A customer wants me to add an expansion board to my industrial computing platform to handle multiple 4 to 20ma loops. Currently, he wants to power the loop and receive data from transducers. I've done a little research, and I've found that typically each receiver has a 250 ohm resistor to turn the 4-20ma range into 1-5V. 12-36V (24V nominal) seems typical for power requirements.
However, the examples for receivers seem way oversimplified. A simple 250 ohm resistor would need to be 6W+ to handle a shorted 36V line, for example.
Here are the problem areas as I see them:
1) Shorting performance. I'd expect the system should handle indefinite loop shorts.
2) Isolation. How important is isolation? I see a note that separate loops powered from the same supply could induce ground loops. This would complicate supporting multiple channels. Currently, I'm ignoring this.
3) Multiple receivers. If they want to chain receivers, 250 ohms seems limiting. Is this still common? Could I use say 50 ohms to reduce the voltage drop? I guess that'd be dropping my noise immunity by a factor of five.
4) Surge events. I think I'll put in a series diode, a parallel diode, and a Bourns surge supression IC per-line.
I'm going to set a goal and say 4 channels of up to 36V loop voltage.
I'm tempted to boost my 12-36V system input to 40V and use a power amp to drop that to the loop current. I could use a power amp like the OPA452 for each channel, which would give short circuit protection and per-channel voltage selection.
Am I overthinking it? Not thinking it through enough? Do I just stick four 250 ohm resistors and ADCs in parallel tied to a 24V source and call it a day? (mostly rhetorical)
Bonus question: What's the best way to transmit 4 to 20ma signals? Should I use something like an AD421?
Best Answer
Shorting. As Olin Lathrop mentions, the power supply (at the receiver) sending power to the sensors should be current-limited. Say the power supply driving the loop is +40 VDC, but current limited to around 30 mA -- perhaps by using a LM317 and a 47 ohm resistor in the "current limit" circuit shown in the LM317 datasheet and the " Simple circuit to limit current through a FET " question. (It doesn't matter if the current limit is in series with the "hi" line going out to the sensor, or in series with the "lo" line coming back from the sensor to the 250 Ohm resistor, or both). If something horrible happens and the two lines going out to the sensor are shorted, the LM317 limits the current to 30 mA through the short and through the 250 Ohm sense resistor, resulting in 7.5 V across the sense resistor (or perhaps only 5.6 V across the resistor with a different configuration of ADC-protection diodes), zero V across the short, and the rest of the +40 VDC is across the LM317 current limiter. That gives at worst 30 mA * 7.5 V = 0.225 W, so a single common quarter-watt resistor can (just barely) handle it indefinitely. The LM317 has thermal overload protection and can withstand the full 40 VDC and 30 mA current indefinitely.
Isolation. The two wires running out to each sensor should be insulated, and each sensor itself should not have any other current-carrying wires connected to it, so each sensor is fairly well isolated from all the others (even if the "hi" lines for all the sensors are connected together at the receiver). The current going into any well-insulated object should be equal to the current coming out of that object -- electrons generally don't jump out of wires -- so I don't see any path for "ground loops" anywhere. The main advantage of current loop sensing is that it's much easier to insulate current-loop wires to give practically zero leakage current than it is to make voltage-sense wires highly conductive to give practically zero voltage drop.
Multiple receivers. No matter what you do, you can't have hundreds of receivers along a loop. I suppose you could sacrifice some noise rejection and use 50 Ohm sense resistors to get a few more receivers before it doesn't work at all. But eventually, you're forced to either (a) use "Current Loop Repeater" devices, or else (b) use "zero voltage drop" current-to-voltage converters aka "transimpedance amplifiers". Alas, some receivers assume that they can connect one end of the 250 Ohm sense resistor to safety ground -- that works fine as long as that's the only receiver that does it; connecting merely 2 such receivers in a loop to a sensor causes bad data.
Surge suppression. Sounds fine, possibly already a bit of overkill. Perhaps add a Zener to protect the ADC from over-voltage and reverse-voltage.