Tech Tip: How and Why to Use a Surge Protection with Your Contactor

Do you really need additional surge protection when connecting a PLC to a contactor coil? After all, many PLCs have output modules with surge suppression built in -- right? In this video we'll take a look at what a surge is and show you a couple live examples and a table of options so you can decide if you need to add surge protection or not. In part 2, we'll take a closer look at each type of surge protection device so you can better understand the pros and cons of each. So what exactly is a surge? When you have a resistive circuit and close the switch, the current comes on almost immediately and when you open the switch it turns off almost immediately. Things work differently when you have an inductive load like a contactor coil. When you close the switch and let current flow, the current doesn't pop up to its steady state like it does with a resistor. It varies as the magnetic field around the coil grows. Once the magnetic field is established, you get a steady stream of current. When you shut off the current, all the energy in the magnetic field tries to keep the current flowing, but the current doesn't have anyplace to go, so voltage builds up kinda like when you shut off a garden hose the water pressure builds up inside the hose. And if that pressure builds up too much or the hose is weak, it will burst. Same thing here. If that voltage gets too big, it will fry the electronics. And the more current you had flowing, the bigger and more powerful that voltage spike will be. Let's do a live example with a contactor that has a DC coil. We'll use a simple switch to control the current flowing through the coil and we'll connect an oscilloscope like this so we can monitor the voltage transitions. We're grounding this end of the coil which is normally at 24 volts. So when we switch it on we expect the voltage at this terminal to go from 24 to 0 Volts, and when we switch it off we expect it to go from 0 back to 24 volts. Let's see what really happens. Here's a live view of the scope. It's set to 50 volts per division with zero volts right in the middle and we expect our turn on voltage to transition from 24 to zero, so lets set the trigger somewhere around here so we can catch that transition. Now if I press the switch, sure enough we see the voltage at that terminal go from 24 to 0 volts -- exactly what we expect. Now that the current is flowing and the magnetic field is established in the contactors coil -- what happens when we release the switch? Look at that. The voltage spiked up to almost 200 volts. My Oscilloscope has its own surge protection built in and its actually limiting the spike -- the voltage really goes up here somewhere. But you get the idea. Its hundreds of volts. And when that huge voltage spike hits your PLC's I/O it can cause an arc to jump the PLC's output relay contacts which will shorten their life and it can destroy a transistor output. So how DO you get rid of that voltage spike -- or "surge" as it is properly called. With an "SPD" -- a Surge Protective Device. By the way, before 2009, these were called a TVSS device or Transient Voltage Surge Suppression device. That term isn't considered proper anymore, but you will still see it used. SPD's simply give the current someplace to go while the magnetic field around the coil dissipates when you open the circuit. That way it can't build up that huge voltage spike. For example, let's add a diode SPD to our switch example. On the WEG mini contactor we are using in this demo it's easy -- you just plug it in. Again, when we press the switch we see the voltage drop from 24 to 0 volts -- no surprises there -- and when we release the switch, the spike is gone! We get a nice clean 0 to 24 volt transition. Perfect. Where did the spike go? Well, the diode allowed the current to circle back around until the magnetic field completely collapsed. Since the current had a place to go, there was no voltage build up or 'surge.' So at the end of the day, do you really want a large voltage spike hitting your PLC and then rely on the PLC's internal surge protection -- assuming it has any. Or would you rather simply kill the voltage spike out at the contactor coil and just not have to worry about - especially since these devices are super cheap and easy to install. On contactors like this Fuji, it just screws in right here. Easy. This table shows you at a glance the various kinds of SPDs available, if they work for AC or DC coils and some of the advantages and disadvantages of each. In general you will find that for DC coils, your best bet is going to be a simple diode type surge suppressor. For AC coils, your best bet is usually going to be a Transorb. Check out part II of this video if you are curious exactly what the difference is between all of these, when you should use one over the other, and some helpful hints on how to get the most out of each type. The video has a bunch of live examples so you can see exactly how each one handles surges. If you need any help, please don't hesitate to call AutomationDirect's FREE, award winning tech support during regular business hours. They will be happy to help you out. Also, don't forget the forums. There are lots of helpful folks there that are happy to share their years of experience with you! Just don't post any tech support questions there because the forums aren't monitored regularly AutomationDirect's tech support.