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I'm looking at ways to design a multi-functional analog input that can take both voltage and current inputs for industrial applications. The voltage signals are in the 0-40V range and the current signals are 4-20mA loops. I have found a few reference designs that seem to accomplish this (usually for 0-10V and 4-20mA signals) by switching on/off the current loop resistive load. Since I want to be able to take in up to 40V, I could add a voltage divider. I found the following schematic for a reference PLC module (enclosed in a rectangle is the part I'm interested in):

enter image description here

Source: http://www.analog.com/media/en/technical-documentation/application-notes/an-1522.pdf

The above schematic is not rated for 40V input signals, but the approach used is similar to what I have in mind. My concern is on how to protect the circuit from user mistakes. I will use this schematic as a reference and forget about the 40V that I want for a second.

There are two basic scenarios that shouldn't happen, but can happen:

1) A voltage is fed when in current input mode (resistive load on): in this case, as long as R1 can handle the power it should be fine. For 10V that is 400mW, so no big deal. If the input signal can't source enough current the system will give a wrong reading, but as long as nothing gets burnt in my circuit I'm happy.

2) A current signal is fed when the circuit is voltage input mode: this case is the one I find more critical. Here is where I'm not sure what happens, and where my question comes. If R1 is not connected, and let's assume we have 20mA coming in, this will go through the voltage divider, and the resistors (assume 1 watt at best) won't be able to handle this power. Also, the output voltage of the voltage divider will be too high, and I'm not sure those clamping diodes will handle it? I'm not sure what happens in this scenario and whether the circuit is protected or if it will just explode. I would like somebody to help me understand exactly what would happen in this scenario, and see if I can extend this approach to work with my 40V signals requirement.

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  • \$\begingroup\$ You can always add PTC polyfuses to protect from over current and TVS from over voltage . Both should limit ok but you ought to keep Differential impedances balance wrt gnd and use a way to shunt CM noise to earth ground so that ground or radiated noise is attenuated. \$\endgroup\$ – Tony Stewart EE75 Jul 18 '18 at 22:28
  • \$\begingroup\$ It seems like under those conditions you would saturate the input of the first or second op-amp, thus giving you an irrational output value. If you clamp the input voltage, that also will give you an irrational output. Try dividing input by ten and re-scale the display. \$\endgroup\$ – user105652 Jul 19 '18 at 0:31
  • \$\begingroup\$ @TonyEErocketscientist, a PTC polyfuse for the voltage divider would have to trigger at 6mA or less (assuming 1 W resistors). For this value they are not so easy to find. About the TVS, are you suggesting adding a second TVS at the output of the voltage divider? \$\endgroup\$ – jmarin Jul 19 '18 at 21:25
  • \$\begingroup\$ @Sparky256, so you don't think something will be damaged during scenario 2? I'm not sure I understand what you say by "dividing input by ten and re-scale the display". \$\endgroup\$ – jmarin Jul 19 '18 at 21:27
  • \$\begingroup\$ You mentioned using a voltage divider. Its ratio need not be 10:1 like I mentioned, as long as the output reading is scaled to match the input. \$\endgroup\$ – user105652 Jul 19 '18 at 21:40
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schematic

simulate this circuit – Schematic created using CircuitLab

Conceptual design for a balanced differential input of cascading clamps to reduce worst case power and provide best case overvoltage protection.

Most CMOS has the 2 stage diode clamp method inside for ESD protect of 3kV from 100pF or 5~10mA DC max.

To reduce Common Mode impulse noise or RF.

schematic

simulate this circuit

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  • \$\begingroup\$ Thanks, the information was useful for checking some aspects of the circuit. \$\endgroup\$ – jmarin Jul 20 '18 at 21:36
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After some more thought, I realized I don't have to worry too much about scenario 2 since a 4-20mA sensor requires a certain maximum load for working properly and effectively run the corresponding current through the loop. If resistor R1 is not present, the sensor will see a relatively high impedance (coming from the voltage divider in this case, or the opamp inputs if the voltage divider was not present), and therefore there will be no current flow. The sensor simply won't work but no damage should occur. The only potential damage happens with scenario 1 so I just have to make sure resistor R1 has a high enough power rating to handle a voltage input.

Thanks to everyone for your comments, answers, and help.

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