Power problems when going through PLC

I am looking for some advice and I hope this is the place. I fairly new to PLC's but I have been working with them for a year and have never had this problem.

I am a teacher and our school decorates a shelter at a park each year. This year we thought we would be smart and use a PLC to flash some lights and run some motors.

I have a set up that uses the Micrologix 1500 PLC with 9 Relays. Relays 0-3 are Omaron with 110V trigger Relays 4-8 are Omaron with 24v triggers. Relays 0-3 work fine but 24v relays do not. When I run them through the PLC it acts like it shorts the 24v power supply out. When I check with a multi-meter not hooked to any of the relays the PLC output shows 24v. Any ideas. I keep thinking it is a wire issue but I have checked and rechecked.

Josh

You should probably post a sketch of how your system is wired. From your description, I'm assuming that your PLC has relay contact outputs, but you should confirm that by telling us the model number of your MicroLogix unit.

Whats the source of your 24VDC? If its the controllers user power out there is only 400ma available. You may need a external power supply. See pic below

Update with wire layout

I have a Micrologix 1500 model 1764-24AWA. I have attached a wire layout that one of my students made prior to wiring the unit. It is a word file and a little crude but it gets the point across.

Could this be the issue.
I was told on another forum to add a grounding strap from the unit base along with the terminal ground that this will eliminate noise.

I have an external 24VDC power suppy for the unit. I have tried two different ones. It just doesnt make since. When I turn the PLC on I measure 24V comming out. I hook up the relay and I have nothing.

Thanks for you help.

Trying to make since of things.
Josh

Where are you connecting your meter leads? What is the current rating of the power supply? What is the current draw of the relays? If you connect 24 VDC power directly to a relay, does it work properly?

Try and hook up the relay coil straight to the Dc power supply and see what happens you should be able to here the relay click if it doesn't work I would double check to make sure the coil's are 24 vdc not vac.

Looking at your drawing, It should work ..?? Check those relays, I bet someone got them mixed up with the 110V ones !

I will have to check the current draw at the park when I get there. I do know that they are all 24VDC relays. They are OMRON MY2N-D2 24VDC Relays. I can hook up all 5 relays in parallel to the powersupply directly and they work. The problem only occures when I hook it up through the PLC. I can put my multi-meter on the output terminal and the common of the relay and it says 24.5V.


Josh

Imracing said:

... I can put my multi-meter on the output terminal and the common of the relay and it says 24.5V.

Click to expand...

Just a technicality, but you should have 24V on the COIL of the relay. Exactly how do you have the relay wired (pin numbers, etc.)?

+24VDC should be on pin 14 (coming from PLC output)
-24VDC should be on pin 13 (coming from -terminal of power supply)

Commons are on pins 9 and 12
NO connections are pins 5 and 8
NC connections are pins 1 and 4

I have the pin out as you stated. You guys have been a help. At least my thinking was correct. I guess I just need to go over everything very carefully. It has to be something minor.

Josh

Imracing,

Is it possible you have counter EMF diodes somewhere in the circuit. If you do perhaps they are connected in reverse.

Put a 24 volt lamp in series with one of the relays. That should help with troubleshooting.

Josh,

My best guesses:

1. The 24V relays are really 24 VAC relays, AC not DC, which will not work with your 24 VDC power supply and will appear as short circuits.

Relays 4-8 are Omron with 24v triggers.

Click to expand...

You need more precision here - 24 VDC or 24VAC coils? There is a diffeence in relays, and they are not usually interchangable.

or
2. The wiring is not really like your drawing, and there could be some overlapping connections between the AC and DC output sections.

According the the Omron data sheet the MY2N-D2 relay has a built in diode. You must get the polarity connected correctly for this relay to function. Refer to page 7. The + terminal is terminal 14. Take note that when you turn a relay upside down and look at the terminal numbers, then turn it back over to insert into the socket that you don't confuse which terminal is really terminal 14.

edit:
Also note how your socket is constructed. While some sockets preserve the relay pin arrangement to the terminals, some other sockets reverse the pin layouts on the terminals, ie when the relay is plugged into a socket, the outboard most pins of the relay (pins 1,4,13,14) are the top inner rows of terminals on the socket, while the inboard pins of the relay (pins 5,8,9,12) are the lower outer rows of terminals on the socket. You need to look at your socket and determine whether it is one that reverses the pin arrangement or not (IME, sockets that reverse the arangement are more common because they are easier to manufacture).

Thanks for all the help. I spent an hour troubleshooting last night and was successful. I had a relay base that was bad. I didnt know they could go bad. It came from the factory that way brand new. It just happened to be the first one and the one I was using to trouble shoot with. I carried it to another room and left it. So I used another one I had and bingo it worked. I just kept changing the cubes. All is well now and the Christmas display looks awsome. Lights are flashing and parts are moving so everyone is happy around here.

Josh