4-20mA to 200-800mA Amp/Driver

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alanled

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Hi, this is my first post as Im a newbie please understand my lack of electronics knowledge!
I have a proportinal valve which requires a 200-800mA control signal the drive card for this valve doesnt exist any more so I have to make something. I want to control this with a 4-20mA output from a PLC. So the proposed circuit would amplify a 4-20mA input and give a 200-800mA output proporionaly. Any help with this would be great! thanks
 
Well, you didn't say what voltage the valve is, so assuming it's 12/24V, the attached circuit should do the trick.

The voltage across R2 is 150-750mV for 4-20mA current. We then add this to 12V/240 (50mV) to give 200-800mV at the positive input of the operational amplifier. The amp turrns on the transistors at its output and current passes through the valve and R3; due to the feedback to the negative input of the amp, the amp will correct the output current until the voltage across R3 is equal to the 200-800mV. As R3 is 1 ohm, this translates to 200-800mA through the valve.

 
I'm curious why you used the 1k/220k level shifter on the front end. Why not just use a 50 ohm resistor for R2? Your circuit leaves ≈50mA through the valve if the input current is interrupted, which might not be good.
 
Roff said:
I'm curious why you used the 1k/220k level shifter on the front end. Why not just use a 50 ohm resistor for R2?
Click to expand...
Sure 50R * 4mA = 200mV, but 50R * 20mA = 1000mV (need 800mV).

Roff said:
Your circuit leaves ≈50mA through the valve if the input current is interrupted, which might not be good.
Click to expand...
Probably not good If it's a problem, use the second half of the LM358 to disable current in this condition.
 
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