MicroLogix 1400 FET Outputs

Hi: I have the "all DC" MicroLogix 1400 (L32BXB) and would like to connect small relays and LED lights to the onboard FET outputs. Of course, the leakage current will turn them on when they should be off. I have added a pull-down resistor to one output and that has made all 6 FET outputs work as I want them to. They do share the same VDC feed. Has anyone done this, with just the one resistor...or know why this works and if it acceptable? Thanks!! RB

Quick question - do you have a zero volt connection to the DC COM 2? If you're missing that, you'll get the symptoms you describe.

Leakage is not enough to turn on a relay coil, and most LED indicators these days have built-in resistors. If you were literally just wiring in a bare LED, then maybe - but you'd definitely not pull in a relay. I'd be looking at the zero volt connection.

ASF said:

Quick question - do you have a zero volt connection to the DC COM 2? If you're missing that, you'll get the symptoms you describe.

Click to expand...

I'm guilty of making that mistake once.

http://www.plctalk.net/qanda/showthread.php?t=92021



-Eric

I've used a few L32BXB units with A-B and Phoenix interposing relays on the DC outputs, along with LED stack lights.

If you wire as ASF said, you shouldn't have any problem.

Guy's at one company were having problems with leakage and pulling their hair out. Asked them if they read the manual... Correctly wired solved their problem.

ASF...I'm not at the location with the PLC now but I know that I didn't connect to DC COM2. That definitely explains some of the weirdness I saw. Normally use the AWAs and BWAs where there is no common required on the "relay only" outputs. That's the best excuse I can come up with for this episode of stupid.
Will connect that wire and report back probably Thursday evening.
Thank You !!!!!!!!!!!!!!!!!

Wire on...problem gone. Gotta love this forum!!!!

No doubt. Installing a MicroLogix 1400 L32BXBA. Had this leakage issue on the
Pilot lights. and found this thread. Very cool!

A few things don’t add up here
First the L32BXB has 6 relay outputs 3 High Speed outputs and 3 Normal outputs
While the 3 high speed outputs may be FET’s I found nothing in the manual stating that. They may well be FET’s but I can’t assume that. Most PLC’s will clearly state if they are using FET’s as outputs
But having said that all FET outputs have very low off state leakage as such would not have a problem when used with LED devices. Typically a FET will have an on state resistance or 100 ohms or less and an off state of over 100meg ohms
I would look at the wiring connections there is a +24VDC and Com for the PLC power and a completely isolated Power connection for the IO, check the manual
From your description it sounds like your IO Com is not connected correctly

GaryS said:

While the 3 high speed outputs may be FET’s I found nothing in the manual stating that. They may well be FET’s but I can’t assume that. Most PLC’s will clearly state if they are using FET’s as outputs

Click to expand...

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https://literature.rockwellautomation.com/idc/groups/literature/documents/in/1766-in001_-en-p.pdf