[time-nuts] HP5328A LEDS driver transistor

Stan Searing timenutstan at gmail.com
Mon Jan 23 16:36:22 EST 2012


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 get
around
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?

Two possibilities for these transistor failures come to mind.  If the clock
controlling
the scan dies, then one digit would be on 100% of the time.  This would
greatly
increase the power dissipation of the unfortunate digit driver).  If the
counter is set for
external reference and no reference is provided, does it fail to scan?

The other possibility is that the transistors were from a batch that was
below par in some area and this increased the chance of failures over
time (incomplete passivation, poor die attach to the tab, silicon fracture
propagation, fractures started from the shock of clipping off the tabs,
poor intermetalics at the bond wires, ...).  A lot of these causes would
not affect all parts from a lot the same, so some parts could continue
working considerably longer than others.

Stan


On Sun, Jan 22, 2012 at 9:01 PM, Charles P. Steinmetz <
charles_steinmetz at lavabit.com> wrote:

> Bob 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!
>>
>
> OK, looking at the schematic 7 of the digit drivers have 7+2 loads (DP and
> annunciator LEDs).  Each segment is driven from 5 volts through two
> saturated transistors and an LED.  Assuming 0.6 + 0.6 + 1.7 V, each segment
> current is 0.21 A and the total digit current with all 9 segments lit is
> 1.89 A, for a digit driver dissipation of (1.13 W x duty cycle).  With 9
> digits, the duty cycle must be less than 11.1%, so the power dissipation
> will be around 125 mW worst case.  As Bob said, any TO-92 will dissipate
> this with no difficulty.
>
> The problem is the current.  You need transistors that can handle 1.89 A
> without sweating.  The MPSU51 is rated at 2A continuous, and is not
> specified for pulse duty so there is no way to know how much if any more
> current it will safely handle at an 11% duty cycle.  The fact that Chris
> has two bad ones may be taken to indicate that they were not hugely
> overspecified in this application.  I would be looking at transistors rated
> for a maximum continuous collector current of at least 3 A, preferably
> more.  Only a very few TO-92 devices fit this bill.  I think my first
> choice would be the ZTX949 -- it has low saturation voltage and is rated
> for pulse current substantially in excess of the continuous current rating,
> and its pinout matches the MPSU51.  I have used lots of them and they are
> very rugged.  In stock and priced from $1.12 to $1.24 in singles from
> Digi-Key, Mouser, and Avnet.
>
> Best regards,
>
> Charles
>
>
>
>
>
>
>
>
>
>
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