Micrologix 1000 DC Inputs

[Guest llrelease]

I need to purchase some proximity switches for use with an AB MicroLogix 1000 PLC, but I am unsure whether to the 24 VDC inputs are sinking or sourcing (i.e. do I need the NPN or PNP type switches)? Can anyone provide some insight? Thanks in advance.

[mgvol 0]

All of the MicroLogix 1000 PLC'c can be wired for either sinking or sourcing inputs. You can even mix types, as long as you keep the DC COM terminals for the different input groups separate. my personal preference is sinking inputs with PNP devices.

[Guest llrelease]

Thanks mgvol. If I understand correctly, you connect voltage (V supply) to the high side of the PNP sensor, connect the output to the digital input, and ground to ground. In the case of a N.O. switch, the output is high when the switch closes. For a N.O. NPN sensor, the connections are the same, but the output would be low when the switch is closed. Is this correct?

[panic mode 246]

yes, npn sensor would pull output low...

[Ken Moore 23]

Try this site for PNP vs NPN http://www.patchn.com/npnpnp.htm

[guillencito 0]

In this forum exist an article about npn-pnp: http://forums.mrplc.com/index.php?act=Downl...&CODE=02&id=141 It was useful for me! Regards

[Guest llrelease]

Thanks to everyone for the great responses. Wow. It's all clear as mud now. It appears that different manufacturers (not mentioning any names... AB) use different definitions. What I conclude from all of the info provided is that if I wire ground (0 V) to the DC com of the AB input, I can use either a NPN or PNP field device. This is true if there is no load (motor etc.). The only difference would be the way the field device is wired (+24 to the emitter for PNP and +24 V to the collector for NPN). However, if I was using the device with a high current load, I would need to be careful that the switch I selected could carry the current, and that the switch was placed in the proper place (high side or low side) with respect to the load. Am I on the right track?

[Guest]

Greetings llrelease, I'm afraid that we need to work on this some more ... you MIGHT have the right idea ... but if so, then you are saying it WRONG ... first things first ... let’s nail down the terminology that we’re using for the DC power supply ... this might seem trivial ... but it MIGHT be contributing to your confusion ... so let’s just nail it down ... we’ll assume that we’re working with a simple "two terminal" 24VDC power supply ... ONE of the terminals from the DC power supply should be marked “+24VDC” ... we’ll call this the “positive” terminal ... most beginners have NO problem with that idea ... next ... the OTHER terminal should be marked “0VDC” ... sometimes that gives beginners a problem ... they instinctively want to mark this “-24VDC” (negative) ... that would be wrong ... the reason: if you were to put a meter from a “+24VDC” terminal to an honest-to-goodness true “–24VDC” terminal, then the meter would read “48VDC” ... the power supply that we (assume) that we’re working with can’t give us a full 48VDC ... only 24VDC ... and so the proper label for the OTHER terminal is “0VDC” ... now that we’ve got the proper labels assigned to the two power supply terminals, let’s come up with some common names for them ... let’s call the “+24VDC” terminal the “positive” side ... let’s call the “0VDC” terminal the “negative” side ... remember that the “0VDC” terminal is not REALLY-AND-TRULY “negative” ... actually it’s ZERO ... but, for our wiring purposes, the name “negative” makes sense ... and we can get away with this “incorrect” name because the “0VDC” terminal IS “negative” WITH-RESPECT-TO the “positive” terminal ... from here on through the rest of this post, I’ll refer to one terminal on the power supply as the “positive” terminal ... I’ll refer to the other terminal on the power supply as the “negative” terminal ... technically this statement COULD be correct ... but only if your power supply could provide a –24VDC (negative) signal ... and so to correct your statement: if you wire ground (0 V) to the DC com of the AB input, then you can use a PNP field device ... no problem ... just hook +24VDC (positive) to the other side of the field device ... if you wire ground (0 V) to the DC com of the AB input, then you can use an NPN field device ... but ... you must then hook –24VDC (negative) to the other side of the field device ... I’m betting that your DC power supply doesn’t really have a NEGATIVE 24VDC terminal ... that’s why I said that your original statement is incorrect ... now ... if your power supply DOES have three terminals (+24VDC and 0VDC and –24VDC), then post again and we’ll go back through this again with all of the bells and whistles ... I sincerely hope that this helps ... let us know if you need anything else ... best regards, Ron Edited 9 Dec 2004 by Ron Beaufort