Help - current sensing switch

Discussion in 'The Projects Forum' started by YT1300, Apr 3, 2015.

  1. YT1300

    Thread Starter New Member

    Apr 3, 2015
    Hi, wondering if anyone can point me in the right direction. I have a circuit that drives a load requiring 100mA to operate correctly. Vcc is a regulated 5v DC supply from a small wind powered generator. The generator is easily capable of supplying the current but I only want my circuit to be able to turn on the load when there is sufficient current available (not just 5v available). In other words I don't want my application to half work. I want it to either come on when I switch it on or if there is insufficient power/current available then not come on at all. Grateful for any pointers in how to do this as I've been scratching my head. Thanks
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    Without connecting a load and drawing current, there is no way of knowing how much current is available.

    You could measure the speed of the turbine, as this would give a good indication of whether there is enough power available, and connect the load only above a certain speed. but, since the output from a wind generator can be very erratic this would lead to often and brief cycling of the load. Furthermore, all the time the generator is turning it will be producing some potential power ( but not necessarily enough to power the load directly) which is not being used.

    Consider using the wind generator to charge a battery and to only connect the load when the battery voltage exceeds an upper threshold (indicating a useful level of charge is present). When the battery voltage falls below a lower threshold the load is disconnected, therefore, allowing the battery to charge before connecting the load again.

    You could achieve this with two comparators and a latch of some kind. the higher level comparator sets the latch and powers the load until the lower level comparator clears the latch and disconnects the load.

    Have a look at op-amp comparator circuits and SR flip-flops.
    Last edited: Apr 3, 2015
  3. DickCappels


    Aug 21, 2008
    A technique I being used in a battery powered glucometer to make sure there was enough left in the battery to complete a test before the user inserted his consumable (read: expensive) test strip is to apply a load to the battery for a moment when power is first applied and notice how much it dipped before giving the use the go-ahead to slip in a test strip and poke his finger.

    You might devise a similar test for your windmill-powered system. If the wind isn't blowing stiffly enough, you should see a drop in voltage when an appropriate dummy load is connected.