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

[Tim Ressel]

I use a modified Howland Current Pump to drive LEDs, especially bi-color.

Here is the schematic:
https://dl.dropbox.com/u/5536133/led_driver1.pdf

--Tim Ressel



----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Cc: 
Sent: Wednesday, September 5, 2012 2:26 AM
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.

Thanks.

Justin




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