On Thu, 1 May 2014 08:10:51 -0400, Douglas Beeson
wrote:

Hi all,

I did a search the other day on zero crossing detector circuits and came across this nice one by Jim Thompson:

http://www.analog-innovations.com/SED/Zero_Crossing.pdf

I think I have figured out how it works, except for capacitor C1. What does it do?

Thanks!

For those of you worried (unnecessarily) but such scare tactics as
"Most LM339s will do horrible goofy things", please see...

http://www.analog-innovations.com/SED/Test_LM339_Input_Range.pdf

Performed on a transistor-level netlist.

I had to go back and refresh my mind as to which input could give a
false logic output. As long as it's only the +IN input that goes
below ground, the LM339 won't output false logic. The -IN going below
ground will, IF -Vbe. (Current _will_ be drawn thru the ESD, just
limit it with an input resistor.)

I'll give the emperor-with-no-clothes the benefit of the doubt with
his statement, "Most LM339s will do horrible goofy things", and chalk
it up to old age forgetfulness... only if you take both inputs closer
to POSITIVE rail than ~1.4V will you choke off the (common-biased)
current mirrors and "... horrible goofy things" will happen. This
applies also to the LM324 and all devices in that same structure
class.

All of these "anomalies" can easily be understood simply by studying
the current mirror structures and what bias "starvations" happen below
-RAIL or close to +RAIL.

Any other questions, simply ask The Master :-} ...

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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| Voice480)460-2350 Fax: Available upon request | Brass Rat |
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