4-20mA Sink and Source Loop Power


Thread Starter


Can somebody explain what 'sink' means in relation to 4-20mA loop versus 'source'?


Michael Batchelor

When you talk about digital components, like proximity switches, a "source" component supplies power on its terminals and responds to the change in the load, while a sink component does not supply power to its terminals and responds to changes in the voltage or current impressed upon it as measured on those terminals.

In 4-20 ma loop parlance, I could understand a "source" component as one which supplies the loop power such as a powered 4-20 ma output card in a PLC rack or a 4 wire pressure transmitter that provides the loop power from its internal power supply.

Likewise, I could understand a "sink" component as a passive receiver instrument such as a 4-20ma input card on a PLC or one of the gazillion 1/4 or 1/8 DIN panel meters on millions of pieces of equipment in factories.

But that leaves a gap for a couple of pieces. In loops which have a 24VDC loop power supply rather than having power supplied by the transmitter, I certainly wouldn't think of that as "source" instrument despite the fact that the PS is obviously supplying the EMF. Likewise, for a loop powered 4-20 ma pressure transmitter you couldn't call it a "source" instrument if the criteria is that it should supply loop power, since it doesn't supply the power. However, i also wouldn't call it a "sink" instrument, since it isn't a receiving instrument.

Plundering around a few supplier's web sites that use the source and sink terminology didn't reveal much of an answer, either.

Michael R. Batchelor

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When you are using a sink system you are controlling your 0vdc for signal and the positive (+5 or +12,+24v etc)are connected as common(not grounded).

Source is the opposite way; your 0vdc is attached
as common and the positive is the controlled signal.

Source and Sink are common used for digital I/O signals (on-off), but in analog signals you have a range with min and max defining your limits.
In 4-20mA, 4mA is going to be zero and 20mA the hi limit of your transducer device and everything below 4mA is reconigzed as Zero, If polarity is changed your controller is going to read a negative signal always below zero.
Might help to think of them as "active" and "passive". A source (active) device will generate a signal without power actually in the loop, a sink (passive) will control the signal level in a loop that does have a power supply.

You might be looking for more than this but;
Sink basically means it acts as a resitive load (it needs a supply voltage from something else in order to act as a load), Source means it’s supplying the voltage for the current.

3 wire device, source- 24V DC from an external source, i.e. from controller marshalling panel... 4-20ma and 0V wire to I/O card.

3 wire device, sink- 24V DC from controller I/O card. .4-20 ma wire to I/O card. 0V wire to controller marshalling panel.

santosh shinde

Sink means to get in, means anything which doesn't generate or source but takes in the current. You can call it as a sink. You can take example of an NPN transistor connected to a +P supply through a resistor and emitter grounded, this NPN transistor then acts as a sink. Because it carries the current inside from a power source which itself doesn't generate.

Ronald Deepak

Thanks a lot Robert !! This is an excellent document capturing the background & basics of 4-20ma loops.