Powering AC LED - old stereo

Discussion in 'General Electronics Chat' started by frusciante89, Feb 18, 2013.

  1. frusciante89

    Thread Starter New Member

    Nov 13, 2012
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    Hi everyone, I have an old Marantz amplifier with a broken lamp. The lamp is rated 8V AC 200mA. However, I used my multimeter to measure voltage and current: Voltage is 7.33 V AC and current is 5.68 mA (first silly question: do I have to use the AC amp or DC amp setting? cause the AC amp setting is reading a zero)
    I want to replace the buld with an led, whose specifications are these:
    http://www.sparkfun.com/datasheets/C...547W2C-A13.pdf
    I suppose I have to use a diode, a resistor and a capacitor, but how can I choose the values?

    Thanks for you help
     
  2. #12

    Expert

    Nov 30, 2010
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    This is a no-no on this site. It's a beginners site and the owners don't want them playing with wall power...unless I have misinterpreted your idea. Please clarify.
     
  3. frusciante89

    Thread Starter New Member

    Nov 13, 2012
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    I need an advice on the values of the components for the LED...
     
  4. #12

    Expert

    Nov 30, 2010
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    and what kind of power are you starting with?
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    It says in the OP that he wants to replace a lamp rated at 8V with an LED. With an 8VAC source, he will need an LED, a diode and a limiting resistor. I usually use a 1K for most indicator usage. The diode will be placed in reverse bias parallel with the LED.
     
  6. #12

    Expert

    Nov 30, 2010
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    There is still the measurement of 5.68 ma DC in a circuit that is supposed to run a lamp at 200 ma. That is enough to run an LED as an indicator but there might be something not working right in the driver circuit. If the driver circuit can not produce more than 5. 68 ma, that would fail to light the original lamp and maybe fool the repairman into thinking the lamp is bad.
     
  7. frusciante89

    Thread Starter New Member

    Nov 13, 2012
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    I did as you told me Bill but the light is very dim... do you know why?
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    Measure the voltage source with the LED/Diode/resistor combo connected. You can then calculate a resistor value that will give you 20ma through the LED.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    What is the source of the 2.2VAC? For illumination of a meter, you will probably need at least 10 to 20 ma to an LED to get enough light out of it. It may be easier to use one of the lamps from a series string incandescent Christmas tree lights.
     
  10. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    use a 1amp bridge rectifier, a 470R resistor and your led.
     
  11. MrChips

    Moderator

    Oct 2, 2009
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  12. BillB3857

    Senior Member

    Feb 28, 2009
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    Wouldn't the 1.4V drop due to the bridge use up most of the available voltage? Not much left when tying bridge to 2.2VAC. Am I missing something?
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    btw, this thread is an old thread and the op has long forgotten about the issue.
     
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