Replacing an NPN Sensor with a PNP Sensor

http://en.wikipedia.org/wiki/Bipolar_junction_transistor#PNP

Depending on the sensor and load currents, you'll probably get away with a pull-down resistor (ie a resistor from the sensor output to ground). Can you give some details?

[Edit: also need voltage thresholds for the load]

Thanks I will give that a try.

Don't try if frequency of the PNP signal is greater than 1kHz or even 200Hz. Relays are meant for slow switching. Perhaps an optoisolator with transistor output looks a much better solution. You need to put series resistance to restrict the current into LED of the optoisolator. 1K ohms 1W resistor in series with LED may just do that for PNP output to source 23mA from 24V output into LED of the optoisolator. Current will reduce if PNP output is of less voltage. Generally 10V to 30V range is permitted and typical value is 24V so that makes 9mA to 29mA dynamic range.

Try HCPL817 optoisolator. Need more help then write to me.

Hi Bolton Alan,

Good answer! I have used that set-up in a pinch and it works just fine. We also started purchasing sensors that could be switched to either mode to use as replacements for hard to find or obsolete units.

Jeff

PNP sensor output stage is current source type while NPN sensor output stage is current sink type. Current sink type needs an external power source to form active stage. PNP type with current source do not require external power to become active.

To convert NPN to PNP type requires a power supply of 24V and a small circuit. I made these for some industries and if you need details then write to me.

Load here can be a relay coil with protective diode in parallel to the coil.

Dear Sir,

I need to use PNP/NPN sensor in my system. But I am confused which one to chese NPN/PNP. My system will be kept in hazardous area.

Kindly reply.

Aarti D. Yardi

What is your sensor going to be connected to?

Dear Aarti,

What kind of Hazard? Explain.

You can use PNP sensor or you can use, NPN to PNP converter. It is that simple. Any problem?

Where can I find an NPN to PNP converter?

You can find it on electronics design magazine

www.electronicsdesign.com

type my name SHYAM SUNDER TIWARI in search box

This information will appear and Aporil 23, 2009 design idea is just for you. Direct link to the page is

http://archive.electronicdesign.com/article/analog-and-mixed-signal/sensor-signal-converter-outputs-both-npn-and-pnp-s

If link does not work then follow above listed way and click on the design idea. This Figure is from that page and part of the published design idea.

• Simple Interface Connects Sensor Output To MicrocontrollerShyam Sunder Tiwari April 20, 2011A way to apply the output of certain sensors to a TTL/CMOS-level iput on a microcontroller. Sensor must have an NPN output stage that can support a p
• Circuit Combines DC High-Voltage Drop Detector, Undervoltage AlarmShyam Sunder Tiwari December 14, 2010Description of a circuit to tell if the high-voltage supply in an industrial smokestack precipitator is functioning properly
• Simplify DC High-Voltage MeasurementsShyam Sunder Tiwari October 22, 2010555 Timer Measures Capacitor charge Time to Indirectly Measure High DC Voltage
• Measuring Stability In A Logic-Gate-Based OscillatorShyam Sunder Tiwari March 11, 2010$jq().ready( function() \{ setupSidebarImageList(); \} );
• Simple Light Sensor Circuit Features High Dynamic RangeShyam Sunder Tiwari December 13, 2009Ambient light is increasingly being used as a source of energy. To help designers of such systems, this circuit accurately measures ambient light
• Sensor Signal Converter Outputs Both NPN And PNP SignalsShyam Sunder Tiwari April 23, 2009A versatile signal-conditioning circuit for use with with programmable logic converters or controllers that can accept only one input polarity
• Low-Power Logic Gate Protects Fluid Sensor From Leakage CurrentShyam Sunder Tiwari March 12, 2009How to make fluid level sensors insensitive to mains-power leakage currents while operating from batteries
• Power Control Circuit Limits On Time To Prevent MOSFET BurnoutShyam Sunder Tiwari February 12, 2009Design Idea for a simple gated-osciallator circuit to limit MOSFET on-time using an An R-C time to limit On-time pulse period length and generates it
• November 17, 2008Staff November 17, 2008PACs Become Task Master
• Superior JFET Biasing Improves Amplifier PerformanceShyam Sunder Tiwari November 17, 2008An N-channel JFET has a low bias current when its gate is biased negative to the source. However, this requires either that the gate voltage be
• Simple Circuit Uses Optical Fiber To Send Relative-Humidity DataShyam Sunder Tiwari April 01, 2002The circuit shown in the figure measures relative humidity in air at the low temperatures found in applications like cold storage of food items and g
• Single-Gate VHF Temperature Transmitter Runs On 3.6 VShyam Sunder Tiwari December 03, 2001This design idea illustrates what may be the simplest and smallest temperature transmitter in the VHF 50- to 100-MHz range. As seen in the figure, th
• Dual Watchdog Timers Improve Embedded PerformanceShyam Sunder Tiwari September 18, 2000This idea describes a new concept that utilizes two watchdog timers instead of the typical single watchdog timer found in most embedded systems. One
• Program Quickly Calculates Transformer Design ParametersShyam Sunder Tiwari June 12, 2000Here's a handy C program for the design and verification of transformer characteristics. Design parameters computed include: required core cross-sect

50 Ohms resistor with PNP transistor need to be 1W or 2W or maxcimum 10W is meant for safety to prevent short circuit by limiting current through PNP Transistor. Picture wrongly shows 100W/200W which is unwanted power rating. Value of 50 Ohms can also be reduced to 1 ohms or 10ohms depending on the maximum current required into load.

If you have 24V 40A relay to drive and its coil resistance is 300 ohms then 10 Ohms resistor is most suited. PTC is used to shut the current when PTC gets heated is a thermal fuse.