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Overcurrent protection for 4-20mA inputs

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Member level 2
Nov 18, 2006
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Itajubá MG Brazil
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Hi all,

Is there any known an simple way to protect 4-20mA inputs for overcurrent ?

I know it is not that difficult to do to the source side of the Loop, but I am looking for to the sensing side, the way it is show at the picture below.

Is a 5.1V zener be the easiest way to get the work done or a circuit to limit the current in the loop could be more efficient to the system (source + sensing side) ?

Any suggestion will be appreciated!

Thank you! :D

From practical experience, I suggest to protect the current input against short to a 24V DC source, which can be expected to power the sensors. Obviously a zener diode without additional means (e.g. a fuse) doesn't offer any protection in this case.

A possible solution, that can be found in some PLC analog modules is a PTC fuse in combination with a mid-scale (e.g. 50 ohm) shunt resistor.

Electronic current limiters or disconnect switches are another way.
Here is an example of protection I wouldn’t do (because of it’s complexity):

But, 5.1 V can be a bit to low, whereas 5.6V Zener should be just enough ..
If you do it together with a 63-100 mA fuse (any type will do, even PCB miniature) followed by 50-100Ohm shunt, this should provide you with sufficient protection for any 4-20mA input ..

Probably you can modify as show below,
analog signal converter,field protection
I have a different question in mind,related to 4-20mA signal. We have hundreds of instruments from different make installed into the Re-heating Furnace for Galvanzing line. The furnace is from TENOVA Hypertherm. It's not been set yet. It's under errection. Their engineer is with us these days. He once told me, we use current for analog input against the voltage input, because they are not suspectible for noise immunity, we can use unshielded cable for this, but on the other hand, our one engineer who was sitting their and listening,added that they had used 4-20mA shielded cable too in another plant. Tenova engineer told me, that we used to use voltage signal in early time like 80's from instrument.Now practice is changing to 4-20mA fashion.That is why, often a resistor space is given in the instrument that are transmitting analog signal, so a person can put a resistor in parallel and get a voltage signal out.He asked me to check myself. I opened ABB,Siemens smart temperature/pressure transmitter and there was space for external resistor to put. Again he was right. Another new thing that was new to me, that I didn't find in book regarding the thermocouple. If we short the inputs of thermocouple in a transmitter, all the transmitter began to display the room temperature and a 4-20mA output is then transmitted corresponding to it. I asked him about this, then he added nothing and leaves a smile.
One new thing that he told me, that in gas/air solenoid valves, we should use DC coils, I opened few different solenoids and see myself a rectifier attach after AC and pulsating DC was energizing the coil. I asked him, as why? He said, it gives them a greater life. AC is not good in this. I again asked him, it just a technique, I don't think AC coil would have greater life comparative to DC coil. He said, look man. I have spent my whole life in instrumentation and furnace making. I'm a multi-skilled, I'm a man who's into electrical/mechanical/designing for decades. I used to do PLC programming and many other things.I had installed & commissioned furnaces in many parts of the world.We have alot of research in this field and he said many other thing. He said. You are a young man,an inexperienced engineer,inexperienced in instrumentation,inexperienced in project,inexperienced in process lines and alot....alot.....alot.....He said, I have devoted my entire life into this field.He said, the thing that I'm telling you are the practices only.Now I have to think twice,thrice before asking any question to him.

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