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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