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variable 4-20mA using 24V power supply

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kurt.brinker

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I am looking to create a variable 4-20mA signal using a 24V power supply. I figured that with a constant 24V, i would need 6000 ohms of resistance for 4mA,2000 ohms of resistance for 12mA and 1200 ohms of resistance for 20mA. I should be able to just use a variable resistor/trim pot to accomplish this right?

I did some searching and all of the schematics seem to have way more components than just a pot.

my question is will my way work and is there any reason not to do it this way. Its just for testing.
 
I am looking to create a variable 4-20mA signal using a 24V power supply. I figured that with a constant 24V, i would need 6000 ohms of resistance for 4mA,2000 ohms of resistance for 12mA and 1200 ohms of resistance for 20mA. I should be able to just use a variable resistor/trim pot to accomplish this right?

I did some searching and all of the schematics seem to have way more components than just a pot.

my question is will my way work and is there any reason not to do it this way. Its just for testing.

hi,
Dont forget to include the resistance of the load in the current loop.
Whats is the purpose of this loop.??
 
it will be going directly to an analog input board.

hi,
This is a very basic circuit that will provide 4 to 20mA, look at the image.
It will into a 0 to 1000R load.
 

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The attached is a 4 - 20 mA source I used years ago. I believe for simple 4 - 20 mA operation the 2N2905 can be eliminated. I used an OP07 but I also think a 741 would work. The 7.5 Ohm resistor should be 1 watt I think. It has been awhile. :) It was designed to operate a current to perssure system for a valve as a manual backuop and it has been out there working for about 12 years. Maybe one of the guys could do a quick simulation for you and verify it.

<Edit> Eric beat me with an even simpler and better circuit. :) </EDIT>

Ron
 

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As Eric noted, you need to take the resistance of the analog input board into account when determining the current. The input resistance will be in series with the resistance you add at the power supply. If it's just for testing and you measure the current, then a simple pot (connected as a rheostat) should work.

The more complex circuits add active devices so that the current is constant, independent of the load resistance. Those circuits approximate an ideal current source.
 
The more complex circuits add active devices so that the current is constant, independent of the load resistance. Those circuits approximate an ideal current source.
Ahh, so the circuit that eric posted will maintain constant current even if the load resistance changes? That makes sense. It would be useful if i was controlling something.

The analog input is part of an algorithm that determines the output of one of the pwm signals. I wanted to change the input and make sure the output also changed accordingly.
 
Ahh, so the circuit that eric posted will maintain constant current even if the load resistance changes? That makes sense. It would be useful if i was controlling something.

The analog input is part of an algorithm that determines the output of one of the pwm signals. I wanted to change the input and make sure the output also changed accordingly.

hi,
This simulation image is with a load from 1R to 1000R.
Remember it is very basic, you can connect a low impedance current meter in series with the load in order to monitor the current.
 

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Hi Eric,
looks like a great circuit i am going to try it! Question though is R1 a 27ohm resistor and R4 a 200 ohm pot? My typical analog board load is 250 ohms for R3
Your response would be greatly appreciated.
Mike
 
Hi Eric,
looks like a great circuit i am going to try it! Question though is R1 a 27ohm resistor and R4 a 200 ohm pot? My typical analog board load is 250 ohms for R3
Your response would be greatly appreciated.
Mike

hi Mike,
The R1 is 27R and the R4 is a 200R variable resistor. Varying R4 will set the constant current value, it will work OK for a 250R load resistor R3.

I split the 27R and 200R , so that the user could not accidentally turn a 250R pot to zero.!

Eric
 
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