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  1. #1
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    Improving LED response when driven from half-wave rectifier



    I'm using the above circuit to drive 2 LEDs as shown. The input is a 10V (+/-5V) PP waveform. One LED gets an inverted and rectified input, the other LED gets a non-inverted, rectified input. One LED shows the positive-going (+V) content of the input waveform, the other LED shows the negative-going (-V) content of the waveform. All good so far.

    I'm finding that as the voltage of the waveform drops below about 6-7V PP - the LEDs are almost unlit. This means that e.g. a +/-2V waveform at the input will barely illuminate either LED.

    How can I get a better response from the LEDs? I don't need them brighter at +5V - but I would like them brighter at e.g. 1 to 3V.

    Would a current source help with this? Can anyone suggest a suitable schematic?

    Thanks in advance.

    •   Alt4th September 2012, 16:47

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  2. #2
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    Re: Improving LED response when driven from half-wave rectifier

    LEDs need about 2V to turn on. You can't expect to put 2V across them and expect them to work, ESPECIALLY if you've also got a resistor in series. You'll need to add some gain.

    - - - Updated - - -

    Actually, it's not clear what you are trying to do here. If you are just trying to light an LED when a +/- signal is present, just put a two resistors, two LEDs and and some protection diodes. You may not even need the protection diodes, depending on the reverse voltage rating of the LEDs.



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