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From: Tilmann Reh
Subject: Re: Pulsing LEDs - need more current, how?
Date: Thu, 02 Jan 2003 08:31:00 +0100
Organization: Autometer GmbH
NNTP-Posting-Date: 2 Jan 2003 07:28:42 GMT
X-Mailer: Mozilla 4.78 [de] (Windows NT 5.0; U)
Frank Petrof schrieb:
> My pulse width is 2mS, and in this application it needs to be that
> short, frequency is between 50 and 85 Hz.
This is not a short pulse. Typical short pulses of that current
amplitude within IR emitting diodes are in the microseconds range...
You should read the datasheet of your diodes carefully to verify
that the diodes can stand these rather long pulses of high current.
> If I go so far as to put a
> pot inline and vary it down to almost 0, still about 50mA max is all I
> can get out of them with this pulse width.
How did you measure the current? Did you really measure the pulse
current, not the average? Did you already check if the driver
transistor really gets into saturation? Why do those diodes take
almost 3 volts at that current? Is the transistor capable to drive
such higher current pulses? Many things to check before we get deeper...
> Can someone help me understand pulsed LED operation and its resultant
> effect on brightness? In order to put 500 mA through the array with
> such a short pulse width, do I need to substantially increase the
> voltage feeding them?
If it really fails due to the total voltage, another option
would be to use two diodes in series each with a separate
resistor. But I doubt this is the problem.
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
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