Interposing Relay(s)

I am looking for some info concerning the use of Interposing relays, I am using a PLC 5 with 1771 OAD outs to control 12 solenoids. I have heard and read that the inductive "flyback, kickback, EMF" could/will smoke the output channels. BUT I have also read (AB wiring and grounding principles) that using a solid state output with a relay would then ALSO require a surge suppressor / snubber. The relay real estate is not a problem, but I don't want to create a maintenance problem by installing extra hardware to fail and mechanical devices at that. I understand the effect of the inductive kick and have no probs fusing / relay / snubbing as needed but why waste the effort if the 1771-OAD CAN handle the solenoids without further hardware. All replies welcome. Thanks folks

Use transistor outputs with relays fitted with flywheel diodes so that you do not smoke your transistors. I have been using this method for nearly 10 years and have not smoked an output yet.
Beware of SSR outputs. Unless the load is high enough they generally do not turn off properly.
I found the Omron G2R relays, with diodes, to be very good and reliable. Have also used Izumi with success.
beerchug

No Problem

I haven't had any problems using AB cards powering solenoids, and have been doing it for the last seven years on thousands of solenoids. I have had the problem where the card will leak enough voltage [no real current though] to fire or hold the relays in when the output is off. We solved the problem by adding a shunt resistor to "produce" a load, but that was several years ago.

I would though strongly recommend wiring all your outputs & any analog inputs to illuminated fuse terminal blocks. Yes, they add a pretty penny to the project, but they will save you [and your customer] tons of time down the road. Almost every plant I work in the field devices are in a "wash down" area, and you can't make something water tight for some operators. When the device grounds out, you only lose the fuse not the card. The cheapest place I have found illuminated fuses is at automationdirect.com - they are a little flimsy but over all fairly good.

Bottom line - shouldn't have any problems with 1771-OAD & solenoids. Stongly suggest using fused terminal blocks.

okiebob

Triacs are sensitive to over-voltages and special care must be taken to prevent any excessive voltages during normal operation, i.e.; Inductive loads. Even though most new Triac output modules come equipped with snubber circuits or use snubberless Triacs, it may be a good idea to use a varistor in your circuit to protect the circuit against possible overvoltages. (Inductive Kick)
You may feel the varistor is over-kill, I would not argue the point.
If you experience spurious firing then you may need to calculate and install your own snubber, otherwise the 1771-OAD should serve you well.

Roger

Output modules are not made of solid gold. Over the years I have used thousands of output points.

Say in the last 5 years I have used 5000 ouput points. At $10-$20 per output for a relay, that is $50000 to $100000. I know of maybe a dozen, tops, of lost output points. Even if the entire modules were replaced at $250 a piece that is that is only $3000.

Using the $50000 figure, I would have to have had 200 bad outputs in that time to justify.

I may be wrong on the cost of an installed relay, correct me if I am.

I tend to use relays on my ouputs to valves and starters as standard practice. This isn't to protect the outputs, though. If you verify the load is within the output specs, as Rick says you shouldn't have many failures. I use the relays for two reasons. First, I don't always supply the field device the output is controlling, and I had a few unpleasant surprises finding 120 VAC control where I tholught it was going to be 24 VDC, or the opposite. Second, I use relays (Turk) with LED indicators and latching manual overrides. This saves me enough time in trouble shooting and enabling manual operation for emergencies to more than make up for the $10 to $20 cost of the relays.

A lot of relays can be purchased with built in or plug in surge suppression if you need this.

Good points, Tom.

I almost always know what I am hooking up to the output.

My main point being, don't "always" do something without knowing why you do it. And if it has a real benefit.

"An engineer is someone who can for a dollar do what any fool could do for two." as the saying goes.

Interesting views on interposing relays. I am often opening and closing ciruit breakers, fuel solenoids etc. Mostly the supply comes from the other device. Relays essential then. Circuit breakers can be nasty. Large inrush etc so relays are for safety as the C/B coils are nearly always 24VDC. Sometimes even good quality relays have trouble putting out the arc.
Yes, LEDs on the relays are very useful when commissioning. I curse and swear when someone else has designed and built the control panels and gone cheap on the relays. 50-100 of them and one is "clicking" in a hard wired circuit. You are down on your knees trying to find the relay that is active. Takes 4-5 times longer to commission the job. I normally have to get in, do the business and get out so everyone can get paid. Enormous pressure sometimes and one needs all the help one can get.
Always use LED indicating lights on switch boards (24vAC/DC types with rectifier built in) and have had no problems whatsoever.
I have worked with high voltage circuit breakers many times and they are usually 48VDC. Extremely nasty things. Have to use a fair sized contactor and connect the 3 poles in series to quench the arc. One of my staff wired the correct sized contactor in parallel on day and we watched the poor little dear burn itself to a cinder due to the DC arc. No one could go near it until the arc went out. That took about 1/2 minute. Set off smoke detectors and all.

There are many point of view in my mind. One is 'standard' output-units 24VDC allways, in anycase. Pneumatic Solinoids 2-5W have controlled directly by transistor-based outputs as well LED-lights.
Valves with more power as hydraulic or AC-solenoids (fie!) needs interface relays as Omron G2R-series as BobB told. And finally motor brake-solenoids need contactor-type relays, usually rectifier is in the motor connecting box.
In principle AC-solenoids need Over-Load realys as Motors, becourse if AC-solenoid stucking, it burning with small inductance.

One other thing I forgot to mention, I have always used interposing relays in every project. Allen-Bradley or Potter Brumfield brand with LED indication and suppression.

Roger

Even me here, i am always relied on having an interposing relay on our d/o cards. We are using four brands of PLCs, Siemens, AMB (Italy), Allen Bradley, and Cegelec (now Alstom) and most of them were using a relay on their output. But as far as inductive load is concern, we never yet experience having a problem on the cards due to static or inductive kick on our cards. Among these four PLCs, only AMB is using snubber on their output relay. Even the design of Siemens for their strapping machine (signode), their are not using any snubber on their system. Thats why most of my drawing here for PLCs specially for the output is having a relay but without any snubber.

By the way, here we are always experience a problem on our communication system. I remember before we remove all the cables of our PLC panel (our company purchased it secondhand in U.K.) including the communications cable, I saw a snubber connecting parallel to its terminal. But I forget to takenote the specifications of that item, so until now we are not installing it. Thats why we have always a problem on our communications, sometimes the watchdog relay failed. Some of them are more than 20 meters apart and yet we are still not using any other devices for this. Is there any amplifier to boost up the power of our communications specially those 30 meters or 50 meters away from the CPU? How can we determine the value of this snubber (just incase needed) to our communications cable?

All of your suggestions will be highly appreciated.

Thanks

What is the communications system? Modbus or what?
beerchug

BobB said:

What is the communications system? Modbus or what?
beerchug

Click to expand...

I dont know if your gonna believe me... We have two mill here, our one mill is not yet automated, we are still in hardwiring processes. But the other one, we have been managed to automate it even up to level 1.5. Our company bought the secondhand PLC Panel as well as the Software in U.K., Sheerness Steel and the software is Cegelec. About the communications, we're not using anything like profibus, modbus or ethernet...we are only using "shielded cable " for our communications. First they laid the cable together w/ the control wires, but after our "free lance" programmer from U.K. found that, he told us to make a separate route of that cable. (maybe due to voltage drop and noise) The problem was solved but not eliminated. Do you think that using "shielded cable" is enough, if not, what other means we're going to do to minimize that problem? Takenote, our manager doesn't want to change it because of cost cutting.... (please refer also to my previous post...)

Thats why on our R1 project, we are proposing using profibus or ethernet form of our communication system. Can you please advice us more on this??? Your advice will be highly appreciated...

Thanks...

The reason I asked what communications network is that there are normally terminators on the network. On Modbus, for example, I use 120ohm resistor in series with a 100nf ceramic capacitor as my terminator. This is as recommended by GE Multilin for their Modbus connection. The cap helps take noise out of the line. That is probably why the "snubber" as you called it was there. To terminate the comms line and reduce noise.