[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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