Learn about the different kinds of surge suppression devices, when to use each and which is the best for your application.
Note: Video states Transorbs are not polarity sensitive. That applies to the AC modules, DC modules are polarity sensitive.
In this video we’ll take a close look at the various SPD’s, their advantages and disadvantages, and even offer some helpful hints along the way. We’ll use this chart from the previous video as our guide. We already saw the Diode SPD in action in the previous video. It took a spike like this and killed it right at the contactor coil so it never got to the PLC. Looks ideal – right? Well, there are a couple drawbacks. First, it’s polarity sensitive and easy to install backwards. And if you do install it backwards you can damage your equipment – so be careful with that. Second, it creates a delay. When you open the switch you expect the coil to de-energize and the contactor to open. Well, if the diode keeps current flowing after the switch is opened, then the contactor will stay engaged until the magnetic field drops low enough to release the contacts which delays the opening of the contactor. And the bigger the contactor, the more current is required to engage the coil, the longer it’s going to take for the coil to dissipate the energy, the bigger the delay. For example, here is a WEG mini-contactor with a 24 Volt dc Coil and this is the surge we get without adding the diode SPD. I’m running a test signal through the contacts so we can see when they actually open. This is the moment in time when I de-energized the coil, and this is when the contacts actually opened. Looks like it took between 6 and 7 milliseconds for the contactor to respond without a diode SPD. If we add the diode SPD then this is what we get. Looks like it took almost 40 msec’s! That’s over 30 milliseconds the diode SPD added for this small contactor. Let’s try the same test a larger 24 volt contactor, like this Fuji, what happens now? Without the diode SPD it takes this contactor about 15ms to open. If we add the Diode SPD, we can see the contactor responded around 100 msec later. So as expected, the larger the coil, the more current has to be dissipated when the switch opens, and the longer the delay that was created by the Diode SPD. Just for kicks and giggles, let’s kick it up a notch. Let’s try the same thing with this big Fuji Contactor. When we release the switch – we expect to see an enormous voltage spike – right? Hmm, what happened? We got a VERY clean waveform with NO voltage spike at all! Well, some manufacturers – like Fuji - know the surge is going to be an issue for you on these larger contactors, so they build in their own surge protection! How about that? You don’t have to do anything special with this guy! And look at this – the off time is under 40ms. That’s really good for such a large contactor – those internal surge protection electronics are actually compensating somehow to help minimize the reaction time of the contactor. If you look closely at the datasheet, you can see that it tells you that it has built in protection and you can even see a block diagram of what’s involved. Ok, let’s get back on track. So the bottom line with Diode Surge protectors is they do a great job, BUT, they only work with DC coils, you have to plug them in the right direction, and they add a delay to the turn off time of the contactor. The more current the contactors coil draws, the longer the delay. Of course since we’re only talking about milliseconds of delay here this is rarely an issue – which explains why Diode SPDs are the go to device for DC Coil surge suppression. What if you want to minimize the delay? Then you add a zener Transient Voltage Suppression – or TVS - diode. This allows current to re-cycle just like before, but now when the voltage falls below the Zener Diodes clamping voltage, the current flow stops. So instead of waiting for the current to dissipate enough to open the contacts, this cuts the current off early which shortens the delay. We’re not going to do a demo of this because this brings up a good question and a better way to do it: And that question is, why not use two back to back zener diodes? Then the voltage can get clamped in both directions and we could use this with DC and AC coils. Well, that’s exactly what a Transorb is. The only downside is they limit the voltage to the clamping voltage of the zener diode, so the voltage can pop up a little, but as long as that is below what your PLC output can handle, you are in great shape. These are reliable, inexpensive and easy to use. And unlike the regular diodes, they are not polarity sensitive. Here’s a Transorb module from AutomationDirect. It’s a convenient way to add 8 lines of protection all at once. You just wire one Transorb in parallel with the contactor coil and it’s protected. Transorbs are NOT polarity sensitive so either way is fine. I’m going to move these cursors to mark where the Transorb limits are. There’s no convenient trigger here so I’m just going to hit the button several times and sure enough, we see that the surges are hard limited to these levels. Perfect. Transorbs are inexpensive , easy to use, reliable and don’t degrade over time. They are ideal for killing voltage spikes out at the contactor before they ever get to your PLC’s I/O. Another type of Surge Protection Device is the Varistor – the most common type being an MOV or Metal Oxide Varistor. As the name implies, it’s just a ceramic material made up of metal oxides – primarily zinc oxide – between two metal plates. When the voltage surge across them gets high enough, the ceramic layer breaks down and the current flows. It works in either direction so it can be used on AC circuits. The good news is these can handle much larger voltage spikes – which is why you typically see them in lightning arrestors. The bad news is, once they have taken a hit, they degrade and can even fail entirely if the spike is large enough. So would you want to use them on a contactor as your only form of protection against surges that occur every time you activate the contactor? Probably not, because every time the contactor coil is energized, the Varistor degrades a little more. If you look at the specs for a Varistor, you’ll see that they are typically rated at around 300% of the voltage you need. That’s to buy you more time before they fail. The problem with that is if you are using a 24 volt coil for example, the closest Varistor may only protect you from 135 volts. That doesn’t help you if your PLC’s I/O can only handle 40 volts! So Varistors are great for single hit lightning strikes, but not so good for protecting your PLC from repeated surges from a contactor coil that’s turning on and off all day long because they degrade over time and the clamping voltage is usually much higher than your PLC’s I/O can handle. The RC type of surge protection is just a resistor and a capacitor in series – a simple filter. These work great – especially for higher frequency transients, but need to be tuned to your circuit for optimum protection. The pre-packaged generic ones that are available work fine, but since it’s a filter, you don’t really have a defined voltage cutoff – the cutoff changes depending on how fast the transient occurs. If the transient is real fast it will work real well. If the transient is slower – not so much. Let’s do an example so we can visualize it better. Here is a Fuji contactor with a 24 Volt DC coil. This is that contactors surge without an RC SPD. This is the surge with it. As you can see, the RC surge protector made the surge last longer, added ringing we didn’t have before and didn’t reduce the surge at all – at least not before my scope limited it. The surge would really go way up here somewhere if the oscilloscope hadn’t gotten in the way … Let’s try the exact same RC surge protector on a larger 24 volt contactor. Here is the surge without the RC Surge protector, and here it is with the RC surge protector. Not a lot of difference and it certainly didn’t really help with the peak of the surge, but at least there isn’t any ringing this time. As you can see, the same RC Surge Protector behaves differently on different sized contactor coils which really highlights the need to carefully select the appropriate Resistor and Capacitor if you want to get optimum performance out of this kind of SPD. Of course, it’s a whole lot easier to just use a Transorb – they work on all coils exactly the same and they actually have a defined hard limit on the surge. So RC type surge protectors WILL attenuate the voltage surge, you just don’t really know how much and it may even add some unwanted side effects if not tuned correctly. These work for AC and DC circuits and are certainly better than nothing, but as you have seen in this video there are better options. We’ve been using Fuji and WEG contactors in this video simply because they have these slick little pre-packaged options. You have also seen the ZIP Link modules with Transorbs which are pretty handy. There is one more option: both the DINnector and KONNECT-IT terminal blocks have diode versions. Here’s an example. You would simply run your wires to your contactor’s DC coil through here, and you now have diode protection automatically where you would be wiring anyway! Easy. So if you are using DIN rail terminal blocks anyway, just get the ones with diodes built in. It’s the least expensive and easiest way to get full diode surge protection for your DC contactor coil. So in summary, we saw that for DC coils, Diodes work great and if you need a faster off time, then you can add in a zener, but why do that when a transorb will do the same job with one component. For AC coils, the Transorb was the best performer. With RC SPDs you couldn’t predict what level of protection you would get because it varies with frequency, and Varistors degrade with every hit they take, WILL ultimately fail, and have a clamping voltage that is typically 300% of where you really want to be which make them not an ideal choice for managing repetitive spikes like we get with contactor coils. Hopefully that gives you a better idea of what your options are and how each one works. Mostly, we hope you understand how incredibly important it is to use SPDs to keep those high energy voltage spikes from ever getting to your PLC. And don’t forget – while we were talking about contactor coils here, this applies to ANY device with a coil – Solenoids, relays, motors, etc. If you need any help, please don’t hesitate to contact Automationdirect’s FREE award winning tech support during regular business hours. They will be happy to help you out. And don’t forget the forums. There are lots of folks there that love to share their years of experience. Just don’t post any questions to Tech Support there – they don’t monitor the forms on a regular basis, so they may not see the question.