Question on zero-crossing circuit

Provides filtered smoothed DC to the comparators

My bad, Bad eyesight.

mistook one for another :-(

Most LM339s will do horrible goofy things, in all four sections, if you pull one input a little below ground. D3 won't prevent that. See the National data sheet, notes 3, 6, and 8. They warn about this *three* times. Also see their ZCD example on page 14.

Good point, no power to the 339s. Probably Spice powers them up, for free. Simulation is wonderful!

Another approach, which I've always thought was pretty, is the one Tektronix uses in their scopes. They heat-shrink a piece of hookup wire to each of the AC input leads, and use the few-pF capacitance to get a line-isolated 60 Hz signal into their low-level circuitry.

That avoids having to have mains voltage on the PCB at all. They probably have to use higher-rated insulation on the AC leads, but it's still a win.

You would have to worry about the resulting phase shift if you need the actual zero crossings as opposed to just line sync and power fail detection, so you wouldn't want to use it for triggering a triac, say.

Cheers

Phil Hobbs

Thank you, John. My interest in ZCD circuits stems from a need to limit "clicks" when an amplified, audio-like signal is gated. I am looking at 13.8 Vp-p and 0.1-20kHz frequency. I want to trigger a uController with the leading or trailing edge of a comparator pulse so that the turnon or shutoff occurs only when voltage is near zero. I will have a look at your circuit ideas.

The Beesons arrived from England in the late 17th century, settled in Pennsylvania (Uniontown used to be called Beesontown), and family genealogy has it that some went south and some went west. I'm a western Beeson, but have since migrated north to Canada.

doug

Thanks for your reply. I do have an actual need, but only at lower voltages (13.8 V max). It has to cutoff closer to zero than the 1 diode drop of Jim's circuit, though. I'm interested in seeing your design if you're willing to post it.

-doug

. . .

. . .

--- From your description it sounds like you have an AC signal which you want to switch into a load, but only after the switch is turned on and then close to when the next AC zero crossing occurs.

Once that happens and the AC is feeding the load, you want the switch to stay ON for some time and then, when it's switched OFF, disconnect the AC from the load close to when the next AC zero crossing occurs.

Is that what you're looking for?

John Fields

This one is kinda cute

Simple, cheap, low power, tight and accurate window.

(Assuming you want an isolated AC line zcd, which you may not want)

I aims to please >:-}

D3 does prevent substrate diodes from conducting. Besides, those of us intimately familiar with the LM339 know why those "horrible goofy things" can't happen in my circuit.

And you don't. So you just throw your usual sand in the air. ...Jim Thompson

More sand in the air. Expands on my comment that you know nothing about how an LM339 works. ...Jim Thompson

Straddle zero, or just one side of zero? What voltage level do you want? ...Jim Thompson

Power supplies, signal levels, AC or DC-coupled? ...Jim Thompson

Amen! And there's not always a handy power outlet nearby to plug a wall wart into.

It's easy enough for me to follow. It looks like most of the other production schematics out there. Your schematic "keeps it real" for novices. The real world often lacks the time to "hand hold" or provide "one on one" answers. So one learns to be grateful for any schematic whatsoever.

It looks like you used LTSpice to draw it. LTSpice is the lingua franca of usenet.

Hot power "keeps it simple" for journeymen. The WARNING - RISK OF ELECTRIC SHOCK sticker advertises that journeymen skills are required.

Jim's very protective about his circuits (good and bad). I wasn't attacking him, I was doing the sci.electronics.basics thing, i.e. walking through the circuit discussing what it does, for the benefit of beginners. To encourage said beginners not to give up, I pointed out out that the circuit looked even more complicated than it was, because the power and signal paths weren't distinguished very clearly. Then he got mad and started calling names, but I can't help that.

Jim's a chip designer, and so works in a different world than most of the rest of us--a lot of his stuff gets made in large numbers and has to work in far less well-controlled conditions that your average instrument, over process corners and not-always-optimal customer designs.

In board-level designs, that frequently leads to using way too many parts, but then board-level isn't his gig.

Cheers

Phil Hobbs

It works without a Vcc supply?

Cool.

Yep. I tend to use too many parts, in discrete designs >:-} But the result is circuits that work come hell or high water. ...Jim Thompson

Yes! Ain't that marvy >:-} ...Jim Thompson