The first thing that comes to mind when the experimenter wants to find out how much current is flowing through the LED is to measure it with a DMM set to read milliamps. This works okay if the LED is connected in series with a current limiting resistor. The DMM will add maybe 10 ohms to a resistance of several hundred ohms, so it is hardly noticeable. But when the 10 ohms is added to a circuit that is driving the LED with pulses, then 10 ohms may affect the circuit and skew the readings. For example, if the circuit is driving the LED with 100 milliamp pulses, then the voltage drop across 10 ohms is… 1 VOLT. Instead of 3 volts across the LED we have 4 volts across the LED and internal resistor. The result is a current reading which is too low and far from what it would be without the DMM.
What to do? Well obviously lowering the DMM’s internal resistance would help. If we look at what is going on inside the DMM when it is reading milliamps, we see a voltmeter across a known resistance, known as a shunt. Well, we can move that known resistance outside of the DMM, and use the DMM as a voltmeter to measure the voltage drop across it and calculate the current. Instead of 10 ohms internal resistance, we use a 1 ohm resistor, and measure the voltage across it. Since 1 amp through 1 ohm gives 1 volt, then 100 milliamps through 1 ohm will read 100 millivolts. Likewise 100 millivolts means there is 100 milliamps flowing in the resistor. And if the resistor is in series with the LED, there is 20 milliamps flowing through the LED and resistor when there is 20 millivolts across the resistor.
Since 1 ohm is 1/10 of the internal 10 ohm resistor, the circuit is disturbed much less, and the reading is much closer to what it would be without the resistor. This 1 ohm resistor is known as a current sensing resistor, even though it would be called a shunt if it were inside of the DMM. We can temporarily put this resistor in series with the LED, or even leave it there permanently if it doesn’t affect the LED too much – 20 milliamps times 20 millivolts is 0.4 milliwatt, which is a very small loss of power in this resistor
Back to experimenting….