Powering from battery instead of USB

Discussion in 'The Projects Forum' started by wiringnoobie, Jul 13, 2012.

  1. wiringnoobie

    Thread Starter New Member

    Jul 13, 2012
    2
    0
    Hello

    I am completely new to electronics, I know very little and am aware that this is probably a really unconventional project but would really appreciate some pointers.

    I have been working on a little project with the intention of fitting the empty shell of a vintage torch with a new torch's LEDs and the addition of a circuitboard taken out of a USB light that glows dark to light - like a moodlight to make the torch look like it's pulsing.

    I have managed to hook up the 9 LED light circuitboard taken out of the brand new torch to the USB glowing light's circuitboard and it works as I would like it to. However, I would now like to run the torch off of batteries rather than off the USB port.


    The new torch came with 3AAA batteries and on the + wire going into the circuitboard with the LEDs on the there was a resistor.

    There were no resistors on the USB light's circuitboard (see attached picture)

    If I were to power the torch using the 3 AAAs I am aware that the current would be a lot higher than the 500ma coming from the USB power. Could I simply use the resistor I've already mentioned that came with the torch? Would it be as simple as moving this to the + wire going into the USB glow light's circuit board?

    3AAAs > Resistor > USB glow light's circuit board > 9 LEDS on a circuit board

    Once again I'd be really grateful for any help.

    Bea

    USB light circuit board : http://bitsy.me/7kw
     
    Last edited: Jul 13, 2012
  2. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    3 AAAs will give you less voltage than the 5V supplied by the USB port, and will probably work just fine. The 500mA rated capacity of the USB port is not relevant - the device will take the current it needs depending on the voltage supplied.

    The problem is this: A USB device will probably work fine at 4.5V as supplied by fresh batteries. Testing this will not put your components at risk. But when the batteries fade to under 4V, there may not be enough voltage to light the LEDs. LEDs won't light at all below a threshold voltage. There is still no risk to your stuff, but it all may quit working well before the batteries are fully spent.

    I'd be tempted to try 4 AAAs in series, but supplying 6V to a device designed for 5V carries a small, but real, risk of damage to that device.
     
  3. wiringnoobie

    Thread Starter New Member

    Jul 13, 2012
    2
    0
    Thanks, this is really helpful I'm so grateful to speak to someone who knows what they are talking about.

    I suppose I should admit that I had tried to power the device from 3AA batteries and I found that the circuit board got physically very hot, I had assumed that this was because the current was too high, but could there be another reason?

    Many Thanks!
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Hmmm, maybe you should post a picture or a schematic. A well-designed board shouldn't really get "very hot". Can you localize the heat to a single spot? Does it get hot when used normally, with USB power?
     
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