Short Question about 1769-OA8/OA16

[liberati]

A very simple question but I have been told different things by different people and wanted to see what the consensus is from the community.

Using a 1769-OA8 or a 1769-OA16 to power things like motors/lights/etc.

One group has said that you should never do it and you should use the output the energize a relay that would control power to said motor/light/etc.

The other group has said you can use it to drive the output but only if the current pull is small.

In the wiring diagram of the 1769-OA8/OA16 it does indeed show a CR ( what I believe to be a control relay ) in between the output of the card and what you are trying to power/drive/run.

The second group that I spoke with has more experience but like I said I would like to have a better understanding of it. I have read the module's spec papers but still don't see any real signs of which way this could go.

Also if I am just blind and missed something completely please let me know where I could have looked.
Thanks,

Just a Greenhorn

 

[Geospark]

Short Question, Long Answer...

Hi and welcome to the Forum!

Ok I'll kick things off...

You will probably get varying opinions here as well.

Some of us prefer to use interposing relays(CR) for all outputs, some like to only use them for specific types of outputs and some don't bother at all. The choice is really yours in the end.

Personally, when installing from new, I prefer to always use an interposing relay. But I also have to work everyday with older outputs that don't have them.

Technically, there is nothing wrong with switching those loads directly, once the output is sufficiently rated to handle it and any adverse effects are sufficiently suppressed.

The main reasons we use interposing relays is to switch higher, noisier loads that the output cannot handle directly and also to isolate the control system from the panel or field wiring. This helps save the output, module, backplane or controller from any adverse effects from back induced transients from the in-panel control gear, or field wiring.

Your particular output modules, 1769-OA8 and 1769-OA16, are 100 - 240VAC solid state TRIAC outputs for switching AC loads. They can draw a maximum of 0.5Amps per output, up to a max of 4Amps, and 8Amps respectively for the whole module. These ratings are more than sufficient to directly drive most low load motor starters, control relays, indicator lamps, etc.

With TRIACS, because they only switch low and not entirely off, there is always a small leakage current through the output. If you're driving larger loads directly, within the outputs current rating, this leakage current usually has no effect. But if you're using interposing relays with a small enough coil current rating, this leakage current can hold a small coil energized after the output has switched low. For this reason, it's recommended to use a loading resistor across the relay coil to absorb the leakage current. For these particular output modules, AB recommend you use a 15KΩ, 2W resistor when supplying 120VAC and for 240VAC use a 15KΩ, 5W resistor.

Another thing to consider when switching inductive loads from TRIAC outputs is surge suppression. If you're wiring TRIAC outputs to control circuits in a panel enclosure and you use an interposing relay, then you do not need surge suppression across the relays coil. As the TRIAC is not classed as a hard contact, i.e. not opening and closing, there is no transient EMI being generated here.

However, if you wire TRIAC outputs directly to control gear, such as a motor starter coil, then there is often a good chance that another source, such as a manual hard-wired switch (hard contact) might be connected in series to the load. Or the output could be wired in series with the switch before the load. In all these cases where a hard contact is in circuit with the TRIAC output, you should use surge suppressors, as close as possible to the load, to save the TRIAC output from the generated transient EMI.

The "CR" control relays that AB illustrate in their diagrams are just to represent a typical load for the outputs. It not really to indicate that they recommend this as best practice.

It's often argued that interposing relays are too costly to implement, but the flip side is, what's the cost when an output module or PLC is blown?

As I said, the choice is really yours in the end.

G.

 

[liberati]

Thanks for the detailed response.

So know I understand why some are fine and others aren't. It's more of better safe than sorry just in case.

So if I understand you correctly as long as I am not pulling over .5A per output or 4 A total I should be able to drive small lights/motors/sensors without issue then?

Also thanks for the quick response, I read your post yesterday but was extremely busy and apologize for not getting back sooner.
Thanks,
Just a GreenHorn

 

[Geospark]

You're ok, we're all busy, which is no bad thing!

If you are choosing to go without interposing relays, that's fine. Just note the following:

Normally you don't try to drive very low loads (25-100mA) from TRIAC outputs. These extra low loads can fall below the minimum holding current and the TRIAC can switch OFF. You can add a bleed resistor in these cases to provide the minimum load.

The current rating of the outputs on these modules is temperature de-rated. The 0.5Amp rating is ok up to 30°C(86°F). This should be fine for most normal working environments. Once you go over the 30°C(86°F) mark it starts to de-rate down to 0.25Amp at 60°C(140°F).

Even if you're well under the 30°C(86°F) mark, try not to load the outputs too near their current rating anyway. This helps to save excessive wear on the outputs and allow for potential in-rush and overload currents.

TRIACS, in general, are more susceptible to electrical interference than relay or transistor outputs. Try to only drive loads inside the same enclosure as the PLC to reduce the potential risk of damaging the outputs.

Don't ignore the surge suppression advice. Depending on the nature of the wiring, inductive loads, and switching frequency of the outputs, you can wear out the outputs quicker than you'd think. It's good practice to just use surge suppression anyway.

Which ever control gear manufacturer you are using for starters, relays or indicator lamps, look into their offerings for surge suppression. They may offer optional or built-in suppression.

I assume when you keep referring to "lights" you mean indicator lamps and not incandescent or fluorescent lighting in a panel or on a machine?

G.

 

[Bullzi]

I tend to use Interposing Relays for most of my outputs. The only ones I don't use relays on are if the output is going to a pilot light on a panel or as a input to another controller/PLC. My rule of thumb is if there is any load over .5A or there is a potential for spikes or noise the output gets a relay.

I will say that most of the projects I have worked on I use Relay Output Cards like the 1769-OW16. I do this for a couple of reasons.
1. I like to use small terminal block relays. The leak current from TRIAC outputs can play havoc with those small coils.
2. I have found that relay outputs offer much more flexibility for my projects. It never fails that the moment that I have a job installed I get a call from the customer saying "could you send a signal from your PLC to the PLC in X location?". With a relay outputs I have my options open. I can install a Interposing relay if I want or I can have them connect it up directly to the output. I also don't have to worry about if the signal is AC/DC, sourced from my PLC or from theirs. I am good either way.

Of course Relay Outputs have some drawback too.
1. They are rated for only a finite number of cycles. So if you have a process that activates outputs 100's of times a day a relay will fail much sooner than a TRIAC output. My applications only have each output activating maybe 50 times per day at the most.
2. If you have isolated relays there is more wiring involved. You have to provided some kind of power to the common of each output. TRIAC Cards usually have one or 2 common points to put the power.

Like Geospark said the choice is up to you.
Hope this helps.

 

[Geospark]

...I also don't have to worry about if the signal is AC/DC, sourced from my PLC or from theirs...

Of course, I forgot to mention another main reason why we use interposing relays...

If you need to switch different AC voltages to the TRIAC outputs, or even DC voltages, then you will have to use interposing relays.

It's good practice, when interlocking signals between different PLCs, panels or systems, to interpose the signals, even if switching the same voltages, so as to isolate the two from each other. Nothing as bad as finding a live wire on an input when you've isolated the MCC. Colour coding is important here as well for interlocking wires.

G.

 

[liberati]

Thanks for all the help.

Yeah all I am trying to do with it is power an alarm light that draws at max 320mA which from the information that you guys have given me seems fine.

Normally I would try and use relays just for safe/good practice and I may still do that. I just like to know what can or can't be done and then even if something can be done why it should be done one was as opposed to another.

Once again thanks for all the great help.
Hopefully it won't be long and I can start giving back to the community.
Just a GreenHorn

 

[Geospark]

...I just like to know what can or can't be done and then even if something can be done why it should be done...

We all have our opinions, whether we're right or wrong, but you should always seek advice when you have conflicting information. The more opinions you receive, the better you can decide for yourself.

Don't think of yourself as a greenhorn, but more of a green bud, destined to blossom into something wonderful. :nodi:

G.

 

[liberati]

Hah, well thanks.
I stick to the GreenHorn until I get some real experience under my belt. After that I'll upgrade my name