Creating Fixed 4-20ma reading

Thread Starter

juniperhillfarm

Joined Mar 25, 2022
2
I have a sensor i'm trying to bypass.
The company says it uses a 4-20ma signal with anything over 15ma opening the valve and allowing me to continue working.

I asked them what voltage it is running and they said 6-12v.

Can i put some resistors in line to allow this signal to come back with the 15-20ma reading? And if so what do you reccommend.

thank you
 

Papabravo

Joined Feb 24, 2006
18,426
If you put more resistors "in line" it will do the opposite of what you intend and increase the resistance which will lower the current. Perhaps a schematic diagram of the sensor and the system it is in would be helpful.
 

crutschow

Joined Mar 14, 2008
29,800
If you are driving the valve using an external power supply, then you can add a resistor in series to adjust the current to the desired value.
Or use a power supply with a current-limit adjustment.
 

Thread Starter

juniperhillfarm

Joined Mar 25, 2022
2
Well the exact circuit i can't be sure of because they don't provide the wiring schmatic. but what I am assuming is it's just a sensor. So it's provided with power going in a loop through a UV intensity monitor.

it has a 4-20ma signal voltage going to a UV intensity monitor. As the signal achieves 15+ma it allows a valve to open.

So all i know is a 2 wire plug that goes to a UV intensity sensor. so i would imagine the signal is generated on the controller and the intensity monitor is in line with it.

but i was hoping i could take the sensor plug out all together and put a circuit in place that would essentially trick the unit into seeing the proper 4-20 signal.
 

crutschow

Joined Mar 14, 2008
29,800
i was hoping i could take the sensor plug out all together and put a circuit in place that would essentially trick the unit into seeing the proper 4-20 signal.
What I suggested using a power supply should work for that.
Just apply the 15+mA to the two valve wires (making sure the polarity is correct, of course).
 

bassbindevil

Joined Jan 23, 2014
422
I'm assuming what you have is some black box that reads that sensor using a 4-20mA loop and then has an output that operates the valve. (If I remember correctly, 4-20 sensors and actuators normally get their operating power from the current loop, so attempting to fool it by applying a voltage isn't necessary.) The sensor could be replaced by a resistor, but the value would depend on the loop's supply voltage and source (and sink) impedance. Measuring the open circuit voltage and dividing by the desired current should get you a resistance to start with, then adjust as necessary. Or use a 3-terminal regulator (LM317) wired as a constant current regulator.
 

dendad

Joined Feb 20, 2016
4,135
If the system is in an industrial control setting, probably the main control box is supplying 24V so a 1.5K resistor in place of the sensor will give you about 16mA.
 

MrChips

Joined Oct 2, 2009
26,113
Use Ohm's Law:

I = V / R

Use a battery and a resistor:
1.5V and 100Ω will supply 15mA
9V and 500Ω will supply 18mA
12V and 600Ω will supply 20mA

You need to take into account the resistance of the receiving end of the 4-20mA loop.
 

strantor

Joined Oct 3, 2010
5,967
Use Ohm's Law:

I = V / R

Use a battery and a resistor:
1.5V and 100Ω will supply 15mA
9V and 500Ω will supply 18mA
12V and 600Ω will supply 20mA

You need to take into account the resistance of the receiving end of the 4-20mA loop.
Typical input impedance for industrial devices with 4-20mA inputs is 250 ohms. I've seen 500 ohms as well but that is uncommon.
 
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