# 4-20ma Problem

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#### bpk

##### Senior Member
I will try to explain this the best I can. I have a building automation system that monitors air quality to turn on/off a fresh air exhaust system. The air quality sensors that were purchased for the job are a loop powered 4-20ma signal. The inputs to the automation system can be configured to 0-5v or 0-10v also. The problem that I have is that in the old sensors 20ma would equal good air quality and 4ma bad. The new sensors are opposite 4ma good quality 20 ma bad. In the automation program it uses an equation such as setpoint less than X activate system. I do not have the software program to make any changes to the system (setpoint greater than X activate system) is what I need. I cant simply reverse the polarity of the sensor because it is loop powered. Any ideas ?

#### don_resqcapt19

##### Moderator
Staff member
You can make the change in a lot of the transmitters, but not all. Is there any kind of set up for the transmitter itself?

#### mike_kilroy

##### Senior Member
is+/-5 or 10v a choice? if so perhaps you could simply put ur 500ohm resistor in prallel across the input in reverse polarity so it goes backwards?

is the 0-5v input true differential? if so couldnt u stick +5v in the dif+ input and the 4-20ma in the diff- input reference to common?

a simple 2n3904 xsistor and 1k resistors would invert it...

#### gar

##### Senior Member
120426-2012 EDT

bpk:

I do not know your parameters but suppose the following:

1. Internal to the automation system is a 50 ohm resistor for current sensing. At 20 MA this has a drop of 1 V. I will assume one input terminal is connected to the system chassis. So call it ground. The second input terminal will be at + 1 V relative to ground at 20 MA flowing into the second terminal..

2. Connect a 1200 ohm resistor to the second terminal. The power rating should be maybe 2 times Imax2 * 1200 or 1 W. The voltage to ground from the input end of the 1200 ohm resistor is 0.02 * 1250 = 25 V.

3. Connect your sensor from the input end of the 1200 resistor to ground. Then your sensor shunts current to ground instead of thru the system input.

4. Provide a constant current source of 24 MA to the input end of the 1200 ohm resistor.

5. When your sensor provides a 20 MA load 4 MA flows to the system input and the voltage across your sensor is 0.004 * 1250 = 5 V. When your sensor is a 4 MA load, then the system sees 20 MA, and the voltage across the sensor is 0.02 * 1250 = 25 V.

6. You can probably build a good constant current source with an LM317T regulator. See p 19 of http://www.ti.com/lit/ds/symlink/lm117.pdf for a current regulator schematic. With 49.9 ohms for the current sensor I got about 23 MA. Using the National 1.2 ohms for 1 A I calculate 50 ohms for 24 MA, but from 49.9 and 23 MA in my experiment I would calculate about 47 ohms. This show you some of the variation from one sample part to a nominal part. For the string I am suggesting above I would use a 30 V DC source and heat sink the LM317. From about 3 V to 30 V across the regulator the current holds very constant.

i do not know if the 317 is still in production, but there should be other comparable parts.

Check my calculations.

.

#### hillbilly1

##### Senior Member
I'm having issues right now with a system that uses both 0-10 Vdc, and 4-20 ma. The 0-10 Vdc controls the dampers, while the 4-20 ma is just for monitoring in the EMS. The 0-10 works fine, As it sits at about 2 volts right now, but the 4-20 goes into "sensor open" in the program because it drops to 0 milliamperes when CO2 is low. I thought that with a 4-20 system, the low end should not drop below 4 milliamperes if its working correctly?

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#### markstg

##### Senior Member
I will try to explain this the best I can. I have a building automation system that monitors air quality to turn on/off a fresh air exhaust system. The air quality sensors that were purchased for the job are a loop powered 4-20ma signal. The inputs to the automation system can be configured to 0-5v or 0-10v also. The problem that I have is that in the old sensors 20ma would equal good air quality and 4ma bad. The new sensors are opposite 4ma good quality 20 ma bad. In the automation program it uses an equation such as setpoint less than X activate system. I do not have the software program to make any changes to the system (setpoint greater than X activate system) is what I need. I cant simply reverse the polarity of the sensor because it is loop powered. Any ideas ?

You could purchase a reverse acting transmitter (signal converter) 4-20ma in 20-4ma out. But changing the program would be cheaper.

#### markstg

##### Senior Member
I'm having issues right now with a system that uses both 0-10 Vdc, and 4-20 ma. The 0-10 Vdc controls the dampers, while the 4-20 ma is just for monitoring in the EMS. The 0-10 works fine, As it sits at about 2 volts right now, but the 4-20 goes into "sensor open" in the program because it drops to 0 milliamperes when CO2 is low. I thought that with a 4-20 system, the low end should not drop below 4 milliamperes if its working correctly?
Sounds like the transmitter is set for 0-20ma and the control system analog input is set up for 4-20ma. Are these two seperate outputs from the sensor or are the Analog Inputs of the BAS and EMS wired in series to the same output from the sensor?

#### stickelec

##### Senior Member
I will try to explain this the best I can...
You were forced to reverse your input logic (and can't change the code) so now you simply need to reverse your output. Let the existing DO drive a relay and use the NC contact for the Exhaust system.

#### hillbilly1

##### Senior Member
Sounds like the transmitter is set for 0-20ma and the control system analog input is set up for 4-20ma. Are these two seperate outputs from the sensor or are the Analog Inputs of the BAS and EMS wired in series to the same output from the sensor?
Your probably right, and yes it has two outputs, one looped between 4-5 RTU's (hence the 0-10 Vdc) and the other to an input on the EMS. The only other setting on the sensor changes the second output to another 0-10 volt output, so I reckon they will have to live with it. Now if I can just get the HVAC manufacture to call me back on how to intercept control of the dampers on the older units! (Trane)

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