An ohmmeter is a device that measures resistance. The basic unit of resistance is the ohm, and it’s abbreviated with the uppercase Greek letter omega (Ω).
This activity will guide you through:
A test circuit cannot have power applied to while measuring resistance. This second, important rule normally applies when using a repair manual, which might direct you to measure the resistance in a much larger circuit that’s part of some device. The device’s power would have to be turned off for an ohmmeter to take a valid measurement.
In this activity you will have an ohmmeter circuit and an LED circuit. You will use the ohmmeter circuit to measure different resistors used in the LED circuit.
Let's once again re-use the setup from First Breadboard Circuit — Build the LED Circuit [1].
The LED circuit may still be on your board. If not, re-build it now.
You will also need the following additional parts, along with some jumper wires:
This ohmmeter circuit can measure resistances in the 100 to 10,000 Ω range.
Individual resistors need to be removed from the circuit to be measured by an ohmmeter. Keep this in mind as you follow the instructions.
TIP: When measuring individual resistors, you can also simply grab the ends of the resistors with the alligator clips.
In the diagram, R1 is the resistance of the part you are measuring. Vo is the voltage that results between R1 and the 2000 Ω resistor. The multimeter module measures the voltage between the two resistors with P1. Then, it uses the R1 = … equation near the bottom of the diagram to calculate the value of R1. For example, if P1 measures 2.97 V, the result of the equation will be 220 Ω. If P1 measures 2.2 V, the result of the equation will be 1000 Ω.
When two resistors are connected end-to-end, they are connected in series.
When voltages like 3.3 V and GND are applied to the ends of two resistors in series, it is called a voltage divider. The name came from the fact that the voltage at Vo is “divided” between the two resistors. In the Voltage Dividers lesson, you will experiment more with this and optionally derive the equation for calculating R1.
Modern multimeters expand the circuit and script to automatically measure anything from a fraction of an ohm to millions of ohms.
Let’s measure a different resistor. Keep in mind that accuracy is best in the 100 Ω to 10,000 Ω range. Outside that range, measurement errors increase, and you’d need a different resistor and some script adjustments.
Imagine you are inventing something, and your design needs a 1220 Ω resistor, or maybe a 500 Ω resistor, but your kit doesn’t have either of those! Not to worry, you can combine resistors in various ways to get resistance values that are not in your kit.
Resistors can be connected end-to-end (in series) or side-by-side (in parallel). Both series and parallel resistances can be boiled down to individual equivalent resistance values. This section will show you how.
When resistors are connected end-to-end, they are connected in series. The total resistance is simply the sum of the resistors. Just add them up, and that’s the resistance.
For 2 resistors, that would be R = R1 + R2.
Example: Calculate the resistance of a 220 Ω resistor in series with a 1000 Ω resistor.
Solution: R = 220 Ω + 1000 Ω = 1220 Ω.
If we have some number N of resistors, the total resistance would be R = R1 + R2 + … + RN.
Example: Calculate the resistance of three 220 Ω resistors in series.
Solution: R = R1 + R2 + … + RN = 220 Ω + 220 Ω + 220 Ω = 660 Ω
In this solution, since all the resistor values are the same, you could just multiply 220 x 3.
Resistors can also be connected in parallel which is side-by-side, with only two connection points where all the resistor leads are connected. (Remember, you can use a row-of-five sockets in the breadboard’s terminal strip to connect resistor leads together.)
In the special case of two resistors in parallel, the equation is R1 * R2 / (R1 + R2). For more than two, the equation is 1/(1/R1 + 1/R2 + … + 1/RN).
Example: Calculate the resistance of two 1000 Ω resistors in parallel.
Solution: R = R1 x R2 / (R1 + R2) = (1000 x 1000) / (1000 + 1000) = 1,000,000 / 2000 = 500 Ω
Example: Calculate the resistance of three 1000 Ω resistors in parallel.
Solution R = 1 / (1/R1 + 1/R2 + …+ 1/RN) = 1 / (1/1000 + 1/1000 + 1/1000) = 1 / (3/1000) = 1000/3 ≈ 333 Ω.
To figure out the equivalent resistance of a circuit with both series and parallel elements, calculate the equivalent resistance of a subcircuit that is either parallel or series first.
For example, if one resistor is in series with a pair of parallel resistors, solve for those parallel resistors first. After that, all that’s left is two resistors in series.
Another example, let’s say that one resistor is parallel to a pair of resistors in series. In that case, solve for the series resistors first. Then, all that’s left are two parallel resistors.
Series
Parallel
Links
[1] https://learn.parallax.com/tutorials/language/python/breadboard-setup-and-testing-microbit/first-breadboard-circuit/build-led
[2] https://learn.parallax.com/sites/default/files/content/Python/Elec/measure-resistance-meter-circuit.mp4
[3] https://learn.parallax.com/sites/default/files/content/Python/Elec/hex/ohmmeter_cyberscope.hex
[4] https://python.microbit.org/v/2
[5] https://learn.parallax.com/sites/default/files/content/Python/Elec/measure-resistance-220-from-led-circuit.mp4
[6] https://cyberscope.parallax.com/
[7] https://learn.parallax.com/sites/default/files/content/Python/Elec/measure-resistance-1k.mp4