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Analoge circuit conversion:- 4 to 20mA & 1 to 5 volts

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Joe McGivern

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I have recently had a situation where I needed to change the analogue current signal output from an instrument from 4/20 mA to 1/5 volts to suit the input to another piece of equipment.

There are converters for this purpose and are readily available for those who have the cash and know where to look.

While doing some tests I had an accident and caused irreparable damage to the converter. I needed a quick fix and connected a 250R resistor across the terminals where the voltage signal was to be connected. By connecting the 4/20 current signal through the resistor it created the desired effect ,ie 1 to 5 volts depending on the level of the current flowing.

It seems to work perfectly but I am concerned that there will be a detrimental effect to either one or other instrument in the long term.

The purpose built converter has several options which make it adaptable to suit other circumstances but "my shortcut" works

If any one can understand what I am doing please comment.

Regards JMcG
 
DOesn't the voltage then become dependent on the current draw the attached device has?
 
dknguyen said:
DOesn't the voltage then become dependent on the current draw the attached device has?
The attached device doesnt draw the current; it produces the current from a power supply which is also in the loop. That is precisely the point. Add a unity gain buffer and viola! A 4-20 ma to 1-5 Volt converter. Make sure you run the op amp from a voltage higher than 5 Volts to give yourself some headroom. You can also call any voltage less than say 0.6V the same as "loop open" which was one of the original ideas behind the current loop in the first place.
 
Joe McGivern said:
It seems to work perfectly but I am concerned that there will be a detrimental effect to either one or other instrument in the long term.

There might be galvanic isolation between input and output that your simple resistor might not be able to provide. How about the load impedance? Just a 5K impedance in parallel with the 250 Ohm would make your signal way off. These converters are expensive for a reason.

Only you would know.
 
Papabravo said:
The attached device doesnt draw the current; it produces the current from a power supply which is also in the loop. That is precisely the point. Add a unity gain buffer and viola! A 4-20 ma to 1-5 Volt converter. Make sure you run the op amp from a voltage higher than 5 Volts to give yourself some headroom. You can also call any voltage less than say 0.6V the same as "loop open" which was one of the original ideas behind the current loop in the first place.


Hi Papabaravo,

whether is it possible by connecting a series resistor of say 250 Ohms in the signal path to get the required conversion --it is indicated in the site at
**broken link removed**
 
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eblc1388 said:
There might be galvanic isolation between input and output that your simple resistor might not be able to provide. How about the load impedance? Just a 5K impedance in parallel with the 250 Ohm would make your signal way off. These converters are expensive for a reason.

Only you would know.
Galvanic isolation and current loops are kind of silly in the same context. In a current loop you have a single supply which connects the transmitter and the receiver. There is nothing to isolate.

Galvanic isolation is what you use when there are two power supplies with no common ground, or a common ground with a potential difference from one end to the other.

I don't understand why you would want to connect the two things, can you elaborate?
 
Not galvanic isolation between the current signals, but between the wanted analogue output signal and the current signal.
 
:eek:
eblc1388 said:
Not galvanic isolation between the current signals, but between the wanted analogue output signal and the current signal.
Except you haven't explained what would motivate you to do something like that. "That's what everyone else does" is not a sufficient justification for makeing such isolation a requirement. What motivates the requiremnent for isolation?
 
I've always used a 250ohm resistor and a rail-to-rail 5V single supply opamp for my 4-20ma converters. Never any problems (application was using thermocouples).
 
This kind of instrumentation is called '2 wire 4-20ma loop powered instrumentation' and is used extensively in petrochemical facilities. The advantage of using loop powered instruments is that there is no calibration errors due to different lengths of signal cable between the field transmitters and the control electronics frequently hundreds and even some thousands of feet apart, plus the same wire pair carries the required power for the field device as well as the measurement value. The 'live zero' value of 4ma also allows your receiving electronics to tell the difference between a valid 'zero' measurement and a opened loop due to a wire, connection or field instrument failure.

It is standard to power the loop from a 24VDC power source in the control house and the return loop is wired to a 250 ohm precision resistor (frequently .1% or even .05%! $) and then back to the 24V common. The house electronics 'reads' the measurement as a 1-5VDC signal and typically converts it via an A/D converter for further action and control.

There are isolators and signal conversion modules that work with these kind of signal loops but they are usually used to solve special compatibility problems rather then required on a typical loop.

So a 250 ohm resistor is the 'industry standard' solution to convert the current value to a voltage value, so you are on the correct path. Typically a instrumentation grade differential input circuit is used to reject common mode noise that can be picked up on these long signal wire runs. Also low pass filtering is sometime used to help with noise and EMI rejection.


Lefty
 
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Papabravo said:
:eek:
Except you haven't explained what would motivate you to do something like that. "That's what everyone else does" is not a sufficient justification for makeing such isolation a requirement. What motivates the requiremnent for isolation?

Why should I justify "what everyone else does" to you if you don't know why? If you don't, you don't.

I didn't say it is a requirement, I said "might be" and only the OP can check if it is his case. Go back and read my post again.

There are situations where the joining of the commons are not desirable. What you have posted suggested that it would never be a real need for isolation.
 
Thanks everyone. I got some good stuff from the replies. Lefty seems to be on the "same wavelength" as I am, or closest to it.

The runs of cable I used were short(all inside the same control panel)but there must be a possibility of picking up noise because there are other AC circuits in close proximity Eg, power supply to a 6 Kw motor and 240 volt control circuits for the same

Regards JMcG
 
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