Contact Switches and Loop-Powered Systems

Hi there,

Very simply put, for analogue loops i.e. 4-20mA @ 24VDC you have two types

1. loop powered

2. non loop powered

What does this mean?

1. The analogue input card will provide the source voltage (Card Source)

2. The instrument will provide the source voltage (Card Sync)

Now both types are in actual fact 2 wire systems (this applies to the signal being measured) In case number 2 it is sometimes refered to as a 4 wire system. This is because the instrument needs an extra two wires to provide power to it. So you have two wires for power and two wires for signal giving you a four wire system. These four wire systems are typically found in instrumentation where the instrument has a higher power requirement. Eg, Nuclear densitometers, magflows, Ultrasonic flow etc. Sometimes these loops might also be refered to as active and passive loops. I.e. the card supplying loop power is card active/instrument passive.

Digital signals work a bit differently. If you have a straight forward switch i.e. a common with a normally open/closed - you will have a two wire system. In most cases your card will provide the voltage and current which you take to the common of the switch and the other side back to the signal channel of the card - 2 wire.

Sometimes you might have inductive proximity switches. 2/3/and 4 wire varient. Again 2 wire you provide the power through the loop. 3 wire is almost the same as 4 wire except you supply an external voltage but the negative is common. 4 wire is 2 wires for signal and 2 wires for supply.

You should always check the wiring configuration of the instrument in question as this will help you to determine what loop type you have as there is much confusion in terminology between the countries. To keep it simple I always refer to the loop as either loop powered or non loop powered.

Regards

Craig