4-20 mA Potentiometer

strebej

Member
Join Date
Sep 2002
Location
Neenah,WI
Posts
24
Does anybody know a good resistance range for a potentiometer that I will connect to a AI / AO module. I want to send the analog output back in through the analog input and adjust using a potentiometer.

Any help is greatly appreciated.

THANKS
Jim
 
OK, I'm confused.

If you want to run a pot to your analog input, just use Ohm's Law. This assumes 4-20 mA I/O. For 24 VDC, assuming a 250 Ohm input impedence, you can use a fixed 1 kOhm or (950 Ohm if you can get it) resistor in series with the pot to limit max current to .020 Amps (24 / (1000+250) = 19.2 mA which is usually close enough. If not you could boost your power supply adjustment a little to 25 VDC). Then a 5 kOhm pot in series gets you down to around 4 mA. (24 / (5000 + 1000 + 250) = 3.84 mA)

If you are looking for voltage I/O, again go to Ohm's Law and your excitation voltage. Remember a potentiometer is really a three wire device, with the reading from the wiper to the legs a ratio of position.

You threw me with wanting to run the analog output through the potentiometer. What is that all about? No matter what you do the input will read the output directly.
 
this might work better than output-to-input

if you're just trying to experiment and "play around" with analog signals, then the project below might give you better results than sending the analog output into the input - although that DOES work for many experiments - anyway, here is a list of Radio Shack parts that will allow you to build a handy portable "4-20 mA signal generator" - basically you put the main pot and the trimmer pot in series with the batteries and adjust the trimmer to give something around 4-20 mA - on each pot, use the middle (wiper) connection and one of the other terminals - the on/off switch is optional but very handy to keep from killing the batteries while the thing is connected but not being used - and don't forget (as Tom Jenkins mentioned) that most analog input modules (4-20 mA current variety) have a built-in 250 ohms impedance - that will have to be considered in any Ohm's law calculations that you do - Note: If you don't want to mess around with 2 batteries (they soon "go dead" if you use them a lot) - then leave the batteries out and hook this up to a 24VDC power supply and adjust the trimmer pot accordingly. Also note: Many Radio Shack potentiometers are not "high specification" grade, if you know what I mean - so you may have a hard time finding one of their "5K" pots which will actually give you a full 5,000 ohms - have fun - play safe - learn a lot -
BATTERY POWERED 4-20mA TRANSMITTER PROJECT

1 piece 270-1803 project enclosure; 5" X 2.5" X 2"; plastic;
2 pieces 270-324 9V battery snap connectors; come 5 per package;
1 piece 271-1714 5k-ohm linear-taper potentiometer;
1 piece 271-342 1k-ohm 15-turn cermet potentiometer;
1 piece 275-613 SPDT submini toggle switch;
1 piece 274-415A 3/4" hexagonal control knob; come 4 per package;
2 pieces common issue 9V alkaline batteries;
1 piece alligator clip; black insulation;
1 piece alligator clip; red insulation;
misc. wire scraps;

 
Look for loop powered panel meters. These will measure and are powered by the 4-20 loop. Datel and Texmate come to mind... I am sure there are others.
 
Are you referring to Ron's instructions for building one, or to the original post?

A cheapie like this one might fill your needs without the hassle of constructing your own.
 
Look for loop powered panel meters. These will measure and are powered by the 4-20 loop. Datel and Texmate come to mind... I am sure there are others.

Watch yourself with these. Many of them have a minimum voltage to power the electronics, induced by a Zener diode. At low current this can look like a high resistance to a transmitter. For example, if the loop powered meter needs 3.2 Volts to operate it looks like an 800 Ohm resistance to the transmitter. If the allowable transmitter load is only 500 Ohms your loop will drop out below 6 mA signal.
 
Last edited:
Hi;
I use ALG223 analog input module. At terminal one i supply 24 v dc and at 19 is negitive. At terminal 2 i found 24 VDC becuase its jumper with one internally in the module for the transmitters. Now i want to simulate the 4 to 20 mA signal.
I think i attached a 250 ohm resistor in series with 1K pot.and connected to terminal 2. Wiper connected to channel 1 means terminal 3 of the module. and other end of pot connect to negitive. Am i right? If wrong guide me plz?
 
You guys are all so funny...

I was thinking of something along these lines..

I was thinking of something more along the lines of ....

http://www.alliedelec.com/Search/ProductDetail.aspx?SKU=5720024&MPN=SP+200

Hey dont get on us - its your project and you are the one not giving us enough details on your wants / needs. We are not mind readers and we do not have crystal ball.

You asked for cheap and Bernie said use what you already have - a VOM. You cant get any cheaper than what you already have.

Dan Bentler
 

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