CHAPTER 11 ■ POWER ON!
But, since there isn’t any return path to the battery except for the one that goes through the LED, the
count of electrons coming out of the battery is the same count as those going through the LED and returning
into the other end of the battery. All the current that leaves the battery must pass through the LED. So, there’s
no reason to move the multimeter probes to determine the amount of current flowing through the LED.
Calculating Current
Here’s a simple formula that allows you to predict how much current a circuit will use before you build the
circuit.
(V / W) × 1000 = mA
Let’s see if the formula matches our test results. For my circuit, the voltage at the resistor is actually 9.15
V and the resistor's resistance is actually 1020 W.
(9.15 V / 1020 W) × 1000 = 8.97 mA
Oh no! Something’s wrong. 8.97 mA is predicted but only 7.2 mA was measured.
Ah ha! The resistor doesn’t use all 9.15 V; the LED uses some. The resistor only uses 7.35 V.(7.35 V / 1020 W) × 1000 = 7.2 mA
Perfect.Minimum Resistor for LED Formula
Here’s a formula that allows you to determine the lowest value resistor you can use to protect an LED. You
need to test your battery’s voltage and test the LED in the multimeter’s diode mode. You need to look up the
LED’s maximum forward current rating on the manufacturer’s datasheet.
(battery voltage - LED voltage) / (maximum LED current in mA / 1000) = minimum resistor
Figure 11-12. Counting current between the green clip and the LED