LED not operating correctly

Discussion in 'General Electronics Chat' started by xstream, Mar 31, 2010.

  1. xstream

    Thread Starter New Member

    Mar 17, 2009
    Circuit and LED Datasheet are attached

    This circuit is used to monitor the status of a 9VDC supply that powers an X-ray Source (XRS) Controller. An Altera Cyclone FPGA is used to control the supply. When the 9VDC is on, the LED will shine and the XRS Controller will turn on. If the LED is off, the XRS Controller is not getting 9VDC.

    My problem is with the LED. It sometimes lights up dimly, or does not shine at all, even though the 9VDC is on.

    As I start to measure the voltage across, I first attach a probe to the anode of the LED. Once this is done, the LED shines bright, as it should! I measure the voltage to be 1.965VDC.

    I then turn off the 9VDC, then turn it back on. The LED shines bright. I turn it off again, then turn it back on, now the LED shines dimly. So I go to measure the voltage across the LED again, this time, it does not light up right away. I get an initial voltage reading of ~1.4-1.5VDC for about 2sec (LED is off), then the voltage jumps to 1.965VDC (LED shines bright).

    Has anyone ever seen this before? Can someone please explain this please?
  2. SgtWookie


    Jul 17, 2007
    You have a bad connection (solder or otherwise) between R63 and D26, or D26 and ground.

    Fix it.
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    I agree, the meter may be giving you a good connection during testing.

    If your solder connections are good, you may have a faulty led. If is a cheap LED, you may have strained the leads into the lens causing the cup to lean to far away, or to close to the anode.
  4. teravenous

    New Member

    Mar 29, 2010
    First you must project a good circuit with counting. A led can be on with 1.5V~2.5V or 10mA~20mA . If you have a source and its voltage is 9V, you must use a serial resistor.

    What is value of resistor ?

    we want that led's current become 20mA. And we have 9V.

    LED's voltage is 2V.

    9V(source)-2V(LED's working voltage) = 7V .

    So we have 7V for resistor.

    V(Voltage)= I(Current) / R(resistance)

    we know Voltage and we know that how many current we want. SO we can calculate resistor's value.

    7V=0.02A/R *

    R= 350 ohm.

    * 20mA=0.002A

    Yes. That's all. If your circuit doesn't work. You must look your solders. Solders musn't be very much. And you must look your circuit's ways . They can be broken off.

    Have a nice day...
  5. xstream

    Thread Starter New Member

    Mar 17, 2009
    After re-soldering the LED, my problem is fixed. This surprises me because the soldering job looked good under the scope.

    Thank you for the responses!
  6. Audioguru


    Dec 20, 2007
    The schematic shows 365 ohms.

    20mA is 0.02A. 2mA is 0.002A.
  7. teravenous

    New Member

    Mar 29, 2010

    I just want to show that how is the calculation. LEDs can work with different voltages and currents. So resistances can be different. I assumed that we have a led and it works with 2V or 20mA.

    oh thank you so much. I didn't noticed this. I think i pushed to zero two times :) Thanks for correcting.