CHAPTER 16 ■ TRANSISTOR SWITCHES
Calculating Current-Limiting Transistors
R17 and R18 are current-limiting resistors. They protect transistors Q7 and Q8 from too much current
flowing through them. The 1 kW resistor values are much lower than the 470 kW current-limiting resistor
used in Figure 16-5. Here’s the formula for determining how much current will flow through the base lead:
((V battery - 0.6 V bipolar transistor uses) / W base current-limiting resistor) x 1000 to
convert to mA = maximum base current
Thus: ((9 V - 0.6 V) / 1000 W) x 1000 = 8.4 mA
Based on this math, 1 kW resistors allow up to 8.4 mA to flow through the base lead. Notice that the
transistor uses up a bit of voltage (0.6 V) for itself. Now that you know how much current is supplied to the
base, you can determine how much maximum current is supplied to the attached circuit.
8.4 mA base current x 75 hFE minimum transistor gain = 630 mA
Identifying Stresses Beyond Official Limitations
There are a couple of potential issues with the choice of 1 kW for current-limiting resistors R17 and R18.
Taking Into Account the LM393’s Worst-Case Minimum Limit
The LM393 comparator officially only guarantees it can provide 6 mA, not the 8.4 mA permitted by the
resistors. The values of R17 and R18 could be made a little higher (more resistive) to prevent stressing of the
comparator outputs.
Figure 16-9. Wiring diagram of a brightness comparator circuit with transistors