Digital output card - PLC


Thread Starter



can anybody how to select proper digital output card for solenoid valves.

i mean what factors to be considered.

I think the digital output card must be able to provide VAInrush power consumption.

Can anybody give more details on other factors to be considered.

I am using 120 VAC SOV and 120VAC digital output card.


Asok Kumar Hait

You are correct, the most important point to check if the output power per channel has the enough capability to drive the solenoid or not. For that you need to consider the inrush power consumption of the solenoid.
Also pl. don't forget to check the voltage drop in the cable (for long distance case) and final voltage available at solenoid end. The end voltage should be within the tolerance limit of the solenoid.

Bruce Thompson

In addition, if you are using a digital output card that consists of relay outputs you'll need to ensure that your outputs are protected against voltage spikes when the contacts are opened. Putting a MOV on the solenoid, or at the terminal strip for the contact output will provide adequate protection.


It is not a good idea to drive solenoid valve directly via PLC DO card's output. In case of any error (from solenoid valve or wiring), digital output's relay or transistor will be damaged. After this damage it is not so easy to repair that output.

Instead of this PLC Digital Output + SSR is a good and safe solution. PLC DO will be more safe cause current drawn by SSR input is in mAs. Current output's of SSR will have no any risk, in case of any error from solenoid and wiring, you can easily change just SSR but PLC and DO card is still safe and operational.

İsmail MISIR

PS: For plc out instead of SSR, it can be used dry contact relay which has a low coil current, enough output contact current. In any case do not forget to use proper fuse.