4-20 mA converter

Thread Starter

Mousivandhossein

Joined Dec 8, 2020
25
Hi,

I designed a circuit as in the picture attached.

could you please tell me that the circuit is good or can be better?
2.png
 
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atferrari

Joined Jan 6, 2004
4,320
Once you do it, you will know if it could work or not (sorry - had to edit what my crappy keyboard sent when posting from my mobile)

Put some effort prior to asking here.
 
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Thread Starter

Mousivandhossein

Joined Dec 8, 2020
25
Once you do it, you Will the flash of it.

Put some effort prior to asking here.
In fact, I put some resistor and capacitor in the circuit by my thought for reliability.
The Diode to avoid mistake connection.
The resistor 2.2K and 100nF cap to avoid any spike voltage on CE of the transistor.
The 220pF cap for compensator.
 

Thread Starter

Mousivandhossein

Joined Dec 8, 2020
25
hi M,
Can you give more information about your project.?
It should be possible to simulate in LTSpice.
E
Hi Eric,

I want to convert a voltage to 4-20 mA current, I know the circuit will work but I have not experience with this circuit and my question about is there a better circuit than my circuit?
 

ericgibbs

Joined Jan 29, 2010
12,883
hi M,
It is important to consider how this 4 to 20mA is going to be used by any equipment that is connected to it. It is connected to -9V.
Also your output current appears in the wrong sense,
ie: with -3V inp you get a 20mA current and at -0.6V its is 4mA,

You said in your first post -3V thru -0.6V for 4 to 20mA.

E
 

Thread Starter

Mousivandhossein

Joined Dec 8, 2020
25
hi M,
It is important to consider how this 4 to 20mA is going to be used by any equipment that is connected to it. It is connected to -9V.
Also your output current appears in the wrong sense,
ie: with -3V inp you get a 20mA current and at -0.6V its is 4mA,

You said in your first post -3V thru -0.6V for 4 to 20mA.

E
Hi Eric,

OK, in fact, the power supply is a single 24 VDC and I need double power supply due to the op-amp, I produced double voltage from the power supply, the 4-20 mA current must enter to negative of 24VDC power supply.

I want to convert output DAC voltage from a microcontroller to 4-20 mA, after DAC I used an op-amp inverter to invert the voltage of DAC.
3.png
 
Last edited:

ericgibbs

Joined Jan 29, 2010
12,883
hi M,
You have to choose a high enough supply voltage to drive the current output stage, its call compliance.
That means there must enough supply voltage to allow for the voltage drop across the Emitter resistor, transistor and 250R load resistance when passing 20mA thru the load resistance

With a 9V supply thats just possible, using a 12V supply will give some overhead to spare. [Compliance voltage]

E
 
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