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Conversion circuits

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Shutterspeed

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I currently have transmitter outputs of 4 to 20 mA, and 10 to 50 mA. Not wanting to buy new analog meters, I am looking to build a simple interface circuit to convert the 4 to 20mA to 0 to 1 mA,or 0 to 1 VDC or the 10 to 50mA to 0 to 1mA or 0 to 1 VDC. Any suggestions would be welcome.

SS
 
I currently have transmitter outputs of 4 to 20 mA, and 10 to 50 mA. Not wanting to buy new analog meters, I am looking to build a simple interface circuit to convert the 4 to 20mA to 0 to 1 mA,or 0 to 1 VDC or the 10 to 50mA to 0 to 1mA or 0 to 1 VDC. Any suggestions would be welcome.

SS

What meters were you thinking of using instead?
 
Any digital or 270 Degree movement, like a panel meter. I have collected some nice 1.75" X 6" EIL, and 1" X 4.25" Bailey instruments. The baileys are brand new.

SS
 
You could use an inverting op amp with two signal inputs and one offset input for this.

For the 4 to 20mA input, run the current through a 50Ω resistor and amplify it by a gain of 1.25.

For the 10 to 50mA input, run the current though a 25Ω resistor and amplify by a gain of 1.0.

Then offset the output by 0.25V to give 0V output with either a 4mA input or 10mA input.

Since this is an inverting circuit it gives an output of 0 to -1V. If want 0 to +1V, you would need to add another inverting op amp with a gain of -1.0 at it's output.
 
The attachment is a design that should work. I just bashed it together on paper so it should be tested before deployment.

edit: oops I just noticed some errors, I will repost with corrections
 

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  • current loop to voltage converter.doc
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Here is the corrected schematic. Design review by other members is welcome.


edit: on further consideration, it would seem that the input resistance of the signal is not R1 in parallel with R3 because the source is a current source, effectively putting 0 ohms across R3, so the input resistance is exactly R1. In this case, R2 would be better set to something close to 500 ohms for minimum error, so perhaps 510 ohms 1% instead of 432 ohms.

I estimate battery life at around 5.7 years.
 

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  • current loop to voltage converter.doc
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Last edited:
Thank you RadioRon and Crutschow for the information.
I just now need to find someone who knows how to repair a Ametek/Dixson bargraph panel meter. Real nice unit, but a buddy of mine hooked it up wrong when I worked at the Plant. Needless to say he let some of the smoke out, and it made its way from the garbage can into my collection.

Shutterspeed
 
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