relay out vs sink/source

[smokym1 0]

I am driving 24VDC solonoids. All use the same supply and commons. What are the decision keys for choosing an output module? A snubbed relay module has a life of 100K cycles. Does a sink/source have more relibility? Cost of relay modules seem to be slightly higher. Am I missing something obvious. Thanks

[Ken Moore 23]

Relay output cards: Handles higher amp loads Usually has multiple commons, so one card can handle mixed voltages. Solid state: Longer life, faster response times, cheaper to buy.

[smokym1 0]

Thanks for the prompt response

[Chris Elston 79]

I would double check your WATTAGE on your coils. Make sure you not going to be stressing your digital ouputs with too high current draw.

[gravitar 3]

IMHO, the benefit to relays is FLEXIBILITY. As it is literally two pieces of metal that either touch or don't touch, the relay doesn't care what you pass through it. (Well, within limits). So if you want to interface two pieces of equipment that probably operate at different voltages (or the same voltage but from a different supply), you would want to use relays. You don't have to worry about PNP or NPN either.. the contacts don't care! Also, many devices expect "contact closure" input devices.. In other words, the device provides the supply voltage AND the input terminals.. you just put the two together to make the circuit. the above are examples where relays are (probably) the best choice. But like you said, they have a finite life span. After all, they are mechanical devices. So if there isn't a clear advantage to using a relay, pick a solid state output instead.

[larry818 0]

The problem I find with both kinds of outputs is that they are very easy to kill. Solid state outputs especially so. If you have them hooked to small relays, and they're close by, you're probably ok. Usually the power supply for either the SS or relay outputs is more than large enough to kill the transistor or vaporise the pcb trace connecting the relay. What I've taken to doing lately is to use a transistor output plc and connect it to a bank of external "ice cube" relays. That way the plc is fairly immortal and the relays are easy to replace or rewire (I always use the 4 pole relays, regardless if I use only 1 pole). I've been stuck on job sites (Jamaica, for example) where I could easily get a new relay but the plc part was at least a week away. I never hook the plc outputs to any wiring that leaves the control enclosure.

[TConnolly 13]

Good advice. Many hydraulic valve solenoids are in the 35-40 watt range. This is just a shade too high for many discrete outputs cards. Some pneumatic solenoids are a bit lower. If you are connecting directly to a device outside the control cabinet I recommend individually fusing the output or using one of the more advanced cards provided by some manufacturers that have onboard self-resetting breakers and current limiting. Edited 11 Aug 2005 by Alaric

[gravitar 3]

Hmm. interesting. So I take it you've had problems even in circuits with "properly sized/rated" fuses on each output line? If so, do you know how the failures occurred?

[gravitar 3]

Hydraulic solenoids are evil.. Especially when they are 24VDC! I gave up and used slave relays on a hydraulic valve bank that I originally intended to drive from a micrologix's internal relay outputs. The cut sheets said they'd work, but long-term reliability just wasn't there. Air valves are usually much more forgiving.. the coils are ususally in the 2-10W range.

[OIDMBTRTECH 0]

I like it when interposing relays are used. I wish every engineer used the 4 pole relays for all interposing connections. That way there is a spare or 2 when one set burns. I would also like it if the load current was checked too. I have seen many burned contacts because the load it higher or the flyback from DC solenoids arc too much. Larry818 In my experiences with plc wiring outside the cabinet, broken conduit, people climbing and using the wiring for steps are amoung the reasons for problems with output cards. I have seen damage done to wiring conduits while mechanical work is being done. Some think conduit is really strong and can be used for anchors, lift points and other things. Having a readily replaceable relay is SSSOOO nice. Some customers just do not understand why you do not have that PLC module in your parts kit.

[TConnolly 13]

I installed two PLCs in Italy about three years ago where I put interposing ice cube relays between the PLC and the hydraulic solenoid valves. Since the plant did not have a strong maintenace force, especially one with any PLC knowledge, I used ice cube relays that had a small mechanical over-ride button to manually actuate the contacts - giving the the ability to in essence to "force" on a device without access to the PLC. There has been a few times since then that I was glad I put those in as they saved me a trip.

[mconner 0]

Hey All, Kind of a side note, What is the excepted norm for PLC SS "relays". NPN or PNP? We have started designing/building a lot of automation cells and we would like to standardize our sensor types. Thanks, M.C

[GerryM 2]

As far as PNP or NPN read the article below. It is very good. Check out this article As far as interposing relays go, I never use them because I haven't had to except in rare circumstances. I never use DC solenoids only AC 120V. I use a fuse and I've never had a problem with blowing outputs. I size the outputs so that they, relay or SS, are very conservative vs. the current draw. I must say if I ever use DC Solenoids I may reconsider. I certainly understand why many of you use them, I just haven't had to yet. As far as choosing relay or ss. I like relays for the flexibility, and ss for their long life and ability to handle short AC coil surges. Don't forget to calculate how long the relay card will last at 100K cycles. You may need ss for that reason alone. Edited 12 Aug 2005 by GerryM

[gravitar 3]

That is a very good article for everyone to read. But as far as NPN devices, I myself don't like them, but they do seem to be the norm in certain regions and/or industries. Take robots for example. I can only personally speak of Motoman robots, but NPN is the standard on the ones we've got. I don't know if that's because they're from Japan or if that's just what robot manufacturers use, but it is a PITA for us to interface to and always leads to longer troubleshooting time, as we recall that the current isn't supposed to flow the same direction as everything else in the plant :) So an individual might have to (or find it advantageous to) use NPN switches for a particular job. Guess it all depends on what you're working on and who you're doing it for.

[entertron 0]

We did an article a few years ago, regarding the advantages of controlling solenoid valves with our solid state output, compared to relay outputs. We then went a step further in comparing our design with other PLC designs. When determining the best method, the answer is - it depends. It depends on how often you are cycling your outputs. It depends on the controller you are using and the capability of that output. Not all controllers and outputs are created equal. When you are dealing with many controllers that offer a transistor output of a half amp or less, then a solid state relay is usually required. You obtain the current driving cabability you require and maintain the solid state reliability. However, you add addtional cost to your application because the controller was insufficient to begin with. A typical PLC relay output is 2 amps. That should suffice for most solenoids, but now you have to concern yourself with life of the relay. The greater the inductive load, the shorter the life of the relay. Solenoids are inductive devices. Here is a link to our article. http://www.entertron.com/press5.htm At the time of the article we used our triac design. The same can be said for our transistor design...both are designed with similar ratings and capabilities. Hope this helps. God Bless,

[larry818 0]

Well, firstly let me say I hate fuses. Usually what ever fault condition that blows the fuse is never corrected before the maintenance drone runs out of spare fuses (if they even bothered to keep track of them) and in goes the foil wrapped fuse. Essentially I view fuses as no protection. I've even done this myself when working in an area that a replacement fuse is not readily obtained. Of course I've fixed the fault first and sent a fuse later, but one can't really delay a repair a day waiting for a fuse. The fault modes are too many to list, since the factory folk are always looking for new ways to short wires. I had a job where a guy was hanging plumbing on a wall with a 12" long drill bit. It didn't occur to the drone to stop drilling when he was in a couple of inches, but drilled thru the 5" wall, 3" space behind the wall, into an enclosure, and into the wiring therein. He managed to connect the 480v feed to the 24vdc source. Oh, yeah, he did manage to drill thru a 120v conduit in the wall. When he hit the 480v he not only toasted most of the stuff in the enclosure and sensors on the machine, but also a lot of the electronic stuff in the office. This is America, however, so he wasn't fired. What I do is to use a breaker protected 24vdc supply and just wire everything downstream to accept the amps of the breaker. Remember, fusing protects wires, not devices. Usually the fault is easy to find, something like a crushed or sheared conduit, and the fault is easily capable of shorting the full amps of the breaker. Giving a maintenance dude a breaker to push is better 'cos they'll push it a few times, watch it trip, and then go looking for the fault without destroying anything. Usually the weakest link in a control circuit is the *maybe* 3 amp pcb trace in the plc, on which usually 4 relays are ganged. Supposedly there are breakers that trip faster than the trace can blow, but I haven't had good luck with them. They are skittish and nuisance trip. Lately I've been using a kind of hybrid to the in/out relay config. I'm using the Mitsubishi FX2NC plc which is transistor output, but ribbon cable connected. I've made circuit boards with replaceable 5amp relays on it (four to a board, and the boards are daisy-chainable up to four in a chain), 5 amp circuit breaker on the common, and pcb traces that can handle 5 amps (about as big as one can go and still connect it to terminal strips). This eliminates all the messy wiring from plc to relays and allows me to use a 10 amp or larger supply and have downstream 5 amp protection, so I can use smaller wires (sensor cords burn spectacularly at 10 amps). Why the plc manufacturers don't offer something similar is beyond me. All the old plcs from the early days (1970s) had monster traces, but still fuses. Just as an aside, it seems similar problems exist in the mechanical end too. The machines I design are all strong enough to withstand the full output of the drive motors, such that if something jams for any reason, the first thing to go is the overcurrent protection. I'm amazed at machinery that's strong enough in drive and weak enough in structure that it can destroy it's self.

[entertron 0]

Larry, Seems like a lot of additional work when there are products out there that can do what you are needing.

[larry818 0]

Such as?

[entertron 0]

Larry, We have one product that handles up to 7 amps (relay restriction) with a common rating of 8 amps. We have another product that incorporates 10 amp relays and is rated for up to 8 amps (common restriction) The relays are not pluggable, due to vibration concerns, however all terminal blocks are pluggable...for easy installation and replacement. Both products come with a 5 year warranty.