Current, the flow of electrons in a circuit, is measured in amps or amperes. In this activity, you will use an ammeter to measure current through a circuit. The trick to measuring current is that the ammeter needs to be connected in series with the circuit. That way, the same current that flows through the circuit flows through the ammeter to be measured. The circle with the A inside is the schematic symbol for ammeter.
In this activity, you will:
This activity re-uses the setup and LED Circuit parts from Measure Resistance, as well as parts for a second LED circuit and additional resistors for testing.
(2) green LEDs
(2) 220 Ω resistors (red-red-brown-gold)
(2) Jumper wires, black
(1) 1 kΩ resistor brown-black-red-gold
(1) 2 kΩ resistor red-black-red-gold
(1) 20 Ω resistor (red-black-black)
The 20 Ω resistor is part of the ammeter. It is important to use the correct one, so look carefully at the color bands so you don't mix them up! The color code for the 20 Ω resistor is red-black-black-gold:
Rcall that the circuit and procedure for measuring resistance with an ohmmeter was different from measuring voltage with a voltmeter. Measuring current with an ammeter also requires its own circuit and procedure different from the other two.
To start, you will need one LED circuit an the ammeter circuit on your board.
For the current measurement, you have to insert the 20 Ω resistor into the circuit. To do this:
IMPORTANT: Remove the 20 Ω resistor after you are done with current measurements in this activity. It is only needed when measuring current, and it could cause errors in voltage measurements later if it’s not removed.
You are now ready to measure current through the LED.
The m in mA is the metric prefix for milli or “1/1000th of”. In this case, the current measurement is 5.108 mA, or 5.108 thousandths of an amp.
The micro:bit, Python script, CYBERscope, 20 Ω resistor, and alligator clip probes all work together to emulate a common multimeter running in ammeter mode. Keep in mind that something akin to the 20 Ω resistor is inside the ammeter. You would not be adding it to the circuit, just connecting the multimeter’s probes in series, just like you did with the alligator clip probes.
The ammeter script’s multimeter module measures the voltage across the 20 Ω resistor by subtracting the voltage measured at P0 from the voltage measured at P2. The multimeter module’s ammeter function then uses I = V / R to calculate current from voltage and resistance. For example, if V is 0.01 V and R is 20 Ω, then I = V / R = 0.01 V / 20 Ω = 0.005 A = 5 mA.
I = V / R is one of the three forms of the Ohm’s law equations, which you will experiment with in the next activity.
Kirchhoff's current law (abbreviated KCL):
“For any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node.” [1]
For example, if the current from three parallel circuits feeds into a single circuit, the current in that single circuit would be the sum of the three currents. In equation terms, three circuits would be:
I = I1 + I2 + I3
… for some number of n circuits, you’d add all of them up:
I = I1 + I2 + … + In
1. Wikipedia. 2021. “Kirchhoff's circuit laws.” Wikipedia The Free Encyclopedia. [5]
Let’s test Kirchhoff’s current law by verifying the current into a node is equal to the sum flowing out. We can do this by connecting another green LED circuit to the ammeter.
Both LED circuits will conduct about 5 mA. If both of them are connected to ground through the ammeter, the sum of the currents flowing through the 20 Ω resistor should be about 10 mA.
In this activity, you:
Links
[1] https://learn.parallax.com/sites/default/files/content/Python/Elec/measure-current-circuit.mp4
[2] https://learn.parallax.com/sites/default/files/content/Python/Elec/ammeter_cyberscope.hex
[3] https://python.microbit.org/v/2
[4] https://cyberscope.parallax.com
[5] https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws