What happens if you overburden a 4-20mA signal?

Question

I have a 4-20mA analogue signal which is looped through three instruments already.   When I add a fourth, there's a small risk the sender will be looking at a total circuit resistance that's greater than the spec.

What will happen?   Will it just saturate at some level below 20mA, or will it go off calibration right through the range?

As yet, I haven't determined if the signal is loop powered or not.   What will be the effect if it is? or isn't?

Thanks!

mariskalen kampf Strudl v.Wurst! · Accepted Answer

It will depend on the power supply voltage, and the total circuit resistance. If you keep introducing resistance into the circuit, it will reach a point in which you wont be able to reach 20mA. 

Measure the resistance of each instrument with an ohmeter and do a quick calculation. Use a power supply that will put out the maximum allowed voltage @20mA.
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none2perdy · Answer

It would be best to try and find the data sheet for the instruments in question. Do a Google search if you have to. If you cannot find any info, you can be guided by two principles:
1) if the source is a voltage source, then the voltage amplitude will sag if you draw too much current
2) if the source is a current source, then the current will saturate if you overload it by decreasing the load resistance too much.

Also, splitting the signal multiple times will be seen by the source is several ways depending on the operational frequency range. Couple of basic cases here:
1) Audio and low frequencies - adding more splits will just increase the perceived load by decreasing the combined resistance.
2) Higher frequencies where transmission line effects come in to play - adding more splits can cause phase shifts that may be seen as increased loads or decrease loads depending on the resultant circuit resonances and modes.

Best bet is to find the data sheet.

Good luck.

What happens if you overburden a 4-20mA signal?

2 Answers