[time-nuts] HP5328A LEDS driver transistor

[Charles P. Steinmetz]

Chris wrote:

>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.

Six or seven of them (depending on the model variation) also drive an 
annunciator LED, for a total of 9 "segments" on those digit drivers.

>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 think you may mean they were selected for current rating, not power 
rating.  In the 5328A digit driver circuit, the required current 
rating is the more demanding spec because it is harder to meet with a 
small transistor than the required power rating.

Some transistors are rated for greater peak current than average 
current, some are not -- it depends on the particular current 
breakdown mechanism of each transistor, and what kinds of 
applications it was designed for.  The MPSU51 is not rated for peak 
current in excess of average current, the ZTX949 is.  But this should 
not be a factor in the 5328 digit driver circuit for either the 
MPSU51 or the ZTX949, assuming a 3.5 V supply, because both are 
rated >= 2 A continuous and the load is < 1 A  maximum -- even if 
this load were continuous, both transistors should survive (if your 
5328 has a 5 V display power supply, the MPSU51's 2 A rating would be 
cutting it close).

Best regards,

Charles