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White LEDs not working on red/white LED torch

Discussion in 'Troubleshooting and Repair' started by acharris_77, Sep 18, 2014.

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  1. acharris_77


    Sep 18, 2014
    Hi group, my name is Anthony and I just found this forum and being a novice at soldering and electronics thought I would post here for some help.

    I have a torch that I use for astronomy that is both a red light and white light LED's. The unit has a switch to change between the red LED's and White LED's. There are 2 LED's of each colour and when I switch to red both LED's light up and switch it to white and nothing lights.

    So I got my multi meter out and tested for continuity across the LED's and the Switch, and when I put continuity to the circuit when in the position for White LED's 1 white LED lights up but the other don't. Do the same to the red and they both light.

    The circuit is powered of a simple PP3 9V battery. I am still a novice at this stuff, but my conclusion is that one of the white LED's have burnt out and shorting out the other one, or I got a crack in a trace or solder, but I can get continuity other than the 1 LED.

    Would one LED blown cause the 2nd one to stop working as it is in parallel or not? Any help is appreciated. Thanks.

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi Anthony and welcome to Electronics Point :)

    Yes, if one LED fails it could cause the other one to fail to light as well.

    Normally LEDs are connected in series, not in parallel, since this ensures that the same current flows through both of them. (Connecting them in parallel is not recommended because the forward voltages of two LEDs can differ, even within the same batch.) Connecting them in series also makes more sense if the supply voltage is 9V because there is only one current path; connecting them in parallel would mean two independent current paths and therefore twice the load current on the battery --> half the battery lifetime.

    If the LEDs are in series, as I suspect they are, then a break anywhere in the circuit will stop both of them. Also if either one fails and becomes open circuit, the other one won't light.

    To really find out what's going on, you really need to draw up the schematic diagram of the torch.

    Or at least show (with a diagram) or describe very clearly the points you connected your multimeter to, when measuring continuity.

    LEDs will not normally indicate continuity with a digital multimeter on resistance range. If your multimeter has a diode test range, you may get a useful indication with the red LEDs but you may not with the white LEDs because white LEDs have a higher forward voltage (3~3.5V typically) than red LEDs (about 2V typically).

    Also LEDs are polarised so you need to either measure the right way round, or measure both ways.
  3. Thirantha


    Aug 31, 2014
    You need to check the switch first for proper functioning. Usually this kind of response is given by the fault in the switch.
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