>> A special case may exist if input s are say [+50 V | float] =A0rather
>> than +50/ground. In such a case if inputs are taken to emitters of NPN
>> optos and collectors all joined, then a sense resistor from the
>> combined collectors to a voltage > 50V (say 60V) will cause all
>> combined collectors to be at 60 V (transistor off OR inout float) and
>> 50V (transistor on and input =3D +50V.
>>
>> Whether it is acceptable for a floating input to have 60V applied in
>> this was is moot in the absence of a proper problem definition.

> I'm confused..

Read my original description and sketch one channel.
I was describing the opposite way of using the opto to what you describe.

You suggest using the input to drive the opto's LED and do X/Y PIC
dive via the transistor.

I drive the LED from the PIC with XY drive and tak the output from ALL
transistors arranged with either C or E commoned (see text ) and E or
C connected one per opto to an input.

My meythod has the following "advantages"

- Load resistance on input can be high - does not need to assume
ability to draw LED driving current.

- Does not need a resistor per input or pair of resistors etc - uses
an OPTO per input but R's etc are common to all circuitry.

- Sample input voltages relative to reference of choice - eg voltage
may be +50/0 or +50 float etc.

The OP's spec is not clear enough to be sure whether input is xxV /
float or + or - wrt ground or able to provide opto level current or
...m. My solution accepts  most of those points.

I do prefer the "inputs drive LED via R" solution but it requires some
of the above to be true.

SO my comments re reverse bias were re CE junction. BE junction is not
accessible or affected.
IF turning on one transistor causes another to have Vce > ~ 5V the off
transistor may reverse conduct. This matters if the input circuit
cannot tolerate this. My various discussions were aimed at avoiding
this.

> there is 100V to 1kV isolation on typical optocouplers.

Yes. Or more. OP has not specified need if any but optos can be bought to s=
uit.

> The LED bit is driven via resistor from 50V line. It can be balanced
> pairs, isolated pairs or common ground.

Yes. Not in my scenario. Good solution if tolerable. Note that at 50V,
dissipation is 50 mW per mA of opto current. And, if say this was a
phone line that current would be far too high to be acceptable. OP
needs to specify.

> Transistor outputs: I've only ever seen NPN commonly. Some vague memory
> of triac/thyristor? =A0Though I suppose PNP =A0or FET is possible.

SCR are available. And FET. PNP I've not seen. Quite often, as bipolar
floats, you could use NPN upside down for reverse polarity BUT here
you cannot as the scenario I describe drives devices into all  4
quadrants at various times.

> The base can be "floating".

Yes. That was how I treated it.

> At more than 5V reverse =A0you could have zener action on =A0BE =A0juncti=
on
> and =A0current =A0via =A0BC.

But, as bad or worse, you get VECO breakdown at 5 to 10 V. Not usual
to use a transistor there.


> But at PIC input and drive levels I think an
> output transistor matrix will work. Examination of a datasheet or device
> would tell. =A0many NPN have zener volts around 6V to 7V, which is why a
> 1N4148 diode across base from ground on RS232 line driving a NPN via 1k
> to 10K is a good idea, in the event you actually meet a serial port with
> ANY -ve swing or indeed -8.6 (typical pumped interface converter from
> +5V) =A0to -12V (original =A0drivers with a real -12V supply).

Yes - for Vbe considerations. For Vce as here doesn't apply.




     Russell

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