PLC 4-20mA pin circuit

S

Thread Starter

SK

I'm working on an automated process to test components that can be interfaced to a PLC, and I need to make a 4-20mA current source to replicate this. What internal circuit does a typical PLC use for its 4-20mA output pins? I have designed several current sources, but I want to be sure that its the same as a PLC's internal source. Thanks.
 
R
There is no simple answer to that, Just make a source with OP amp. a transistor and a feedback resistor. If you just need a few fixed points, a 3 terminal voltage regulator like an LM317 makes a great constant current source.

Regards,
Roy
 
2, 3 or 4 wire?
Active?
Passive?
Single channel or multiple channel?
Isolated what-to-what or none?
0-20 functionality or 4-20 only?
 
Y

Yeasir Rahul

Great suggestions from Roy.

Different PLCs will use different internal circuits. So, you cannot make an absolutely general test circuit. Just ensure the followings for your test source.

1. It should be a constant current source.
2. It should be ground-referenced, not floating.
3. It should be "sourcing" current, not "sinking".

The most obvious circuit would be a V-to-I converter, controlled by an analog voltage. Most PLCs internally use 1-5 volt control voltage and convert that to 4-20 mA current.

Another option is using a DAC with current output. More expensive, but much simpler to use and control from a digital system.

Cheers
Yeasir Rahul

www.voltsmith.com
Control System Integrators
 
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