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I'm building a device to read 0/4 - 20 mA loop currents using a 250 ohm precision sense resistor. The power source inside the receiver used for driving the sensors is current limited to 30 mA. Suppose that due to some fault or operator error, an external 24 volt supply (that is not current limited) is directly connected to the system input thus putting 24 volts across the 250 ohm resistor. This would result in 2.3 watts of dissipated power. Is there any way to protect the 250 ohm resistor from such voltages or am I forced to use a high watt rated resistor?

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  • \$\begingroup\$ How about a 50mA quick blow fuse in series with the 250 ohm resistor? \$\endgroup\$
    – Andy aka
    Commented Apr 29, 2016 at 12:22
  • \$\begingroup\$ Usually it blows off. In any case you would need an aditional dissipating component if you divert the extra current with a kind of crowbar circuit. I guess you won't find precision 250 ohm resistor for large power. \$\endgroup\$ Commented Apr 29, 2016 at 12:26
  • \$\begingroup\$ Could use 8x 2.2k, a 3.3k, and a 16.5k all in parallel = 250Ω. 2.3W / 8 = 290mW each, so use 1/2 Watt. \$\endgroup\$
    – rdtsc
    Commented Apr 29, 2016 at 12:29
  • \$\begingroup\$ @Andy aka I'm looking for something that would provide continuous protection. \$\endgroup\$
    – hadez
    Commented Apr 29, 2016 at 12:31
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    \$\begingroup\$ It can be an electronic resettable fuse - use your imagination - mouser.co.uk/Circuit-Protection/Thermistors/… \$\endgroup\$
    – Andy aka
    Commented Apr 29, 2016 at 12:34

1 Answer 1

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A differential Op Amp measuers the voltage on resistor, if it is above level it disconnects for certain time the entire loop.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ This seems like a good idea. I'll have to perform some simulations to see if it's okay within my circuit. \$\endgroup\$
    – hadez
    Commented Apr 29, 2016 at 14:51

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