CHAPTER 16 ■ TRANSISTOR SWITCHES■ Note There are a number of potential outcomes when a circuit design exceeds the minimum and maximum
values listed on a component’s datasheet: The first scenario, and the most likely, is that the component
will continue to perform even moderately beyond the official limitations. This is because the manufacturer
understates capabilities in the official datasheets so that the manufacturer can confidently guarantee
performance within the specified ranges.
The second scenario is that the component would simply not provide any greater amount, but wouldn’t be
damaged either. For example, the comparator would simply provide 6 mA even though the resistors would allow
up to 8.4 mA.
The third scenario is that the component would not work at all, or at a significantly reduced level, but would not
be harmed. For example, a chip run at lower than minimum voltage might slow down, provide incorrect output,
or temporarily cease operating.
The fourth scenario is that the component would wear or age more rapidly. For example, a motor run faster and
hotter than the limits might slowly become less and less efficient and then fail earlier.
The last scenario is that the component would fail utterly, perhaps being destroyed or significantly damaged.
For example, a 20 mA LED dies when run at 200 mA.
The trick is in knowing how a component is going to behave outside the official limits. Although you can take an
educated guess, there’s usually no way to be sure in advance.
((V battery - 0.6 V bipolar transistor uses) / mA desired base current) × 1000 to convert
from mA = W desired current-limiting resistor value
((9 V - 0.6 V) / 6 mA) × 1000 = 1400 W
Oops. That didn’t take into account that the battery might start as high as 10 V.((10 V - 0.6 V) / 6 mA) 1000 = 1566 W
If you happen to find some resistors at or above 1566 W, you could design the circuit for the worst case.
However, according to the datasheet, typically the comparator provides 16 mA. So, 1 kW resistors limiting
current to 8.4 mA is fine. Additionally, as the battery voltage declines with use, the current will also drop.
Looking Out for Heavy Current Through the Attached Circuit
The other issue with an 8.4 mA base current is that some manufacturers only guarantee their 2907A can
provide a maximum of 500 mA through the collector to the attached circuit. The 8.4 mA base current was
calculated as being able to drive 630 mA.
It gets worse. The transistor’s datasheet claims the transistor’s gain could be as high as 300 hFE.
8.4 mA base current × 300 hFE transistor gain = 2520 mA collector current
2520 mA? A TO-92–encased bipolar transistor supplying that much current is going to melt!