Interposing relays

[Malador]

I've read two or three post on this PLC Talk where it was stated that 24 VDC powered relays were preferred for interposing relays. A few questions come to mind after reading that. I did Google search and consulted some text we have at work. but didn't find anything on the topic.

Wouldn't 24v AC be better, because of the zero crossing?

If 24V is preferred over higher voltage 120V because of reduced arcing, wouldn't 12v be even better?

[Zane_Terblanche]

I believe in general most of us use 24vdc relays on our outputs purely because most of the PLC we use feature 24VDC outputs.

[Malador]

Originally Posted by Zane_Terblanche

I believe in general most of us use 24vdc relays on our outputs purely because most of the PLC we use feature 24VDC outputs.

That makes sense. I was also thinking that a lot of PLC's use 24vdc Power supplies, so it you'd all ready have it in your panel. I knew i was probably over thinking this, but thought i would ask anyway.

[Zane_Terblanche]

There may well be other more technical reasons why. Hopefully some of the other guys give their opinions too.

[g.mccormick]

The voltage of the relay needs to match whatever voltage the control for the relay is.

If you have a PLC that has a 24vdc input, and you want to know when a 120vac signal is on, you'd use a relay with 120vac coil. The 120vac signal would energize the relay coil. 24vdc would go through the contacts of the relay and go to the 24vdc input of the plc.

If you want to turn on a 120vac with a 24vdc plc, you'd use a 24vdc relay. The plc 24vdc output would energize the 24vdc coil. 120vac would go through the relay contacts to the load.

[Geospark]

Originally Posted by Malador

Wouldn't 24v AC be better, because of the zero crossing?

I'm not sure if your fully understanding the concept here?
A 24VDC output cannot energise a 24VAC coil on a relay.

Originally Posted by Zane_Terblanche

I believe in general most of us use 24vdc relays on our outputs purely because most of the PLC we use feature 24VDC outputs.

To simplify this even further Malador. You use whatever voltage the output card is switching.


You probably know this, but to be clear:
The current rating of discrete outputs tends to be low as they are usually only transistors or micro-relays. So field devices such as solonoid valves or large contactor coils would potentially place too great a load on the low current rated outputs. So we use interposing relays to bear the load instead. These relays amplify the current rating that the output cannot handle by providing higher current rated switching contacts. So the output energises the low load coil, which switches the higher rated contacts that handle the high load field devices or panel contactors. As long as

a)the wattage of the interposing relay's coil is low enough to draw a smaller current than the output is rated for

and

b)the rating of the relays switching contacts are high enough to handle the current of the controlled device

then all is well.

There is no "What voltage is best?" when selecting the interposing relay coil voltage. Once your output voltage has been selected at design stage your interposing relay coil voltage is now fixed. What you interpose(switch) through it, however, is entirely up to you.
If you have a preference or regulation as to what output voltage you want/have to switch at, then you can select whichever PLC output voltage you prefer or have to.
This tends to be mainly 24VDC, but could also be 12VDC, 12VAC, 24VAC, 110VAC or 240VAC, and others around the globe I'm sure.

Same goes for inputs - if you have a field signal of 24VAC, but your input is 24VDC, you would energise a 24VAC relay coil with the signal and switch 24VDC through its contact to the PLC.

G.

p.s. sorry g.mccormick, you got there before me...Or whatever g.mc said Malador!

[Malador]

Originally Posted by Geospark

I'm not sure if your fully understanding the concept here?
A 24VDC output cannot energise a 24VAC coil on a relay.........

I was mainly thinking about relay outputs. I apologize for not stating that in the original post

Originally Posted by Geospark

I'm not sure if your fully understanding the concept here?

There is no "What voltage is best?" when selecting the interposing relay coil voltage. Once your output voltage has been selected at design stage your interposing relay coil voltage is now fixed. What you interpose(switch) through it, however, is entirely up to you.
If you have a preference or regulation as to what output voltage you want/have to switch at, then you can select whichever PLC output voltage you prefer or have to.
This tends to be mainly 24VDC, but could also be 12VDC, 12VAC, 24VAC, 110VAC or 240VAC, and others around the globe I'm sure.

G.

The reason I asked is because I've seen it multiple post where it's stated that ideally interposing relays are 24vdc. I couldn't see an obvious reason for this, so I asked. You statement suggest it's preference, which is the best explanation I think.

[Malador]

I was also talking about using interposing relays for the purpose of carrying higher current loads, rather than providing a different voltage. I really didn't articulate my question well I guess.

[blueyedpeter]

Some of the older Hitachi hizac plc's are very old and there relay outputs have been switching 240 volt output for almost 30 + years now .
And back then plc implementation was a little scratchy on the how too side of things , but never the less they are still clocking along even now .
Even though they are well past the use by date .And lower voltages are sometimes better but they need more interfacing with the real world , through relays and then there's more connections to break down with .
But as it was pointed a lot of current switching devices are 24 volt dc if not all , there are only a limited number of intrinsically safe components that will work on less than 24 volts , usually its around 6 to 12 volts for hazardous locations or environments . Any how enough of the woffle from me cheers and have a good day .

[iant]

Originally Posted by blueyedpeter

Some of the older Hitachi hizac plc's are very old and there relay outputs have been switching 240 volt output for almost 30 + years now .
there are only a limited number of intrinsically safe components that will work on less than 24 volts , usually its around 6 to 12 volts for hazardous locations or environments . Any how enough of the woffle from me cheers and have a good day .

I am confused - interposing relays are generally used due to the current limitations of relay or SSR output cards - Hitachi Plc's are very old but when we were designing control systems we still looked carefully at the current.
As far as Intrinsically Safe - that uses a barrier, the field device is about 8V the primary side of the barrier is what ever it is purchased at - personally I would keep to 24Vdc.

As far as the OP's 24Vac / 24Vdc selection
some plc's have a limited current DC supply available(1A max)

[Geospark]

Ok Malador, understood. You were asking what interposing relay coil voltage should you switch through a relay output to reduce arcing on the relay output's contact.

Originally Posted by Malador

I was mainly thinking about relay outputs...
it was stated that 24 VDC powered relays were preferred for interposing relays...
wouldn't 24v AC be better, because of the zero crossing?

Sorry to reverse paraphrase your statements Malador, but I still think your a little confused about this topic.

Whether outputting to 24VAC or DC coils on interposing relays, zero crossing is not relevant to RELAY type outputs. Under normal operation, when the output goes off, the relay output will open an electro-mechanical contact and the output voltage will be off. It does not require a zero cross as the interposing relay coil is not forming part of a circuit that needs to be driven low.

Zero crossing is only relevant when dealing with TRIAC type outputs. They are solid state outputs used for AC loads. They require the zero cross to switch the output to the low state, not off or open, just low. This is why you can still get leakage current on triac outputs and close to full high state voltage at the load. Zero crossing also has nothing to do with arcing.

So when selecting which interposing coil voltage to switch through a relay output, most select 24VDC as they usually have that power supply available in the panel. You would have to put in a transformer or suitable PSU to supply any other voltage if not already available.

Originally Posted by Malador

I was also talking about using interposing relays for the purpose of carrying higher current loads...If 24V is preferred over higher voltage 120V because of reduced arcing...

With inductive loads, like relays, solonoids and contactor coils, when opening a circuit under current the voltage can spike. This creates the arcing. Without surge protection, relay contacts can pit from arcing, generate electrical noise and sometimes even weld closed.

If switching outputs from relay or transistor outputs to interposing relays, or direct to the field you should add a diode for supression across the load as close to it as possible. This prevents a high back voltage to the PLC which could potentially damage the output, or worse, the PLC itself. If using triac outputs you would add a snubber which is a resistor/capacitor(RC) supressor. The capacitor is needed to slow down the rate of rise of the voltage as the contact opens, so as to prevent the back voltage spike from firing the triac high. Also both prevent back EMF's who's resulting current could damage the PLC output. The particular interposing relays we use have this built in.

Originally Posted by Malador

I've seen it multiple post where it's stated that ideally interposing relays are 24vdc...

In my opinion, to say that most use 24VDC outputs may not be correct. What would be more correct to say is that the most used are 24VDC outputs.

Again, it's horses for courses. At the design stage you, your company's specification, the manufacturer's specification or guidelines, or a customer specification, depending on your business, decides what voltage(s) is(are) required to switch the intended panel or field device outputs. It may be decided to use a certain coil voltage for starter contactors, another voltage for solonoid valves, another voltage for sensors and micro-switches, another voltage for indicator lamps/horns.

Obviously, as most try to use extra low voltage to keep the installation as safe as possible and where higher less safe voltages are not necessary, they use 24VDC devices as these are widely available for that reason. But many devices are 10-30VDC rated or 100-240VAC or 10-275VAD/DC, for instance. This is so that manufacturers can provide for all preferences.

In some older installations I've worked on in Ireland, there have be 24VDC, 120VAC, and 240VAC output cards, all in the same rack, driving different groups of devices, as mentioned above. No interposing relays. That was their preference.

Interposing relays were seen as a luxury as they took up too much space, extra wiring, and added cost. Nowadays, PLC's/PAC's, cards/modules and relays have become smaller, reducing their footprint in an installation and lessons have been learned on the hazards of not protecting or isolating outputs (or inputs for that matter). So now most installers use interposing relays as a default precautionary measure, regardless of whether the output load is big or small and even when the voltage is the same. Whether they realise it or not, these efforts are primarily to protect the PLC, not the cards and modules.

So the different discrete output types are transistor, triac and relay.

If an output is switching a low DC load directly, you would typically use a transistor type output. You usually get more transistor outputs per card as the circuitry is smaller than relay.
For slightly higher loads you would use a relay type output here, but they cannot handle high speed repetitive switching as they are mechanically driven and life span tends to be shorter.
If switching a low AC load, you would typically use a triac. but these are very prone to those voltage spikes. Also as they never actually drive to a safe off or open state, they can be hazardous. They can also be held on by their own load in certain circumstances, even though the PLC has switched the triac output off.

The golden rule is to keep the PLC wiring in the PLC panel. Limit the load the outputs drive, and interpose and supress those loads.

If your dealing with arcing on the contacts of the interposing relay, then use supression on their loads.

Originally Posted by iant

some plc's have a limited current DC supply available(1A max)

Usually you would have a 24VDC PSU as part of your PLC chassis. This should only be used to power the PLC and its backplane, or I/O modules fed through that backplane.

If using alot of relay outputs, switching 24VDC, you would put in a seperate 5A or 10A 24VDC PSU just for this. Feed each relay output card from this PSU seperately through a breaker. Also feed each output through a fused terminal with a quick blow fuse rated for the intended load. This PSU can also be used to supply 24VDC input devices.

From my experience these methods are a good guideline, but everyone has their preferences.

G.

[Malador]

Originally Posted by Geospark

.........
Sorry to reverse paraphrase your statements Malador, but I still think your a little confused about this topic.

Whether outputting to 24VAC or DC coils on interposing relays, zero crossing is not relevant to RELAY type outputs. Under normal operation, when the output goes off, the relay output will open an electro-mechanical contact and the output voltage will be off. It does not require a zero cross as the interposing relay coil is not forming part of a circuit that needs to be driven low.

Zero crossing is only relevant when dealing with TRIAC type outputs. They are solid state outputs used for AC loads. They require the zero cross to switch the output to the low state, not off or open, just low. This is why you can still get leakage current on triac outputs and close to full high state voltage at the load. Zero crossinghas nothing to do with arcing...._
G.

I guess I was confused, I did not understand zero cross with triacs. I was thinking about zero crossing as it pertains to high voltage breakers.

[RussB]

Originally Posted by Zane_Terblanche

There may well be other more technical reasons why. Hopefully some of the other guys give their opinions too.

All the opinions you get will be worth all that you pay for them.
24 VDC has been a standard for a long time, the main reason it is used is because there are many devices to choose from and because they are made in large quantities the pricing is kept in line.

Good ol' capitalism!

[Geospark]

Originally Posted by RussB

All the opinions you get will be worth all that you pay for them.

Did you just zero-cross me RussB?

Originally Posted by Prov.11:14

Where there is no guidance, a people falls, but in an abundance of counselors there is safety

[blueyedpeter]

I wouldn't have thought that interposeing relays would have been such a hot topic , i think i might have to take up smoking to give me something to do on the weekends . The mention of intrinsically safe was only mentioned because of an alternate view on the world , we all know that once you step into an older factory you can and will find any combination of voltages it was not that long ago that 32 volt ac was OK .And with modern times out with the old and in comes the new , 24 volt dc ac who cares , and yes at the design stage where ever you should find your self the choice will be yours and yours alone. There are many considerations to take into account , so chose wisely and you may inherit the earth . Good luck and cheers