Just found this thread: Overcurrent protection for 4-20mA inputs

But it seems to focus on limiting the current at the source. My device is intended to be connected to various 3rd party voltage source / transmitters of which I won't have control over. I assume just a fuse won't do it as a short circuit may blow the ADC pin before the fuse opens

Hi Mark.

I would like to ask you for a little background on your project.

I work in an industrial setting and am familiar with control instrumentation (I'm an I and A technician)...However I predominantly work with lab analyzers. It's been years since I calibrated a transmitter so I'm rusty. But the key question I have, is this project for an industrial or biomedical application, or is it a personal project you're doing at home?

Reason being, I know my company has strict policies and guidelines about modifying equipment or processes...i.e "don't". Not so much a problem when your trying to control the temperature of bathwater, much bigger deal if your supplying 200 lbs of steam to a boiler or reactor.

So maybe some background?

Sceadwian, in this application I do not think he is trying to protect the load from the source, but rather the source from the load.

You are correct that the ADC is what he is trying to protect. I misread. Usually fault sensing in 4-20 mA loops is for shorts.

And that is what usually happens. This is not a circuit board, in most cases it is hundreds of feet of cable in conduit, transmitting signals from a local transducer to a remote PLC or controller, and from that PLC to a control valve actuator. Sometimes short circuits occur, thus the limiting resistor.

I'm thinking that voltage clamping could cause problems in these kinds of circuits due to their nature.

Originally Posted by ke5frf

i.e. the brand and function of the transmitter and sensing element (honeywell, foxboro, rosemount? thermocouple, dP cell?) the process being sensed, (fluid, level, flow, catalyst, furnace, tank, vacuum?) and the control element. PID. PI. or just P?

I'm asking as much out of curiosity as anything, but I'd feel better if you said your working on a system for baking gingerbread for Christmas

A quick Google search led me to this technical bulletin:

http://pdfserv.maxim-ic.com/en/an/AN3195.pdf

Looks like it discusses fault protection for a 24V source and A/D converting the 4-20 by sensing across a 2 ohm resistor , making it 8-40 mV, and the amplifying it and converting it to a .8 to 4.0 volt signal.

Maybe this kind of circuit is what you're after?

I'm trying to setup a PID loop to maintain feedwater level for a 15.5Mbtu steam boiler

Kidding. This is not for control, this is for a remote monitoring system for industrial / waste water processes. kind of like SCADA without the "C".
I looked at the Maxim doc before, but it seems to be a solution for limiting the excitation voltage (source). This system will be installed by a third party, they will provide the power supply and / or transmitter

Originally Posted by Sceadwian

Either way clamp diodes will work. He said he wanted to protect the ADC which is what is reading the voltage dropped by the resistor, I read that as trying to protect the load, as the source of the current is an external device.

I'm not quiet sure what you're protecting against though, you need to protect from reverse voltage of the sense resistor that's about it. If you can apply 24V directly to the ADC then that means your have an exposed circuit board or a bad connection layout. You can't protect a circuit from stupid people if it's all available like that.

If we are talking about a powered transmitter, then that would be current limiting, no big danger, but a lot of 4-20 systems have a separate power supply with a passive transducer ==> http://www.omega.com/Manuals/manualpdf/M3939.pdf If the run to the sensor shorts, the full voltage of the power supply would be present at the positive side of the sense resistor, no?
This is also the connection point of the adc pin, which can only take 5.3 volts before I breach the factory smoke containment system.

I re-did the math and I must have made an error before. With the resistor at the high end of the 1% tolerance, the transducer would have to exceed 5% over spec for the ADC pin to climb over 5.3V. I was getting 5.6-ish before.

OK, here is how it could cause a problem:

A 4-20 mA signal is an analog representation of a transducer. The transmitter is calibrated (zero and span) to represent the high and low range of operation for the particular process. For example, a CONTROLLED process that ranges from 1 psi to 100 psi would be 4 mA=1 psi 8mA=25 psi 12 mA=50 psi 16 mA=75 psi 20 mA=100 psi

There would be a setpoint somewhere in the middle of that range where the process is operating normally, perhaps 50 psi.

If a disruptive condition causes a pressure increase, say of 25 psi, the transducer would sense it and adjust its output to 16 mA. At the PLC, the algorithm of the microprocessor senses a deviation from its set point. It calculates an appropriate response and sends another signal via a seperate 4-20 loop (which is also calibrated) whose responsibility is to adjust a CONTROL VALVE. In this case, the control valve would want to open wider to relieve the pressure in the system. So, the steady control condition of 12 mA loop current would hold the valve actuator at 50% travel. The upset requires it to open, for simplicities sake lets say 25% to relieve the 25% increase in pressure. So, the valve opens to 75% travel, relieving the pressure and then eventually the whole system gets back to equillibrium.

OK, I explained the system to show you how clamping it could cause trouble. If, anywhere along the circuit, the transucer is bypassed or shunted, the entire loop could be thrown out of whack and the valve actuator could slam shut or be thrown full open, creating an overpressure condition or a dump. In a system with $100,000 or more of product or raw materials, or explosive, etc etc, this could be a big deal.

Also, these PLCs often have multiple loops operating off of a single supply. If one loop gets clamped and it effects the power supply voltage for all the loops, an entire process could be sent into chaos.

Generally speaking, these PLCs have many safeguards and back-ups systems, but the point is valid.

I'm just saying, and I'm not a Process or Electrical engineer, but I'm not sure its a good idea.

Originally Posted by ke5frf

OK, here is how it could cause a problem:

A 4-20 mA signal is an analog representation of a transducer. The transmitter is calibrated (zero and span) to represent the high and low range of operation for the particular process. For example, a CONTROLLED process that ranges from 1 psi to 100 psi would be 4 mA=1 psi 8mA=25 psi 12 mA=50 psi 16 mA=75 psi 20 mA=100 psi

There would be a setpoint somewhere in the middle of that range where the process is operating normally, perhaps 50 psi.

If a disruptive condition causes a pressure increase, say of 25 psi, the transducer would sense it and adjust its output to 16 mA. At the PLC, the algorithm of the microprocessor senses a deviation from its set point. It calculates an appropriate response and sends another signal via a seperate 4-20 loop (which is also calibrated) whose responsibility is to adjust a CONTROL VALVE. In this case, the control valve would want to open wider to relieve the pressure in the system. So, the steady control condition of 12 mA loop current would hold the valve actuator at 50% travel. The upset requires it to open, for simplicities sake lets say 25% to relieve the 25% increase in pressure. So, the valve opens to 75% travel, relieving the pressure and then eventually the whole system gets back to equillibrium.

OK, I explained the system to show you how clamping it could cause trouble. If, anywhere along the circuit, the transucer is bypassed or shunted, the entire loop could be thrown out of whack and the valve actuator could slam shut or be thrown full open, creating an overpressure condition or a dump. In a system with $100,000 or more of product or raw materials, or explosive, etc etc, this could be a big deal.

Also, these PLCs often have multiple loops operating off of a single supply. If one loop gets clamped and it effects the power supply voltage for all the loops, an entire process could be sent into chaos.

Generally speaking, these PLCs have many safeguards and back-ups systems, but the point is valid.

I'm just saying, and I'm not a Process or Electrical engineer, but I'm not sure its a good idea.

I get all that five by five. We do process controls with PLCs etc. and are familiar with the concepts and pitfalls (just this summer I cracked an expensive mold because I plugged the "A" molds thermocouple into the controller for the "B" mold. The A controller kept trying to drive the mold temperature up but was not seeing the correct feedback, by the time I noticed, the mold had overshot 40C. Stuff happens )
This is not the situation here. This is a monitor system for reporting tank levels, freeze protection, power fail, Etc. at remote unattended locations. No control, not tapping into loops used for the process, just calling the cavalry.

I figured you had to know how control loops work since you were working with a transmitter here

I posted the quick and dirty tutorial on the subject for the sake of future Googlers or current readers who may not know what a 4-20 loop is predominately used for.