[time-nuts] HP5328A LEDS driver transistor

Chris Albertson albertson.chris at gmail.com
Sun Jan 22 23:51:34 EST 2012


On Sun, Jan 22, 2012 at 3:18 PM, Robert LaJeunesse
<rlajeunesse at sbcglobal.net> wrote:
> Specifying the digit scan transistor is a bit tricky, and often power
> dissipation is just not a concern. Why? Consider the duty cycle. A transistor
> switching one amp (a LOT for 7+1 segments) with a 0.6V saturation dissipates
> 600mW, but only when on. Given a 1/8 duty cycle that's 75mW average. Any stinkin
> TO-92 will handle that power all day long! Only an ultra-conservative design
> would use an exposed tab transistor, as expected for HP!

Ultra-conservative and "overkill" but two of the nine failed.  OK it
is 30 years old (made is '83, I think)

These were in air.  The tabs were not screwed to anything.  I think
looking at how this display works it was likely used for many
different instruments, not just the hp5328.  They must have sold many
thousands of these
>
Chris Albertson
Redondo Beach, California




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