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Single transistor 4-20mA Source

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electrocad

New Member
Hello All! This is my first post to the forum and I wish to construct a 4-20mA Source.
I made one many years ago and it was a very simple circuit powered by a 4½V battery
and controlled by a pot'meter.
I would appreciate any pointers to a necessary circuit diagram.
Thanks in advance.
Malcolm J
 

JonSea

Well-Known Member
Most Helpful Member
An LM317 adjustable voltage regulator will make a very simple current source. See the diagram below.

If you replace the resistor shown with a 60 ohm resistor in series with a 250 pot, you should cover the range from 4mA to 20mA. If you don't care about strictly limiting the output to the 4mA to 20mA range, a 500 ohm pot in series with a 50 ohm resistor will give a slightly larger range, limited to about 25mA max.

SmartSelect_20190225-123759_Drive.jpg
 

ChrisP58

Well-Known Member
You need to know what load voltage range the current source needs to work into.

The minimum dropout voltage of some circuits will severely limit the range of your output voltage.

If you're only starting with 4.5 Volt battery, you may not have much left at the output.

For instance, the datasheet input to output spec number for the 317 is 3 Volts. Subtract the 1.25V for R1, and you only have 0.25 Volts left. And zero when the batteries age.
 

crutschow

Well-Known Member
Most Helpful Member
Here's the LTspice simulation of a two transistor constant-current circuit.
It requires a little less than a volt of headroom
Pot U2 adjusts the current from 4mA to 20mA.
A possible disadvantage of the circuit is that current is nonlinear with respect to pot wiper position.
If you want a linear adjustment with position, then you would likely need to go to a more complex op amp circuit.

1551151723739.png
 
Last edited:

electrocad

New Member
Thanks to everyone that took the time to respond to my query.
The 2 - Transistor circuit looks to be the what I was looking for,
I assume that the load goes between rload and -4.5V, and also
the "Bottom" of R3. I seem to remember that I had a 0-30mA
meter in series with the load.

Thanks again,

Regards

Malcolm J
 

crutschow

Well-Known Member
Most Helpful Member
I assume that the load goes between rload and -4.5V, and also
the "Bottom" of R3.
No.
The load replaces the resistor Rload.
With a 4.5V supply, its maximum allowed resistance is about 175 ohms.
 
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