Today is...
Thursday, November 27, 2014
Welcome to Control.com, the global online
community of automation professionals.
Advertisement
Featured Video...
Featured Video
Wiring and programming your servos and I/O just got a lot easier...
Advertisement
Our Advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive
PNP vs. NPN outputs
What are the advantages/disadvantages to using a PNP vs. NPN sensor?
By Darren Ahlstrom on 26 March, 2002 - 4:42 pm

What are the advantages/disadvantages to using a PNP vs. NPN sensor?

By Ken Elphick on 27 March, 2002 - 10:59 am

The PNP sensor:- This is a sensor whose output pulls up to the positive supply rail when it senses a metal target. Thus any attached load to the sensor output must be connected between zero volts & the output of the sensor to operate.
This type of sensor is very vulnerable to short circuits to earth ( zero volts), a common fault if the wiring chafes/ becomes damaged. Often it will fry under this type of fault.

NPN sensor :- This is a sensor whose output pulls down to the negative ( 0 volts) supply rail when it senses a metal target. Thus any attached load to the sensor output must be connected between the Positive supply rail & the output of the sensor to operate. This type of sensor cannot survive a short up to the positive supply rail ( a very rare occurrence!). Shorts to the negative rail (zero volts) will not damage it at all & it can tolerate this indefinately.

Other factors affecting choice :- If there is a PLC attached then :-

a/ If the input device of the PLC registers a logic high/true state when left open circuit then this type of input is best served by an NPN sensor. This will pull the PLC input low when a target is sensed.

b/ If the input device of the PLC registers a logic low/false state when left open circuit, then this type of input is best served by a PNP sensor. This will pull the PLC input high when a target is sensed.

Ken

By Rick Daniel on 29 March, 2002 - 1:28 pm

Another point ...

NPN sensors are current sinking devices and PNP sensors are current sourcing devices. You can't connect a current sourcing sensor to another
current sourcing input (like TTL for example), it just won't work unless you provide a path to ground. Likewise a current sinking sensor must be connected to a current sourcing input. So you have to know something about the input circuitry of the device you're trying to read the things with.

Rick Daniel
Intelligent Instrumentation

By GordieGii on 21 March, 2012 - 5:16 pm

.
.
.
> a/ If the input device of the PLC registers a logic high/true state when left open circuit then this type of
> input is best served by an NPN sensor. This will pull the PLC input low when a target is sensed.

> b/ If the input device of the PLC registers a logic low/false state when left open circuit, then this type of
> input is best served by a PNP sensor. This will pull the PLC input high when a target is sensed.

Every PLC I have ever used (Mitsubishi, Allen-Bradley, Omron) showed a logic zero/false/off when left open and a logic one/true/on when shorted to the rail it was designed to be shorted to.

Gordie

By Steve Myres, PE on 27 March, 2002 - 2:05 pm

When using grounded (NPN) signals, an unintentionally grounded signal wire will be interpreted as a logic true, and can cause unexpected and potentially dangerous results. It's like putting your light switch on the neutral
conductor.

By Craig Blondin on 27 March, 2002 - 3:44 pm

npn is faster

By Hakan Ozevin on 27 March, 2002 - 4:57 pm

> npn is faster<

Are you sure about this? Is npn faster than pnp?

By Curt Wuollet on 1 April, 2002 - 11:54 am

Without getting into semiconductor physics, pnp transistors have been more difficult to fabricate for speed. But the difference shouldn't be anything you'ld notice in PLC timeframes unless the designer hosed up or extremely low cost devices were used. I've measured switching times on thousands of transistors. If they are noticably slower it was because the base drive was not optimized.

Regards

cww

>> npn is faster<
>
>Are you sure about this? Is npn faster than pnp?

Tecnologically speaking npn is faster because n is lighter then p

By Rocco F. Dominick on 29 March, 2002 - 11:28 am

Refer to Rockwell Automation Selecting Sinking (NPN) vs. Sourcing (PNP) - I/O Modules, Document No. 12812 for your information.

Regards,
Rocco F. Dominick
Butech, Inc.
550 South Ellsworth Avenue
Salem, OH 44460
Phone: 330-332-9913 EXT:267
Fax: 330-337-0800
Web: www.butech.com
E-mail: rdominick@butech.com
These opinions are my own and are offered on the basis of Caveat Emptor.