[time-nuts] HP5328A LEDS driver transistor

[Chris Albertson]

> I am surprised the LED current is so high.  I always found diminishing
> returns above 20 mA but some LED types are better than others for
> multiplexed applications where the peak to average current is high.  8
> x 20 mA also conveniently stays below the bond wire limit if you drive
> 7 segments plus the decimal point directly with a single IC without
> multiplexing.

HP does drive all the segments at once. They multiplex one digit at a
time.  So worst case is the transistor has to drive all 7 segments and
a decimal point.

I checked after running for a while and these transistors don't get
even close to warm.  I think they were selected for peak current, not
average.

I do not know the history of this unit.  Why did two of these fail.
Maybe the power line is hit by lightening, who knows Maybe the
transistors were defective from the factory?   But this kind of
problem seems to be very rare

Onemore odd thing that leads to the factory defect theory:  the two
that failed had a different style of ink marking.  The numbers were
the same but obviously different lots.  Also the surface finish on the
metal tab was different.   So 7 identical (same lot) continue to work
fin after 25+ years and two that are different failed after some time.

At 50 cents to a buck each I'll buy a few of each suggested type.
I'll find a use for all of them eventually.   Now to start my next
hp5328 repair thread......
Chris Albertson
Redondo Beach, California