How do I stop my circuit from turning on and off 100s of times at low voltage?

Discussion in 'General Electronics Chat' started by moeburn, Aug 25, 2013.

  1. moeburn

    Thread Starter Member

    Aug 16, 2013
    31
    0
    Just the question:

    Is there any way I can make a range, a little higher than the circuit's minimum input voltage, so that the circuit won't turn off until it gets below 2.4 volts, but it won't turn back on unless the voltage is ABOVE 2.6 volts?

    The Back Story:



    Hello! I have built an AA->USB charger, that allows you to use rechargeable AA's (or alkalines, or NiCads, or AAAs, or 3.7v li-ions, basically anything between 2.5 and 4.5 volts), and using a DC-DC boost converter, outputs 5 volts to a USB socket.

    The circuit works great! Sort of. When the batteries reach the end of their charge, and they are right at that threshold voltage level, the circuit turns on and off a few hundred times per second. It's kinda helpful, in that because my phone makes a 'beep' every time you plug or unplug a charger, my phone will squeal like a dying chipmunk when this threshold level is reached, and I'll conveniently know when my AA batteries are dead.

    Unfortunately, if I'm not around when this happens, it actually kills the phone's battery. Because when the batteries are low enough, the charger turns on and off a hundred times a second, which means the phone/tablet's CPU has to load the 'beep' sound from memory, and play it, and stop playing it, and also load the 'charging' lightning bolt icon, and display it, and then remove it, all a hundred times a second. So basically, if I don't catch my phone the minute it's done charging from this AA charger, it will start to very quickly discharge.

    My Question:

    Is there anything I can do to prevent this? For example, the thermostat dial on a space heater doesn't tell the space heater to "turn off when it gets this hot, and turn off when it gets this cold", it tells the space heater "turn off when its 2 degrees WARMER than this, and turn ON when its 4 degrees COLDER than this". It makes a range. Is there any way I can make a range, a little higher than the circuit's minimum input voltage, so that the circuit won't turn off until it gets below 2.4 volts, but it won't turn back on unless the voltage is ABOVE 2.6 volts?

    That would be the ideal fix, I think. But if that's too complicated, I also thought of using a delay circuit. Just make it so that when the voltage is high enough to turn the circuit on, it actually doesn't turn on until a 2 second timer has passed. This won't fix the issue, but it will reduce the severity of it. So at least my phone will only be beeping once every 2 seconds, instead of 100 times per second.

    Anyone out there have any idea how I can do this?
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,341
    1,024
    What you are looking for is hysteresis on the voltage sensor (a comparator?). Hysteresis makes the trigger on point and trigger off point a little different so that the control snaps rather than oscillates.
     
    KJ6EAD likes this.
  3. moeburn

    Thread Starter Member

    Aug 16, 2013
    31
    0
    Yes! Thank you! I have never heard that word before, but knowing it certainly makes it easier to research! Unfortunately, I don't think there is any voltage sensor in this circuit; I think the DC-DC boost converter IC just requires 2.4v (or whatever the number is, I can't remember exactly) to simply turn on. So I would have to add a voltage sensing hysteresis circuit that triggered a transistor or power FET or whatever to allow current to flow out of the batteries when the top end of the voltage range had been reached, and stopped the current flow when the bottom end is reached.

    Know of any circuits like that that are smaller than a loonie (about the size remaining in my altoids tin)? :p
     
  4. JohnInTX

    Moderator

    Jun 26, 2012
    2,341
    1,024
    Try something like a system supervisor that is typically used to control a microprocessor system when the voltage drops. Look around the list to see if there is one with the voltage point and output you need. These have hysteresis built in as well as a precision reference. Other manufacturers have them as well.

    Good luck!
     
    KJ6EAD likes this.
  5. moeburn

    Thread Starter Member

    Aug 16, 2013
    31
    0
    perfect, thanks!
     
Loading...