So I've looked through a lot of the discussions on Linear actuators here, and none seem to apply. I've got an off the shelf Linear actuator (Datasheet can be found here http://www.windynation.com/cm/Actuator Manual_R3.pdf). I'm building up a TV lift. You'll notice this actuator is very basic. Its UP and DOWN controlled by the polarity of the 12VDC applied. It has integral limit switches, and no feedback or anything fancy. I'm looking for the lowest part count method of driving this from an MCU Pin. (I'm going to in the end drive it with a PIC). From the MCU I want to be able to drive UP and DOWN. I thought of driving two relays each from a separate MCU Pin, and handling the interlocking in the MCU firmware, but that leaves a risk that if for some reason both pins drive I should dead short the load. I've heard people talk about using a DPDT relay with a CENTER OFF, but honestly I've never been able to find such a part in any catalog..... Looking for any guidance.
Thanks!

 

You interlock both the controller AND the relays.
Done all the time with PLC control.
Max.

 

You only need to drive a single DPDT relay for up and down control. Due to the way the relay works it would be impossible to drive up and down at the same time.

 

You only need to drive a single DPDT relay for up and down control. Due to the way the relay works it would be impossible to drive up and down at the same time.

Don't you need a second relay to switch power? DPDT handles polarity switch and preventing shorts, but the DPDT would have the motor always spinning in one direction or the other. So you use the DPDT for direction control and a second relay to turn motor power on and off (or BJT, MOSFET, etc. with appropriate protections for inductive loads.)

 

This explains it well.
Even if both relays are accidentally on, both sides of the motor get +12v. No short, no problem.
reversing the DC motor with relay - YouTube

 

Don't you need a second relay to switch power? DPDT handles polarity switch and preventing shorts, but the DPDT would have the motor always spinning in one direction or the other. So you use the DPDT for direction control and a second relay to turn motor power on and off (or BJT, MOSFET, etc. with appropriate protections for inductive loads.)

I don't think so, the linear actuator should be ok with power applied permanently. When it reaches the end of its travel the limit switches cut power to the motor until the polarity is reversed.

 

I don't think so, the linear actuator should be ok with power applied permanently. When it reaches the end of its travel the limit switches cut power to the motor until the polarity is reversed.

Ah, I see! I was assuming we needed to be able to stop the actuator at any arbitrary position within its range of motion. If we assume it should always move all the way to one extreme or the other, then I think you're right.