massive failure

It looks like, for the few microseconds during which the LED is recharging, the comparator is linearly coupled from output back to input through a low-gain path; this doesn't matter because the comparator has more gain. It also has plenty of phase shift, so NFB tends to create pretty PWM waveforms rather than smooth operation.

The LED isn't really forced to charge, it's just acting as a series coupling capacitor in a momentarily unstable loop.

If R3 were smaller, you'd couple less, but you'd also have less signal, which means you need more gain anyway, same problem.

Two things you could try: positive feedback applied via capacitor, which will increase the minimum pulse width from ~100ns to something more useful; else, perhaps an op-amp, wired so as to force the LED to a constant voltage. The error voltage generated in the loop corresponds to the LED current blips. The advantage would be, you can force the LED to some particular condition, such as constant terminal voltage, which may have slightly different behavior (response, time constant, whatever).

Tim

-- Deep Friar: a very philosophical monk. Website:

lup?

g,

=A0It

ms

ing

l;

ge.

Well I had an opamp before the LT1013(?) comparator.

The idea is to reduce R3, you are right. At 100k ohms it's a passive quench. lower R's and the LED just stay's on for a random amount of time. Here's the patent number for an active quench.

U.S. Patent No. 4963727

George H.

Pardon my french..but WHY the multiple waits .. wait countdown for ANOTHER wait which pops up a bullshit BOT "downloader" .. In computer terms, NO BYTE! One look at the circuit and o knew what the result would be.

Hi Robert, are you complaining about the image hosting site?

=A0"One look at the circuit and o knew what the result would be."

Oh, Well you are a better man than me then. I guess I needed my face rubbed in it. :^)

George H.

Hide quoted text -

Not really..if one sees certain undesirable feedback results enough times, one starts to recognize the pattern in their sleep.