CHAPTER 14 ■ VARIABLE RESISTORSCurrent-Limiting Resistor
In Figure 14-15, R1 is called a current-limiting resistor. Because all of the other resistors are variable, there
might come an occasion when a combination would resist so little that lots of electricity from the battery
would be wasted.
For example, if R2 were dialed down so that 0 W was supplied to one pair of photoresistors and that pair
was looking at a bright floor (around 100 W each) then only a total of 200 W resistance (0 W + 100 W + 100 W)
would exist through that path. The formula for determining current is:
(V / W) × 1000 = mA
Thus: (9 V / 200 W) × 1000 = 45 mA
But, by adding a 470 W resistor (R1), the worst case becomes 670 W (470 W + 0 W + 100 W + 100 W).Thus: (9 V / 670 W) × 1000 = 13 mA
With R1, this branch of the circuit draws ¼ the power in the worst case, yet provides the same function.
R1 doesn’t have much of an effect in the average case because R2 is dialed to around 10,000 W. In that case,
the current drawn is less than 1 mA for each path.
If you’re careful to not adjust the potentiometer to either end of its dial, then R1 is technically
unnecessary. Still, it’s nice to know R1 is there so that you can dial the potentiometer to any value without
possibility of component harm or extreme battery waste.
When you create your own designs, be sure to add a minimum-value fixed resistor to any path that is
otherwise controlled by variable resistors.
Test Points
The schematic (see Figure 14-15) features test points, labeled TP1 and TP2. No components physically exist
at TP1 and TP2; they’re just good spots to attach a multimeter for testing voltage. In fact, it turns out that
these test points are great spots to test voltage.
Calculating the Voltages
When the battery is connected, the electricity flows through R1 to R2 and then splits into two paths.
Considering just one path for the moment, the electricity then flows through R4 and R3 and then it finally
returns to the battery.
We know the voltage starts out around 9 V and always ends up at 0 V when it reaches the other end of
the battery. Along the way, how does the voltage get apportioned between the components?
- Add up all of the resistance in the path. We’re going to assume the photoresistors
are looking at bright light (100 W) for this example.
R1 W + R2 W + R4 W + R3 W = total W of the path
This example: 470 W + 10,000 W + 100 W + 100 W = 10,670 W- For any resistor of interest, divide its resistance by the total to determine what
share of the voltage it will use.
R1 W / total W = R1's share
This example: 470 W / 10,670 W = 0.044