PDA

View Full Version : bs2 4-20ma



Tobias
02-25-2010, 12:45 PM
What is the best way to get a value on a bs2 from a transducer with a 4-20ma output? I have a ad converter working with a 1-6 volt output transducer, what is your plan?

Thanks, Tobyhttp://forums.parallax.com/images/smilies/hop.gif

Tracy Allen
02-25-2010, 01:31 PM
A 250 Ohm resistor converts 4--20mA to 1--5 Volts, then to same AD converter as your other transducer.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com (http://www.emesystems.com)

Tobias
02-25-2010, 02:23 PM
I can follow you through the math and theroy could you also provide a schematic for the hookup and the output wire.



Thanks, Toby

Tracy Allen
02-25-2010, 03:04 PM
Stamp with ADC
Power (12 Volts?) ----> 4-20 mA
O--------------------------------------------------------------o
|
ADC input ------------
O---------------; sensor
| -----------
250Ω | |
O-----/\/\/\----o----------------------------------------------o
Common <-------- loop current
resistor converts
4-20 mA
into 1 to 5 Volts for ADC



This is for a loop powered transducer. Some transducers have their own power supply and simply produce the 4-20 mA on a pair of output wires.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com (http://www.emesystems.com)

Tobias
03-07-2010, 01:30 PM
I have a trancducer with a red wire which is 24v and a black wire which is ground and the green wire is 4-20ms output do I put in a 150 ohm resistor inline to ground a read of the voltage from the input end of the resistor?



Thanks, Toby

Tracy Allen
03-08-2010, 01:07 AM
Hi Toby,

You are probably correct. That is a typical 3-wire 4--20 circuit. If you use 150 Ohms, the output voltage will be 0.6--3.0 V. With 250 Ohms, it will be 1--5 V.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com (http://www.emesystems.com)

Tobias
03-11-2010, 11:44 AM
I have a 12volt power supply and a volt meter with a 4-20ma output transducer and a 250ohm resistor can anybody tell me how to hook up the circuit and get a voltage read out?

Thanks, Toby

Tracy Allen
03-12-2010, 01:09 AM
If it is a loop powered transducer:


+12 ---[transducer]---A----/\/\/\----B---- battery common
...resistor is 250 Ohms, connect voltmeter(-) common to B, voltmeter(+) to A.
...voltmeter can be Stamp ADC(-) and ADC(+)



If it is a separately powered transducer:


+12----------|transducer power|-------A---/\/\/\-----B---,
|---------------------------'--- battery common

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com (http://www.emesystems.com)

Tobias
03-12-2010, 10:08 PM
What do I do with the signal output wire?

Tracy Allen
03-13-2010, 01:31 AM
Hi Tobias,

If you have three wires, they will (probably) be power, signal and common. In that case follow the second diagram above with the 250 Ohm resistor and the signal output wire is the point "A" in the diagram. You use your voltmeter or Stamp with ADC to measure the voltage across the resistor. The resistor converts V = I * 250, that is, 4-20 mA to 1-5 Volts. It is really quite simple!

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com (http://www.emesystems.com)

$WMc%
03-13-2010, 01:34 PM
Toby

I work with 4-20ma Transmitters all day long.What Tracy Allen suggested should work.
Some transmitters will only take so much current.Adding and another 250 ohm resister might help.

Is Your transmitter smart? Hart,MODBUS,etc.

Do You have some specs for Your Transmitter?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
The Truth is out there············___$WMc%___······················ ······ BoogerWoods, FL. USA