# 4-20mA loop voltage to high

I have a 0-16 bar pressure probe connected to a module called Adca UD-720. The probe works on a 4-20 mA loop and the readings are ok. This module also has a 4-20 mA output and I need to connect this output to a data acquisition module in order to get the readings remotely. But when I connect the output to the module, the module goes into protection mode. When I measured the output I couldn’t read any current, but I got a 14.7 voltage, which is way too high. The module can only support an input up to 10 V.

So, how can I reduce the voltage below 10 V, and still be able to read the current? i was thinking to use resistors, but i do not know what resistors to use.

thanks.

So, how can i reduce the voltage below 10v, and still be able to read the current? i was thinking to use resistors, but i do not know what resistors to use.

The standard load resistor for a 4/20 mA loop is 250 ohms. This will give a peak voltage of 5 volts across the DAQ inputs for 20 mA. I'd use a quarter watt resistor to be safe on power.

Picture source

• Thanks for your answer. The data acquisition module has a 120 Ohm impedance on the 4-20 mA input. But when i connect the output to the module it gets into protection. so i put the multimeter on the module terminals with the wires connected (with 120Ohm load) and i get 14.7 volts. if i connect a 9 v battery to the module i get the 9v reading on a 0-10v channel but on 4-20mA i get "out of range" wich is correct, beacuse the current is way more than 20mA. as soon as the voltage is higer then 10v, the module can't read it any more. – Leonida-Alexandru Diaconu Jul 27 '18 at 9:42
• Then you have something wired incorrectly or your 4/20 mA module is putting out a lot more current than you think. – Andy aka Jul 27 '18 at 9:50
• the + of the output is connected to + of the aquisition module and - to -. what could be wrong? – Leonida-Alexandru Diaconu Jul 27 '18 at 10:19
• I have no idea what could be wrong other than either the 4/20 mA output current is too high or you have wired something incorrectly. – Andy aka Jul 27 '18 at 10:38
• Just connect a 250R accross your ouput (with the ADC disconnected) as check what voltage you get. Do you have a datasheet of this barpressure probe? – po.pe Jul 27 '18 at 11:28

It may be that your initial voltage from the probe is triggering the module's protection before the internal resistor can pull it down to a "safe" level. Try putting 120 ohms in series with the module; it will cut the initial voltage by half, but shouldn't affect the current significantly.