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'Total Current from a 12C508?'
1998\01\07@223721 by Markwint

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Hello All,

This is my first posting to the list, so please excuse me if this is a
previously discussed topic.

I'd like to drive an IR LED with a 12C508, sending out a bit stream on a 40kHz
carrier. I'm wondering if I need to use a transistor to drive the LED, or if I
can just use one or more output pins in parallel. I notice that the data sheet
shows each pin on the '508 can source or sink 25mA -- can each pin supply this
much current simultaneously, or is there a total limit for the chip as a
whole? Ideally, I'd like to be able to send 100mA through the LED (in pulses
of 12 usecs each).

Is this possible if I tie several output pins in parallel?

Thanks,
Mark Winters

1998\01\07@225839 by Andrew Mayo

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face
Use a transistor. Although you can parallel port pins to do it, the
voltage drop at maximum load current will be significant, and you may
not,depending on your battery voltage, have sufficient voltage swing to
drive the LED reliably (you need around 1.5V across the LED but this can
be as high as 1.8V or so depending on LED tolerances). Anyway, you're
stressing the chip with thermals. Using a transistor, you can drive the
IR LED very hard. I have experimented with a system which fed 700mA peak
for 12uS to an IR LED (a small tantalum capacitor of 47uF or so provides
sufficient charge storage to supply this sort of current). An SM3181
transistor has an Ic peak of 5A, comes in a TO92 case and has around
0.1V Vce at 1A - a superb device for this sort of application.

Also using a transistor, you can put two IR LEDs in series with a 5V
supply, doubling the light output without requiring more LED current.

{Quote hidden}

1998\01\08@090637 by Andy Kunz

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face
>I'd like to drive an IR LED with a 12C508, sending out a bit stream on a
40kHz
>carrier. I'm wondering if I need to use a transistor to drive the LED, or
if I

Use a transistor discharging a cap for a brighter IR flash.

==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

1998\01\08@224449 by Markwint

picon face
Andrew,

Thanks for the reply. Using the SM3181 transistor sounds intriguing. Any idea
who carries them? I couldn't find it with a quick check in my Digi-Key
catalog...

Thanks again,
Mark Winters

----------------------------------------------------------
Date:    Thu, 8 Jan 1998 16:57:45 +1300
From:    Andrew Mayo <andrewspamKILLspamGEAC.CO.NZ>
Subject: Re: Total Current from a 12C508?

Use a transistor. Although you can parallel port pins to do it, the
voltage drop at maximum load current will be significant, and you may
not,depending on your battery voltage, have sufficient voltage swing to
drive the LED reliably (you need around 1.5V across the LED but this can
be as high as 1.8V or so depending on LED tolerances). Anyway, you're
stressing the chip with thermals. Using a transistor, you can drive the
IR LED very hard. I have experimented with a system which fed 700mA peak
for 12uS to an IR LED (a small tantalum capacitor of 47uF or so provides
sufficient charge storage to supply this sort of current). An SM3181
transistor has an Ic peak of 5A, comes in a TO92 case and has around
0.1V Vce at 1A - a superb device for this sort of application.

Also using a transistor, you can put two IR LEDs in series with a 5V
supply, doubling the light output without requiring more LED current.

{Quote hidden}

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