# Lab1: Measuring voltage, current and resistance

## Introduction

Taking measurements on a circuit is an essential skill when working with electronics. Even for the simplest problems usually the first checks will be similar to the following:

• is there enough voltage for the circuit to work (dead battery?)
• is there current flowing (loose connection?)
• is the current flowing what you would expect or perhaps more (short circuit?) or much less (loose connection?)

Taking measurements will also give you invaluable and often necessary insights in the working of a circuit. So, for example, to know how long your project will be running on a battery it is necessary to know how much current the circuit uses.

The below experiments may seem like a boring task to do but they are basic tasks you need to be able to carry out by yourself if you want start building and understanding circuits. As with all skills, the real fun starts only after mastering the basics. Read and try to understand the information in the linked documents and look for other sources of information if the provided links are not explaining it well for you. There is also a book available at the Interaction Station called Electronics for Dummies which may be of help during below experiments. We will go through the results of the experiments during the next class.

Note 1: If you are unfamiliar or have forgotten about metric prefixes like mega, kilo, milli, micro etc. please refresh this here: Metric Prefixes and SI Units

Note 2: be careful with your units!! Saying there is a voltage drop of 1 volt (V) across a resistor while you are measuring a millivolt (mV) means you are a 1000 times (3 orders of magnitude) off.

## Description

In this lab you will use a Digital MultiMeter (DMM) to take measurements in a simple circuit.

If you don't know how to use the WiKi yet note them down elsewhere so you can note them on the WiKi after the WiKi introduction!

For the current and voltage measurements you need to create a simple circuit on for example a breadboard consisting of a battery, led and a resistor:

Remember an LED has polarity. If you hook it up the wrong way it won't light up. The long leg is the anode or +, the short leg is the cathode or -. See https://learn.sparkfun.com/tutorials/light-emitting-diodes-leds#how-to-use-them for more details on LEDs and how to use them.

### Measuring Voltage

What you need (ask at the Interaction Station):

```* Multimeter
* 9 Volt battery (5V in the tutorial)
* 9 Volt battery clip for in a breadboard
* 470 Ohm resistor (1K Ohm in the tutorial)
* LED
```

Note 1: instead of the 5V breakout board the tutorial uses, you are using a 9V battery. This means your measured values will differ from the results in the tutorial. Your resistor value and LED forward voltage will differ as well.

### Measuring resistance

What you need (ask at the Interaction Station):

```* Multimeter
* 3 random resistors
* 3 resistors of the same value
```

Resistors come in a wide variety of values, tolerances, wattage and packages. The most common (hobby)resistor is the 1/4 Watt resistor with a tolerance of 5% (meaning the real value of the resistor can be 5% lower or higher than the indicated value). These resistors are usually beige in colour. At the Interaction Station you will probably get resistors that are light blue. These are still 1/4 Watt resistors but have a tolerance of 1%.

Resistors come in specific values determined by their tolerance range. The range for the 5% tolerance is called the E24 range. The E96 range is for the higher precision 1% tolerance range. For an overview of corresponding values and other ranges see for example: http://logwell.com/tech/components/resistor_values.html

The value of a resistor is encoded on the resistor with color bands. You can use a resistor color code calculator like: http://www.allaboutcircuits.com/tools/resistor-color-code-calculator/ Resistors of the E24 range (5% tolerance) have a 4 band marking. The blue resistors of the E96 range have a 5 band marking. Be sure to select the right one (4 or 5 strip in the calculator).

Suggested reading for the following experiments: Resistors in Series and Parallel

#### Series resistance

• Add three resistors in series
• Calculate the total resistance
• Measure the total resistance, does it agree with the calculated resistance?

#### Parallel resistance

• Add two equal resistors in parallel
• Calculate the total resistance
• Measure the total resistance, does it agree with the calculated resistance?
• What can you tell about the total resistance?
• Repeat with the three equal resistors.
• Add the three random resistors in parallel and calculate or measure the total resistance
• What can you tell about the total resistance in relation to the resistances of the individual resistors? (hint: is the total resistor larger or smaller than any of the individual resistances).

#### Extra (Optional)

If you know that the resistance of a conductor can be expressed by R = ρ * (l / A) where ρ is called the electrical resistivity and is constant for the specific material of the conductor, l is its length and A is the cross-sectional area. Can you explain the results of your parallel and series measurements?

### Measuring Current

What you need (ask at the Interaction Station):

```* Multimeter
* 9 Volt battery (5V in the tutorial)
* 9 Volt battery clip for in a breadboard
* 470 Ohm resistor (1K Ohm in the tutorial)
* LED
```

### Measuring voltage and current in a series circuit

What you need (ask at the Interaction Station):

```* Multimeter
* 9 Volt battery (5V in the tutorial)
* 9 Volt battery clip for in a breadboard
* 3 resistors random resistors
```

• Add three resistors in series and connect to the battery
• Measure the battery voltage
• Measure the voltage across each resistor
• Measure the voltage across each pair of two resistors
• What can you tell about the voltages measured above in relation to the total battery voltage?

• Using Ohms law for current (remember: I = V / R) calculate the current through the individual resistors. Hint: Remember you should not use the total battery voltage here but the voltage dropped across the resistor and the resistance value of this resistor.
• Using Ohms law for current calculate the current through the total resistance. Hint: first compute the single total resistance of the 3 resistances in series and use the measured battery voltage.
• Measure the current through the circuit. Does it agree with your calculations?

### Measuring voltage and current in a parallel circuit

What you need (ask at the Interaction Station):

```* Multimeter
* 9 Volt battery (5V in the tutorial)
* 9 Volt battery clip for in a breadboard
* 3 resistors random resistors