How to drive proportional hydraulic solenoids with two separate controller 12 V output and 400 to 1500mA drive current


I do not know how to start but I describe my problem:

I have PWM / milliamp control solenoids, which actuates proportional hydraulic system/movement. I have 4 solenoids total. Their drive voltage is 12 V battery and drive current is 0,5 to 1500 mA

They are connected to electronic controller which connects both of solenoids wires: solenoids one wire is not connected to ground and both wire. This system works OK.

Controller also monitors, that solenoids are connected and if there is open circuit, there is 3, 9 V voltage together with error code.

If everything is ok, then there 3.9 ma current through and no error code. :)

Now is my issue: 

I would like to use existing solenoids be driven by another controller. Use controller 1 OR controller 2 to drive solenoids.

Both controllers do it separately OK. My simple plan was to make Y cable with diodes in both supply wires and this way guarantee, that one controller output does not go to another controller input.

Return wires were simply connected together with Y, no diodes in them

I made bench test with 18V cordless drill battery and 10W light bulb. Everything worked OK, as I predicted.

When I put wiring to machine, then during either movement actuation hydraulic system jumped with full power, no proportional drive.

Measured with oscilloscope and amp clamp, that there was 12 v voltage and around 1 amp current flow through wire, which was not active controller.

Now I think, that there is possible to use  relays for switching or is there possible to use optocouplers or another type of isolators ?

I understand, that I need to make both circuits galvanically isolated.

Control logic is: controller 1 is primary, control 2 secondary. When control 2 is active, there is 12 V signal present. When control 1 became active, then it overrides control 2 output.

My initial idea is to use DPDT relays and probably transistor as switch to drive relay coil power.

Is there any better ideas?

Any comments / ideas, guidelines Thank You in advance!


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2 Replies

  • Hi Andrus,

    I'm not sure if it solves the problem you describe, but we do have devices that can be used to drive solenoids called low side switches. These are simple devices that can drive an array of inductive loads, each one requiring a control signal to enable / disable. Our most popular devices are the ULN2003A and TPL7407LA. There is also the DRV101 and DRV102, used for PWMing solenoids and valves specifically.


    Alek Kaknevicius

    To find the latest information on Power Switches, visit

  • In reply to Aleksandras_Kaknevicius:

    Hi Andrus,

    I am not sure if I understand your application and the requirement very well. Is it possible to share a simplified block diagram of your application and explain the issue that you are facing in relation to the block diagram?
    Based on the your inputs, I will be able to suggest if your application requires any isolators. Looking forward for your inputs, thanks.

    Koteshwar Rao