massive failure

I made this 'brute force' circuit to try and quench the LED spad.

Massive failure. I made an oscillator (again :^)

+V bais--1k ohm---+-R1-+ | | | \| _ |--R2- ^ to comparator. R | 3 | +---+ GND

R1 ~ 10k, R2=100-1k, R3=10k-100k

Here's a scope shot,

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Chan 1. is the base and chan. 2 the signal into the comparator.

Recharging the C of the LED fires the comparator off again.

Reading stuff by S. Cova,

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(A nice patent, I didn't write the number in my notebook. I've got it at work. number tomorrow)

It seems I need to make the recharging 'common mode' (to the comparator) while the signal is differential.

George H.

Reply to
George Herold
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+input? -input? What comparator? If it's LM393, et al., what pullup?

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).


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Reply to
Tim Williams









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.

Reply to
George Herold

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.

Reply to
Robert Baer

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.

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Reply to
George Herold

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

Reply to
Robert Baer

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