Episode # 54: 5 vs 24-volt Logic for PLCs?

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The Motion Control Show

When designing a new system, you may be forced to decide whether you are using 5 or 24 volts for your logic for your sensors, feedback or other devices connected to your controls.  I am Corey Foster at Valin Corporation.  Let's see what we can learn.  If you have any questions or want to reach out to us, reach out to us here at this email address or reach out to us at TheMotionControlShow.com.  We're happy to help.

Here is a controller on the right and some device on the left which could be a sensor or some other device that has an output.  You can see here that this device has a voltage and it ranges from 5 to 24 volts, goes into the PLC, the PLC or controller then sinks it to ground.  Well, what happens if this device here actually pulls it to ground?  You can see this device is pulling it to ground here.  This particular controller has an input pull-up.  It has 5 volts on board, so it pulls up to 5 volts.  But this could also be pulled up to an external 24 volts to make this a 24-volt input here for this output. 

Now 5 volts is older.  A lot of older products ran on that, but 24 volts is definitely the newer way that the world is going.

Why is that?  Well, if you look here: 5 volts, if you have electrical noise, a 5-volt electrical spike for example, it can cause an extra count here if you have a square wave for example.  Or here, this one could actually take away this square wave.  Electrical noise can be a problem.   Well, we have that same voltage spike on the 24-volt signal, it is not going to cause nearly the same problems.  This is only 5 volts out of this in 24 volts.  24 volts is definitely more resilient to electrical noise. 

If you look at this system here, we have a PLC with some FETs going out in Pulse and Direction, or Clockwise/Counterclockwise, out to a motor driver.  These are 24 Volt signals.  Well, what happens if this same PLC is going to be used with a drive that takes 5 volts?  Well, some drives will allow you to put in resistors in order to cut that 24 volts down and to control the current.  However, as we saw in the previous episode, a lot of drives are differential and you can't just make those changes with resistors.  The compatibility of 5 volts versus 24 volts is something you should look at beforehand.  A lot of products, sensors, and feedback devices will have options for what voltage you are going to use.

I hope this helps.  I am Corey Foster at Valin Corporation.  Again, reach out to us at this email address here or this website.  We are always happy to help.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.