? Simple, use an intermediate stage to increase the control signal to the appropriate signal levels.

For example, I have a relay that requires 24VDC @ 10mA that I want to control from my USB-1608G. I used a Fairchild Semiconductor 14N05L N-Channel power MOSFET to drive my signal to the relay. This MOSFET can handle up to 14 Amps, 50 Volts DC.

Here is the circuit I used:

NOTE: be sure your relay has a back EMF diode (a.k.a. flyback diode) built in, or add one externally. To add one externally, use a diode such as a 1N914, 1N4148 or 1N4004 installed across the relay’s coil terminals such that the cathode is attached to the same terminal as the power supply. This will allow it to conduct current in a circular loop from the positive potential at the bottom of the relay coil to the negative potential at the top when the relay is de-energized.

To control AC voltages such as 110VAC, 120VAC, 240VAC or higher, you can use the relay contacts shown in the above circuit to control your AC or next stage relay.