>> So, at best, each LED is on 1/28 of the time.  This is neglecting the
>> time between turning off one segment and turning on the next.  If you
>> have an LED rated at 10mA displayed 1/28 of the time, do you need 28
>> times the power?  Surely not.  The LED couldn't handle it and we can
>> rely on persistence of vision to some degree.
>
> Correct. LED datasheets usually have a duty cycle to amp rating chart.
> 1/28 is about 3.5 percent duty cycle.

Often hobby LEDs can be impossible to identify, and therefore the data
sheet can't be checked.  The maximum continuous current rating of an LED
is limited by heat.  The instantaneous maximum current the device itself
can handle is significantly higher.  You can therefore use high currents
for short bursts as long as the average is within limits.

As a rough rule of thumb, figure an unknown T1 3/4 LED can take an
instantaneous current of 10x it's continuous rating.  The continuous
rating is normally 20mA for such an LED, so you can put 200mA thru it for
short periods of time.  At such large ratios, figure the average should
not exceed the continuous rating over any 10mS interval.  In this case, it
means 1mS on at 200mA, 9mS off.

However, the LED gets less efficient at higher currents, so the average
current needs to be derated a bit when the maximum/continuous ratio is
high.  At 10x, figure no more than 15mA average, so that would really be
750uS on, 9.25mS off.

Again, these are all rules of thumb only for when a data sheet is not
available.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu