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'[EE]: Pic driving Optocoupler & Led with same pin?'
2000\08\31@054956 by Graham North

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Hi

I currently have a few boards with pics driving triac driver optocouples.

For fault finding purposes I thought it would be a good idea to put an
ordinary Led in
parallel with the optocouple.  However when I did this neither the led lit
nor the optocouple
triggered.

Between the pic pin and the optocouple is a 330 ohm resistor.  I tried
reducing this but it
made no difference.

If the optocouple is removed the Led lights fine.

Any ideas?

Regards

Graham North

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2000\08\31@060214 by Michael Rigby-Jones

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Placing different LEDs in parallel is always a bad idea.  Due to the
differing foward voltages, only one LED (the one with the lowest foward
voltage) will light.  In your case I would have expected either the LED or
the opto to work, but probably not both.

If you wish both to work, each LED should have it's own current limiting
resistor, assuming the driver can supply enough current to supply both LEDs.

Regards

Mike

> {Original Message removed}

2000\08\31@062314 by Muhammad Irfan(TECHNOMAN)

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Try placing LED in series instead of placing it in parallel to optocouple
and reduce series resistance.
Did you check if LED is removed optocouple works fine? It seems your
optocoupler's LED gone short circuited.
Irfan
{Original Message removed}

2000\08\31@062934 by Graham North

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Thanks for the reply Mike.

I am almost certain that neither lit.  I will check.  I understand what you
say about the different forward voltages.
I will try the two resistor setup.

Thanks

Graham North
       {Original Message removed}

2000\08\31@062938 by Graham North

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Thanks Irfan,

Why didn't I think of that!  Good idea.  If the led was removed the
optocouple worked fine.

Thanks for the suggestion I will try it.

Graham North
       -----
       From:   Muhammad Irfan(TECHNOMAN) [SMTP:spam_OUTtechnokhTakeThisOuTspamCYBER.NET.PK]
       Sent:   31 August 2000 16:50
       To:     .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
       Subject:        Re: [EE]: Pic driving Optocoupler & Led with same
pin?

       Try placing LED in series instead of placing it in parallel to
optocouple
       and reduce series resistance.
       Did you check if LED is removed optocouple works fine? It seems your
       optocoupler's LED gone short circuited.
       Irfan
       {Original Message removed}

2000\08\31@065056 by Michael Rigby-Jones

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Be carefull.  How much voltage overhead do you have?  If you are driving the
opto with 5 volts or less, then you will not have a lot of overhead with two
LED's in series.  For fault finding I guess the optimum would be an LED in
parallel and one in series.  The parallel led would tell you that the driver
was operating, and the series device would indicate that the opto was
working, or at least not open circuit.

Mike

> {Original Message removed}

2000\08\31@071626 by Arthur Brown

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Put the led in series with the led in the optocoupler and lower the resistor
to 100R to 200R

Regards Art

//----------------------
Beware of Programmers who carry screwdrivers.

{Original Message removed}

2000\08\31@090350 by Olin Lathrop

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> For fault finding purposes I thought it would be a good idea to put an
> ordinary Led in
> parallel with the optocouple.  However when I did this neither the led lit
> nor the optocouple
> triggered.

Different LEDs have different forward bias on voltages.  It is generally a
bad idea to paralell two diodes of any type for this reason.  Your external
LED has a lower on voltage, therefore it gets all the current.  To fix this,
use a transistor, spare get, or whatever is convenient to drive the external
LED indirectly from the opto input signal.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olinspamKILLspamcognivis.com, http://www.cognivis.com

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2000\08\31@175215 by Gennette, Bruce

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Why don't you temporarily put a resistor in parallel with the 330R to reduce
it a bit so that a (slightly) higher voltage and current can get through to
power the pair of LEDs? Take a few seconds to calculate the required
resistor so that the opto coupler specs aren't exceeded.

If you need to design a test point into your circuit boards just use a short
link - fit the test LED during construction, then, after testing, replace
the LED with a link. LEDs in series get the same current, but reduced
voltage - they should still work, but at a reduced light output - your triac
driver may be a few nanoseconds late switching as the light output rises to
the trigger level a little later, but that should not be a problem for
testing purposes.

Bye.

       {Original Message removed}

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