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Fernando_PLC February 16th, 2018 05:46 PM

No power in Output Modules - SLC 503
Hi everyone.
I am new here in this page. This is my first post. I hope to receive the best suggestions and tips wanting also to collaborate according to my experience.

I bought an used SLC 503 with controller and 8 modules (3 out of them outputs). I am also working with a Virtual Machine.

To test the outputs I did a small program. When I run it, everything is OK except that there is no power in the outputs activated. The ladder runs OK, the module (physical) shows the leds of the activated outputs, but when checking the voltage with a multimeter in the outputs, there's no voltage at all!

See the pic here

Thanks in advance for your help and comments.

Steve Bailey February 16th, 2018 05:51 PM

Output modules require external power. You will need to supply DC to it. The LEDs get their power from the backplane, but the field devices do not.

Aabeck February 16th, 2018 05:56 PM

First, are you expecting the output card to provide the power? Or do you have power wired into the proper terminals?

Also, is it a relay, TRIAC, DC source, DC Sink or TTL output?

A TRIAC only switches AC, DC Source only controls the positive of 24VDC, DC Sink only controls the negative, and TTL outputs are 3.3 to 5VDC. 120VAC to DC outputs would burn them up instantly. Also pull; the cards and check for obvious damage - blown transistors, TRIACs, diodes, etc.

Fernando_PLC February 16th, 2018 10:18 PM

ey Aabeck,Thanks a million for helping me. As a beginner and also with some basics in electronics, I usually struggle to solve a problem .
You're right. I thought the output module would provide the 24VDC.

Now, in the module it is written "Output Voltage 10-50 VDC". So, does it means that I have to add an aditional VDC voltage supply? If so, 10-50 VDC means, I could use a power supply of 12,24 or 48 VDC?

NOTE: I got some 4PTD relays, 24 VDC, 120 VAC since I thought I could energize the relays with the output of the module (24VDC)... but now I know the module itself is not a voltage supply.

I've also checked the output cards and eveything's ok.

Once again, thanks.


Module 1746-OB16

Fernando_PLC February 16th, 2018 10:22 PM

You're right

Originally Posted by Steve Bailey (Post 769011)
Output modules require external power. You will need to supply DC to it. The LEDs get their power from the backplane, but the field devices do not.

You're right. I had no idea. That changes everything. Tomorrow I'll go to a school to see the wiring of a similar PLC and have a better idea.

Thanks a million,:yeah:

Aabeck February 17th, 2018 05:09 AM

Yes, the DC voltage can be anything between 10 to 50 volts.

Being -OB16 they are DC Sourcing outputs, so wire the positive to the VDC terminal, and negative to the DC COM terminal.

For testing (and low power requirements) the SLC power supply has 24 VDC outputs on it,

Geospark February 17th, 2018 05:26 AM

Hi Fernando and Welcome to the Forum!

The best advice I can give you, as a beginner to PLC's and here on the Forum, is to always provide the full catalog number for the hardware in question within your posts. That way we can better and more quickly understand what you actually have and then provide specific advice as to how you may achieve your goals.

Your photo shows a 1746-OB16 output module, but if readers don't view the photo, or cannot view it, then they will not know which output module you are having issues with. It's better to type it here so we are all clear.

For the 1746-OB16 10-50 VDC DC SOURCE transistor output module wiring, refer to page 24 of the following document...

SLC 500 Digital I/O Modules Installation Instructions

What you are missing, if I'm viewing your first photo correctly, is simply the 24 VDC supply, as Steve has advised. The supply should be wired to the VDC (+) and DC COM (-) terminals of the module. These should be indicated on the terminal designation sticker on the door of the module, if the correct sticker for this module was applied originally. The stickers can be mixed up at installation or I've come across module doors that were all removed for wiring and then incorrectly refitted, mixing them up. Just something to watch for here or in the future.

You should not need to go to the school to view how another module is wired to finish the wiring, but then again, it never hurts to see a working example for reassurance.

Another thing to watch for is the input modules. I'm not 100% sure but again, from your first photo, they all appear to be DC SINK? If so, then they will receive a positive (+ 24 VDC) signal on each input point, as I'm sure you would expect. But if any input modules happened to be DC SOURCE, or you run into any in the future, then they would receive a negative (- 24 VDC) signal on each input point. This is as opposed to the DC SOURCE output module you are currently working on, which is sending a positive (+ 24 VDC) signal to the field devices. Being SINK or SOURCE has a bearing on how a module's supply is wired to it. Refering to the document above is a better way for you to see some examples of this.

Input Modules:
DC SINK = Receive + signal
DC SOURCE = Receive - signal

Output Modules:
DC SINK = Send - signal
DC SOURCE = Send + signal

It's easy get these mixed up and many users regularly do.

That's a very basic introduction to the Sinking and Sourcing characteristics of these particular manufacturer's I/O modules, but hopefully it points out enough to keep you on track.

EDIT: Apologies for any repitition Aabeck. I just saw your last post as I finished this.


Aabeck February 17th, 2018 10:30 AM


Originally Posted by Geospark (Post 769041)
Apologies for any repitition Aabeck....

Repetition is the key to learning something, otherwise grade schools K-12 could be cut down to about 3 weeks!

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