As part of a larger design I am trying to monitor current through a wire and cut it off if it exceeds a threshold. After the current has been turned off it should remain off for a minimum time period and only turn back on when a reset button is pressed.
The circuit below is my current design idea. I'm looking for any reason why this wouldn't work, as well as hearing any suggestions for better designs (excluding software solutions).
- The current sensor will output a voltage proportionate to the current through the wire, causing the op-amp to output a high signal when it exceeds a REF voltage.
- This will trigger the monostable circuit to produce a high signal for a set period, setting the data pin low and turning on the Latch Enable (LE) to read in that low.
- The high-pulse also switches the Output Enable (OE) pin on the D-latch, the Output goes to high-impedance and the NFET is turned off by R3. (Possibly unnecessary but it doesn't hurt)
- Once the pulse has finished the LE pin is driven low and the data pin is high. I think I might have to introduce some small delay for the data pin here so that the D-Latch doesn't read in the high-going data pin but instead keeps its value latched at low?
- The OE pin is released again and the D-latch should output its latched-to state of low. Once the button is pressed the data pin is read again and the output switched on, allowing current to flow.
Added R5, C1 to provide a delay of 2ms at data pin. This is the time to charge from 0 to 2V (minimum V_IH @ 3.3V supply) with 3.3V applied. Data pin must be stable for only a few ns after LE goes low so this should be plenty of headroom
Added R6, R7 to provide 25mV of hysteresis for the op-amp which corresponds to a change of 100mA through the load