Micrologix 1400 FET Outputs

Secpcb

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Join Date
Jan 2015
Location
Detroit, Michigan
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I have a project I want to do with a ML1400 [1766-L32BXB Ser B] listed as having relay outputs. However 6 of the 'relay' outputs are FET outputs.
A couple of the outputs will be wired for 120VAC [OK so far] but the rest of the outputs will be 12VDC. [All inputs will be 12VDC which the unit is rated for]

I have read through the manuals I downloaded from AB.com and nowhere does it have any spec's on the FET outputs, like the operating voltage - only that they are DC only.

Will the FET outputs work on 12VDC? I will need to use at least 3 of them and all of the relay outputs for this project. Or will I have to wire these to the 24VDC powering the PLC and add interposing relays to control the 12 volts?
 
Flopro,

I read through that page, and that entire manual.

It only shows the power supply voltage - is this the input voltage for the FET's incorrectly labelled? And does the [-15%, 10%] mean 24 volts less 15 % then within 10% of that?

I took it as the 24VDC the PLC runs on as it says 'power supply voltage'.
 
Note that the FET output group requires both + and - supply connections. So the must be output circuitry besides just the FET and it requires a specific voltage range (shown at the top of that particular chart) to operate correctly. I read it as 24, +10%, -15% range. So I make it as 20.4 to 26.4 volts.
 
I think you would be OK with 12 VDC on the FET outputs
FET's are nor like other transistor outputs they are for high speed use but will work at slower speed.
As long as you do not exceed the current ratings.
if you look at the manual then have a 1VDC max voltage drop at 2.5 A but the max current ratings is only 100ma for outputs 2, 3 and 4
it goes up to 1.5 VDC max drop for the others
just make sure that you do not exceed the current ratings
it looks to me like you could use the FET's on a 5VDC source and it would work
 
Bernie and Gary,

Thanks. I guess I am going to have to hook this up on the bench and put a test program in. The outputs will not be over the current rating.
 
It would be interesting to see a schematic of the output circuitry. If it were just an optoisolator attached to a FET then GaryS' guess might hold.

But typically there are many driver components before the FET. Thr preciseness of the stated voltage limits leads me to believe that may be more the case and a low voltage may not have the desired effect (possibly partial turn on of the FET.)

I guess we'll see with your test. It shouldn't hurt it (fingers crossed)
 
Guys,
This bugged me so much I stopped by the shop and brought it home.

Turns out outputs 2, 3 & 4 work on 12VDC very well. BUT: outputs 5, 6 & 7 will not power a 12V relay and only light a 12V bulb very dimly.

If I don't have to add another output I could use the 1st 3 FET outputs on 12V, if I do have to add another output I will have to change them to 24V and add interposing relays.

I'm thinking of getting 6 slim DIN 24V relays, putting them next to the PLC and wiring them to the 6 FET outputs in case of future need.

Thanks for the much needed help.
 
FET's are strange devices they change resistance values when turned on the resistance drops to just a few ohms but when off the resistance is as high as 100 meg ohms. and they switch very quickly. if the 12 vdc relays don't pull in when turned on then I would guess that the voltage drop across the FET when it's on is to high. if the 12 v is a little on low side it may not have enough to pull in the relay. try a different relay a solid state relay may work.
as for voltage range a few years ago I had to hook up a flow scale with a pulse output. just 2 terminals no voltage rating or polarity shown I needed 120VAC I call tech support and was surprised when I was told that the range was 5 to 240 VAC or VDC and it was a FET. I hooked it up to the 120VAC no problems. I am still trying to find out what chip they were using
 

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