The  circuit was intended to sense current to a model train.  I liked it for
that purpose.  But it may not suite your needs.  What it does do is since AC
or DC current, can withstand any current or voltage, and only affects the
circuit with a 1.2 V drop.  Witch is nothing at all condersiddering today's
inflation.

The two diode/rectifier pairs can be as delicate or as robust as needed.
They are acting as a rectifier and small ones can handle a amp.  It is a
case of  "I don't gota show you no stinking badges!"  But you do need a load
in the circuit to protect the power supply, else it would be basically a
short circuit.

The diodes in the opto's are basically led's and if you insure that the
voltage across a led will never exceed 1.2 V,  do you still need a limiting
resister?  Does a limiting resistor limit the current or does it use the
current to limit the voltage?

If there is not a load on the circuit, then the simplest implementation
would be to parallel a small diode (or a led if you want a indicator)
(reversed bias) across the opto's diode and put a limiting resistor in
series with that parallel circuit.

Bill

----- Original Message ----- 
From: "David Minkler" <mink@luxtron.com>
To: "Microcontroller discussion list - Public." <piclist@MIT.EDU>
Sent: Friday, October 22, 2004 9:25 AM
Subject: Re: [PIC:] AC zero-crossing circuit idea (LM339 hints?)


>
>
> Bill & Pookie wrote:
>
> >Think it takes 0.6 volts to get a diode to conduct.  Therefore two
> >diodes/rectifiers in series will drop 1.2 volts.  By putting the two
> >diodes/rectifiers , biased the same as the opte's diode, in parallel with
> >the opto, you limit the forward voltage to the opto to 1.2 volts.  And
you
> >may not need a current limiting resistor.
> >
> >
> What limits the current through the series pair?  How do I get control
> over the current through the opto diode?  What if the opto's diode needs
> more (or less) than two diode drops to respond correctly?  What about
> temperature effects?  Current through the opto's LED is what you are
> really trying to limit.  If you limit current through it, the opto's
> diode will manage its own Vf.  A properly sized resistor is a cheap
> reliable solution.
>
> >Then a single diode/rectifier biases in the opposite direction across the
> >circuit will limit the reverse voltage across the opto's diode to 0.6
volts.
> >
> >
> Certainly a good idea if significant reverse potential is possible.  You
> still need to limit current through that diode.
>
> >Or....  you can replace the single diode/rectifier (to limit reverse
> >voltage) by putting a second opto/(diode/rectifier pair) across the first
> >but bias in reverse direction.  This will protect both opto diodes from
> >excessive forward and reverse voltage.  And give you another pulse.
> >
> If you need it.
>
> Dave

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