PNP or NPN output circuits

[Gambrinus]

Ungrounded 24VDC? I just thinked about it. Hm, in this case NPN is safe like PNP, may be.

 

[BobB]

Ungrounded 24VDC? I just thinked about it. Hm, in this case NPN is safe like PNP, may be.

Pretty close. Have a look at a Japanese machine with NPN inputs. I would be very surprised if the DC supply is grounded. I have never seen it in any Japanese machines I have been involved with.

 

[Gambrinus]

But if occasionally - of ungrounded 24VDC grounds, how can you know about it?

 

[NetMan]

... the only benefit I see to NPN over PNP is the current capability. NPN transistor output can sink more current than a PNP transistor can source. In electronics ... "negative" logic is often used. ie. - to illuminate a LED ... you switch the ground connection on and off and leave the positive connected to the LED via your current limiting resistor. This example, however, is typically on a pc board and it only lights a LED. For a control system, wired though cabling and conduits, I see the dagerous implications. Maybe it's a carry-over from the electronics industry.
Just my 2 cents.
- Tony

 

[teh]

pnp problem

i face the problem now, my ge fanuc output module have some 6 volt

to the device ( here i connected to 24 vdc relay )when cpu didn't

activated it. this cause the relay always energize all the time !!

How to solve the problem?? grounding ? how?

regards

 

[Clay B.]

What Dave and Bernie said. Check out this link:

http://www.mrplc.com/kb/index.php?page=index_v2&id=44&c=38

Interestingly, I just received a spec from a new customer (Japanese-based automotive subcontractor), and they require NPN devices. Hmmm.

Paula

I worked for Matsushita (parent company for Panasonic) for 3 years. Every sensor we had in the plant was NPN. Same with the switches and outputs. Bad part was we had several automated welding processes. Ever seen a welding inverter lock on?

Imagine a robot going to home position with the Mig welder still running. Needless to say, even though it was not the Matsushita standard we did change to sourcing outputs. Wondering whether a inverter was going to shut off when it was supposed to was not something I liked to think about.

Also another point on which is better to use..PNP or NPN...imgine the poor maintenance tech who has to troubleshoot this circuit.

And thanks for the article I really enjoyed reading it. It reminded me why I had that argument so many years ago.