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4-20 MA sensor Protection
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Please be positive and constructive with your questions and comments.

4-20 MA sensor Protection

by TomasReabe on Fri Sep 09, 2011 1:15 pm

So what I need to do is protect My ADC during a sensor failure.
For me the worst sensor failure is when it failes and acts like a short.
I am also trying to limit the voltage drop created by the protection circuit.
If I am willing to have a .7V voltage drop I could something like this. http://www.eleccircuit.com/constant-current-source-led-drive/
But I am thinking that by using something like this I can get down to a .05V voltage drop.
Circuit.jpg
Op Amp Current sink used as current limit
Circuit.jpg (47.09 KiB) Viewed 5466 times


The problem I can see using a current sink is when it limits the current the Micro controller could see 24 V on the high side of the sense resistor for the Micro controller.
I am looking at using a Current source to solve that problem. But is there a problem with using a Current source as a current limiter?
is there something else I should be looking at?
A PTC does not like a narrow current band and High temp range. I would like to be able to work from 0C up to 85C
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Re: 4-20 MA sensor Protection

by zener on Fri Sep 09, 2011 5:07 pm

Your ADC is not ground referenced? It is floating/isolated? If so then I don't see how the MCU could see the 24V.

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Re: 4-20 MA sensor Protection

by TomasReabe on Fri Sep 09, 2011 5:22 pm

Your ADC is not ground referenced? It is floating/isolated? If so then I don't see how the MCU could see the 24V.

Sorry my ADC is not floating the sysem shares the same ground but the ADC and micro have a regulator to get VCC down to 5V.

The ADC is the one from here http://forums.adafruit.com/viewtopic.php?f=31&t=12269.
The data sheet is here http://ww1.microchip.com/downloads/en/devicedoc/22088c.pdf.
From what I can understand the inputs need to stay within a range of -.4v to +5.4V if I have VCC set to 5V

So when everything works it goes just fine, But when the sensor shorts I see to high of a voltage on my ADC does not like it. I am using the other chanels and they go bonkers.
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Re: 4-20 MA sensor Protection

by zener on Fri Sep 09, 2011 5:46 pm

So the "bottom" of the 50 ohm resistor is connected to ground?

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Re: 4-20 MA sensor Protection

by uoip on Fri Sep 09, 2011 6:07 pm

Pardon this naive response, but I think I'm missing something obvious here. Why not limit the current by putting a pull-up resistor between the 24V supply and the sensor? Eliminate everything below the 50 ohm sense resistor and connect its low side directly to ground, and connect the sensor's high side to 24V via something like a 200 ohm pull-up? If the sensor is shorted, the resistors will make a voltage divider that will give a bit less than 5V to the micro. If the sensor tries to pass 20mA, you'll have 1V across the 50 ohm sense resistor, 4V across the pull-up, and 19V across the sensor.
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Re: 4-20 MA sensor Protection

by TomasReabe on Fri Sep 09, 2011 10:45 pm

So the "bottom" of the 50 ohm resistor is connected to ground?

No, the ADC conects where shown using CH+ and CH- and the ADC looks at the difference between CH+ and CH-. But the ADC is conected to VCC and GND and the data sheet says CH+ and CH- need to stay within VCC and GND.
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Re: 4-20 MA sensor Protection

by TomasReabe on Fri Sep 09, 2011 11:02 pm

uoip wrote:Pardon this naive response, but I think I'm missing something obvious here. Why not limit the current by putting a pull-up resistor between the 24V supply and the sensor? Eliminate everything below the 50 ohm sense resistor and connect its low side directly to ground, and connect the sensor's high side to 24V via something like a 200 ohm pull-up? If the sensor is shorted, the resistors will make a voltage divider that will give a bit less than 5V to the micro. If the sensor tries to pass 20mA, you'll have 1V across the 50 ohm sense resistor, 4V across the pull-up, and 19V across the sensor.

Not at all Naive.
The Sensor needs 10V to run and I am hoping to run saftly on a 12V system I loose .5V in the 50 ohm sensor. So I only have 1.5V for the Current Limiter. I want to loose the lowest amount of voltage that I can.
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Re: 4-20 MA sensor Protection

by veggek on Sun Sep 18, 2011 9:00 pm

TomasReabe wrote:So what I need to do is protect My ADC during a sensor failure.
For me the worst sensor failure is when it failes and acts like a short.


I am following the suggestions in this thread

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1283882387

including the addition of a zener diode at the Arduino ADC input (last post in above thread).

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Please be positive and constructive with your questions and comments.