[sdiy] Improving LED response when driven from half-wave rectifier

[Paul Perry]

If you have + and - rails available, I expect you could just run the signal 
to the base of  a PNP and a NPN transistor, which themselves then drive the 
relevant led.

paul perry Melbourne Australia

----- Original Message ----- 
From: "Justin Owen" <Subject: [sdiy] Improving LED response when driven from 
half-wave rectifier


>I figure some of you would have crossed this bridge already so...
>
> I'm using 2x LEDs to show the positive and negative-going halves of an AC 
> waveform. The input (10V (+/-5V) PP) is duplicated with one signal being 
> half-wave rectified and then fed to an LED to show the positive-going part 
> of the waveform, the other signal is inverted, then half-wave rectified to 
> show the negative-going part of the waveform. All good so far.
>
> Problem is that as soon as the waveform cycles below the LEDs forward 
> voltage drop the LED no longer illuminates - so, e.g. a 4V PP waveform 
> will not illuminate the LEDs at all. I'm using a 330R series resistor off 
> the half wave rectifier FWIW.
>
> How would I get the LEDs to illuminate down closer to 0V PP? At 0V PP they 
> should obviously be dimmed because there is no signal - but I'd like them 
> to display a better small signal response.
>
> I'm figuring biasing the half-wave rectified output upwards by 1.5-1.8V or 
> so would be one solution - any others?
>
> ...and secondly - is there going to be any real advantage in swapping the 
> 330R series resistor on each LED for some type of transistor current 
> source?  The LEDs are just a visual indicator - they don't represent a 
> specific measurement of any type.
>
/listinfo/synth-diy


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