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Activity 1: Reading a transformer voltage with a multimeter

In this activity, you will learn to use a multimeter to read DC voltages while learning about a new part called a transformer.


Transformers are used for converting one voltage to another using induction.


Parts Required

A digital multimeter capable of measuring AC and DC voltage. (Warning: Confirm voltmeter specifications to make sure you are not exceeding maximum ratings of your multimeter. A suitable Microcontroller and or DC power sources such as a battery, or wall wart.

The fundamental design for a transformer has two electrically insulated coils of wire next to each other. The magnetic field that is created through the flow of current in the first coil (primary coil) induces current to flow in the second coil (secondary coil) through magnetic inductive forces.


An AC adapter or a wall wart. 


When electron current flows in metal wires, it also creates a magnetic field directly around the wire that the electrons are moving through. This process, called induction causes other electrons in the conductors near the magnetic fields to move, as well. As you recall in Chapter 2, when a magnetic field cuts through a wire, a current is induced to flow. Two James Clerk Maxwell (1831--79) most important laws are:

Moving electric charges create magnetic fields

Magnetic effects can accelerate electrical charges

So electron flow in one wire can cause electron flow in another wire. This magnetic field around the wire is exploited in many electronic devices, especially transformers.


This is what the interior of a transformer looks like.


The ratio of coiled turns in the primary coil versus the number of coils in the secondary coil determines the ratio of input to the output voltage. If there were 100 turns in the primary coil with 100 volts coming in, and 50 turns in the secondary coil, the result would be 50 volts out.

With the transformer AC input coil takes 115 volts AC available from the wall and steps the voltage down to 17 volts. Conditioning circuitry on the interior of the transformer then converts the 17 volts AC into a clean 5 volts DC.

In Europe and parts of Asia, they are using 220 volts AC, but the transformers still convert the power to about 17 volts DC on the output, and then this voltage is regulated to 5 volts, the voltage that most microchips use.

Introducing another New Part

A multimeter is an important tool for measuring electrical characteristics of parts and circuits. While we cannot see the electrons in wires directly, (though we can see photons, which are part of the electromagnetic spectrum) we can use the multimeter to allow us to read various voltages. A meter is also an important tool for debugging and finding out why things sometimes do not work. This will often be the first tool you turn to so that you can confirm voltages are what you expect or to determine where wires appear to be connected or not.


Note: if you are using another meter than be aware: some digital multimeters are auto-ranging and may not have to be set. Also, make sure hold buttons are not activated in other meters, as these can confuse beginners. When using a multimeter, you must pay close attention to current and voltage measurement limits. These can be found in the instruction book that came with the meter. Caution should always be taken when using a multimeter, as the metal probes at the tip can easily be shorted between wires or other electrical parts.


The reason we use meters is that we should NEVER assume a voltage is a particular quantity or + or – in relation to what the circuit needs to receive. You must ALWAYS use your meter to confirm these voltages and the polarity, and you will be rewarded with circuits that do not fizzle, pop, and burn.


Polarity refers to the + or – side of a DC source of voltage. You will be measuring polarity later in this chapter.


Note: Be aware of absolute maximum ratings of this meter and use the small pamphlet that came with the meter to confirm what those are before you measure any voltages outside the range of this book.