connecting 4-20mA device to analog input
I picked up a laser displacement sensor on ebay.
This device outputs 4 - 20 mA from the sensor. I will be connecting it to my 24 VDC power supply. So, I assume this current is at 24 volts?
The galil 1900 board analog inputs are for 0 -10 volt. How should I condition this signal to read it properly?
Red Lion controls makes a universal converter that I've used before with no problems:
This unit can be configured to accept a 4 - 20ma current input and ouput 0 - 10v DC and can be powered from your 24VDC.
The laser probe voltage will be 24 Volts. However, the Output from the probe will be a 4 to 20 milliamp CURRENT. This current is a stand-alone signal, and is not required to be referenced to any other source.
Originally Posted by Karl_T
The only signal conditioning device you need to purchase is a 500 Ohm precision resistor. (.01 percent tolerance if you can find one.)
Across the 0 - 10 Volt Analog input on the Galil 1900 board, connect the 500 Ohm resistor. Also across these same terminals connect the 4 to 20 ma signal output from the laser sensor. (The polarity is inportant.)
The above connection will have the 500 Ohm resistor in PARALLEL with the ANALOG INPUT. The 4 to 20 ma. signal will flow from the sensor thru the 500 Ohm resistor and back to the sensor. The 0 to 10 Volts is created as a VOLTAGE ACROSS THE 500 OHM RESISTOR ACCORDING TO OHM'S LAW.
To find resistor value to use: Desired output Voltage (10 Volts) divided by max current (.020 Amps) in current loop = 500 Ohms.
.004 Amps (4 ma) X 500 Ohms = 2 Volts at Galil Analog input Bd. (This is Low end of linearity adjustment) OFFSET
.006 X 500 = 3 Volts
.008 X 500 = 4 Volts
.010 X 500 = 5 Volts
.012 X 500 = 6 Volts (This is the Mid-point of linearity Adjustment.) Utilizing OFFSET, MID-RANGE, and SPAN Adjustments.
.014 X 500 = 7 Volts
.016 X 500 = 8 Volts
.018 X 500 = 9 Volts
.020 X 500 = 10 Volts. (This is High end of linearity adjustment.) SPAN
I worked on instrumentation for several years and you can use different values of resistors as above to interface to almost ANY desired analog input.
The more precise the resistor, the more precise the input to the Galil 1900.
Hope this saves you some money. The resistor should be a .5 watt unit. A quarter watt resistor would be maxed at 0.2 watt @ 20 milliamp.
Last edited by CJL5585; 09-02-2006 at 04:04 PM.
Reason: To Add power rating of resistor.