Monitor power/voltage loss

Discussion in 'General Electronics Chat' started by Rmauser, Jan 26, 2014.

  1. Rmauser

    Thread Starter New Member

    Feb 12, 2013
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    Hello, I've got a circuit where a uC is monitoring for a loss in input (5V wall DC adapter). I have a large capacitor involved so the circuit can stay powered for a brief time when input power is lost and perform a few functions. I have the circuit working fine with a schottky and the uC input tied in behind the schottky so that the capacitor doesn't backfeed power to the uC sense pin when power is lost. The issue is the forward voltage drop of the schottky is affecting load stability (need as close to 5V as possible and the 0.34v loss of the current diode is too much). Does anyone know of a way for the uC to sense a loss of power without the capacitor backfeeding to the pin? I've attached a basic schematic of what I have going on here.
    [​IMG]
    Thanks for any help!
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Is U1 the ucontroller?
     
  3. Rmauser

    Thread Starter New Member

    Feb 12, 2013
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    0
    Hi, yes U1 is the microcontroller
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Have you looked at the internal schematic of V1? It is possible that its reverse leakage current is negligible compared to 1A. You could measure it.
     
  5. RichardO

    Well-Known Member

    May 4, 2013
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    You can replace D2 with a PMOS FET. Connect the FET with the drain to the battery, the source to capacitor and the gate to ground. Note that the source and drain are backwards from how a FET is normally connected. This is correct. In this configuration, the body diode of the FET is forward biased.

    Choose a FET with an on resistance small enough that the current flowing through it causes a negligible voltage drop. Make sure to select a logic level FET so that it will turn on from the 6 volt battery.

    I would add a 10K resistor between the battery and the voltage sense input to limit current under adverse conditions -- for instance if the battery is connected backwards.
     
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