[time-nuts] HP5328A LEDS driver transistor
davidwhess at gmail.com
Tue Jan 24 00:37:58 EST 2012
On Mon, 23 Jan 2012 17:27:32 -0500, "Charles P. Steinmetz"
<charles_steinmetz at lavabit.com> wrote:
>>The 5328A manual I have shows a 3.5 V supply (looks like it is used just
>>for the display). But with a bit lower Vce sat drops, I'd guess you
>>1 V across the 10 ohm segment resistors. Or only about 900 mA when all
>>9 segments are on (but at a low duty cycle). Did HP also use a 5 V version
>>of the driver in some 5328A counters?
>Interesting. I have 3 different sets of schematics for the
>5328A. The most legible of these (thus the one I consult first)
>shows a 5 V supply for the display. The other two show 3.5 V, as
>yours does. I cannot recall ever measuring the display supply
>voltage in a 5328.
>Even with 900 mA per digit, I'd want a transistor with >/=2 A maximum
>collector current for the digit drivers -- so I still think the
>ZTX949 may be the best readily-available choice.
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
I would use a 2 amp or greater transistor just to keep the gain high
although I know those Zetex transistors much better about that. I
think Fairchild has a similar line of high beta, high current, low
dissipation transistors now as well.
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