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I am planning to make a 24-volt DC circuit breaker with MosFET, when there is earth leak in the load (DC motor), or a human touches the terminal (24 Volt DC) with hand, to simulate AC, earth leakage detection relay operation. Currently used is a small shunt resistor with op-amp current sensing circuit as below, connected Arduino to drive a buzzer when leakage current reaches 1 mA. But I am not able to detect current changes. The supply voltage used is 12 Volt DC. The Op-amp used is 741.

Design Assumption.

When incoming current equals to the outgoing current through sensing resistor or difference between them equal to nearly zero, no leakage is happening in the circuit.

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    \$\begingroup\$ (1) Please edit your schematic to show relationship between Vsupply, GND, V+ and V-. (2) You need to show the component values for all resistors, and load current. (3) Why are you using a 741? Are you aware of the input and output voltage swing limitations? \$\endgroup\$
    – Transistor
    Mar 9, 2020 at 12:29
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    \$\begingroup\$ somebody touches the terminal with hand drive a buzzer when leakage current reaches 20 ma When a human touches a live point and 20 mA (or more) flows then that human might be dead already before that 20 mA is reached. In principle that circuit can work but only under the right circumstances. There are a lot of details you need to get right for that circuit to work. My guess is that you lack the experience for that (but feel free to prove me wrong). \$\endgroup\$
    – Bimpelrekkie
    Mar 9, 2020 at 12:44
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    \$\begingroup\$ How would you differentiate motor load from leakage in that circuit? \$\endgroup\$
    – winny
    Mar 9, 2020 at 12:45
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    \$\begingroup\$ Example of a "tiny" detail: when no current flows, what will be the voltages (relative to ground) be across the 1 Ohm \$R_{sense}\$? What voltages will there be at the inputs (and output) of the opamp? Can that happen when the opamp has a supply voltage of +/- 12 V? I suggest you search for "high side sensing" and "current sensing" circuits and figure out how they work. Starting from such a "principle schematic" will not teach you how to get the details correct to make it actually work. \$\endgroup\$
    – Bimpelrekkie
    Mar 9, 2020 at 12:48
  • \$\begingroup\$ when incoming current equals outgoing current or there is no difference exist, or difference nearly equal to zero, no leakage exist to ground, \$\endgroup\$
    – Cathode
    Mar 9, 2020 at 12:51

2 Answers 2

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Op amp used 741

If your op-amp positive supply is 12 volts then high side current measurement is impossible when the monitored supply voltage is as high as 24 volts even if potted down 1:1 by R1 and R2: -

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This is because the input voltage range of the 741 does not extend to either its positive or negative rail. You'd need a rail to rail op-amp and, to be safe, you must ensure that R2 is a little less than 1 kohm in case the 24 volts drifted upwards a tad.

Alternatively, use a high side current monitor like the INA139 from TI: -

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Design Assumption.
When incoming current equals to outgoing current through sensing resistor, or difference between them equals to nearly zero, no leakage is happening in the circuit.

No. If incoming current equals to outgoing current through sensing resistor then all you are proving is that there is no leakage to earth from the resistor. It doesn't tell you anything about where the current goes after that.

All your circuit can do (if you remove all the other design problems) is measure the current through RSENSE.

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Figure 1. Points at which to monitor current for earth leakage.

If you really want to check for leakage you would need to measure the difference between the current at (1) and (2). You might be able to do this by using running two conductors through a Hall-effect current transducer so that the two currents normally cancel out.

Others have already pointed out some of the other problems with the circuit.

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Figure 2. OP's version 2.

In principle, this idea would work but note that you have not got the circuitry to work out the difference between the two current sense amplifiers and this will have to work whether the error is positive or negative.

The big problem with this approach will be with the precision matching of the high and low-side shunts and amplifier resistors. This will be even more difficult if your power-supply voltage is higher than the op-amp and ADC maximum voltage.

Related:

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  • \$\begingroup\$ Load is connected between, high side and low side current sensing opamp differential amplifier configuration, and comparing values(High side & Low side output) with Arudino, make the circuit working \$\endgroup\$
    – Cathode
    Mar 10, 2020 at 13:26
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    Mar 10, 2020 at 13:34

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