Blue Print instruction on Essential Electrical Skills – Part 1

Essential electrical skills

The should knows

Voltage

Voltage can be defined as pressure supplied to an electrical circuit, pretty much the same as water pressure supplied to a tap or valve. This voltage is invisible and measured in Volts (V).

Imagine a vehicle battery as a tank containing a pump constantly supplying pressure to a self-sealing valve at the positive terminal. The electrical pressure acting outwardly from the positive terminal of the battery is normally about 12 Volts (12V). The negative terminal of the battery is the low-pressure (0.0V) top-up filler for the tank/pump. Nothing happens when the battery is in this state because the valve is closed. A length of wire (hose) attached to the negative terminal with its open end in air, makes no difference.

If the open end of this wire is then connected to the valve at the positive terminal, thus joining the two terminals together, the valve automatically opens, allowing electricity to easily flow to the negative terminal. This flow is called Current. As can be imagined, because the pressure difference is large (12V) and with no restrictions in the hose, the current flow will be very high. (Burned fingers anyone?)

Current

All materials, including those used in electrical wiring is made up of atoms. Each atom has three main parts, Protons, Neutrons and Electrons. Every material has different numbers of these parts, which determines its electrical characteristics. The protons and the neutrons are grouped together in the centre of the atom making up the Nucleus. The important bit of the atom for someone wishing to understand current is the electron, which continuously orbits the nucleus, held in place by magnetic attraction. When an external pressure (voltage) is applied to an atom, it acts on the invisible bond between the nucleus and the electron, forcing the electron to jump away. The moving electron cannot avoid hitting another atom, forcing its electron to jump away as well. This process continues as long as electrical pressure is available and is similar to what happens when a snooker player Breaks a pack of snooker balls. The quantity of electrons jumping through a conductive material is defined as the current flow.

If you were able to count the electrons moving through a conductive material at any point, and timed it, you would be able to work out the current flow. Just as water flow through a hose can be measured in say, litres per hour, flow of current can be measured in electrons per second. For simplicity, 625 trillion electrons moving past a single point in a conductor in 1 second is called one Amp (A).

It is this process of electrons passing through an electrical consumer (motor, bulb etc.) that actually makes the consumer do work, i.e. a bulb glowing or a motor rotating. Up to a point, the more electrons (Amps) that flow through an electrical consumer, the more work will get done. This current flow must be controlled as too much flowing through a consumer can result in damage.

Resistance

As we have already seen, current flow is the amount of electrons passing through a conductor over a time period. If a high current flows, lots of work can be done i.e. a starter motor cranking an engine. Lower power devices will be damaged if the current flow is excessive and some sort of restriction to current flow must be provided. In electrical terms this is known as Resistance (R). This resistance may be a component incorporated into the device itself or as a separate component in the circuit. Some components have natural resistance in the materials from which they are constructed – lamps are an example of this. Materials used in electrical circuits have different resistances, enabling them to conduct (carry) electricity at different rates. A good conductor will have a low resistance; a poor conductor will have a high resistance. Resistance is measured in Ohms (Ω).

Measuring current

Although not commonly used, an ammeter can provide a good deal of information about a circuits condition. To measure current, the ammeter is connected in series with the circuit or part of the circuit you wish to test. The ammeter becomes part of the circuit and measures all current passing through it. Any resistances (consumers) in the circuit will prevent high current flowing, just as they would when in normal operation.

The ammeter should never be connected in parallel with a consumer on the circuit. If that consumer is the only resistance in that circuit, the ammeter creates an effective short circuit (an easier route for the current to take through little or no resistance) and excessive current will by-pass the consumer directly through the ammeter to earth. At best this will lead to a blown fuse in the ammeter, damaged circuits / ammeter or even a fire!

The must knows

The four key rules

With a clear appreciation of voltage, current and resistance, it should be easier to understand the testing of voltage in a circuit.

Note: Throughout the following, reference to a black probe refers to the negative or earth probe on a voltmeter. Reference to a red probe refers to what is commonly known as the positive probe. These colours may vary on different equipment.

Four key rules can be applied when using voltmeters to test circuits:

Rule 1

A digital voltmeter displays the difference in voltage between where you place the black probe to where you place the red probe.

Rule 2

The voltage after the last resistance in a circuit will always be zero providing current can flow.

Rule 3

Volt drop will only occur across a resistance if current can flow.

Rule 4

The Volt drop across a resistance in a series circuit is in direct proportion to the comparative resistances values.

Proving the rules:

Rule 1: Example 1

A digital voltmeter displays the difference in voltage between where you place the black probe to where you place the red probe.

The red probe is probing the earth part of the circuit where there is 0 Volts. The black probe is probing the same section of the circuit where there is also 0 Volts. There is no difference between these two measurements; therefore the meter will display 0V (no difference).

Rule 1: Example 2

The red probe is probing the circuit where there is 12 Volts directly from the battery. The black probe is probing the same section of the circuit where there is also 12 Volts. There is no difference between these two readings so the meter will display 0V (no difference).

Rule 1: Example 3

The red probe is probing the circuit where there is 12 Volts. The black probe is probing the circuit where there is 0 Volts. The meter displays the difference between 12V and 0V, which is 12V.

Using this rule to measure voltage at any point in a circuit, place the black probe on earth, (a point in the circuit which is ultimately connected to the negative earth terminal, where 0 Volts is present) and place the red probe wherever you wish to measure the voltage.

Rule 2: Example 1

The voltage after the last resistance in a circuit will always be zero providing current can flow.

If we assume current flows from the positive post of the battery to the negative post, the first resistance for current to flow through would be the lamp. More importantly, the lamp is also the last resistance that the current flows through. Therefore any part of the circuit after the lamp will have 0 Volts present.

Rule 2: Example 2

We now have three lamps placed in a row (in series). The current will flow through lamp 1 first, then through lamp 2 and finally through lamp 3, which is the last resistance in the circuit. Therefore, any part of the circuit after lamp 3 will have 0 Volts present.

Rule 3

Volt drop will only occur across a resistance if current can flow.

There is now a break in the circuit after point c. This means current is no longer able to flow anywhere in the circuit. Rule 3 states that a voltage drop will only occur across a resistance when current can flow. In this case therefore, none of the resistances (the lamps) can cause a drop in voltage. In this example, if the black probe of the voltmeter were placed at a point in the circuit attached to the negative post of the battery and the red probe anywhere else in the circuit up to point c (the lamp side of the break), 12 Volts would be displayed.Rule 4

The Volt drop across a resistance in a series circuit is in direct proportion to the comparative resistances values.

It should now be clear that voltage measured anywhere from the battery positive post to Lamp 1 will be 12V. It is equally clear that measuring voltage anywhere between Lamp 2 and the negative post of the battery will produce a reading of 0V. This means that the voltage drop caused by the resistances of lamp 1 and lamp 2 together must total 12V. If both lamps have the same resistance, they will both restrict current flow the same amount, which will in turn lead to an equal voltage drop. In this case a drop of 6V will occur at each lamp. The voltage measured between the lamps will be 6V. (12 minus 6 = 6)

Note: The actual resistance is irrelevant as long as they are the same. Comparative values means the individual resistances compared with each other.Rule 4 states that the volts drop across each resistance is relative to its comparative resistance. This means that if you have more than one resistance (consumer) in a series circuit, the highest resistance in that circuit will create the largest volt drop and the lowest resistance the lowest drop. The total volts drop across all of the consumers must add up to the supply voltage value. In this circuit the resistance of lamp 1 is 5Ω and the resistance of lamp 2 is 1Ω.A simple formula can be used to calculate voltage drop across resistances in a series circuit when you know all the resistances. This can be used as one way of finding out whether an extra, unwanted resistance is present (poor connection etc). After calculating what the voltage drops should be and then measuring the actual values, any discrepancy will be easily seen.

* Note: When calculating the total circuit resistance of a series circuit you simply add all the resistance values together.

This formula has allowed us to calculate how much of a Voltage drop is going to occur across each Ohm (Ω) of resistance, in this case 2V per Ω. The 5Ω lamp 1 will cause a 10 Volts drop and the 1Ω lamp 2 will cause a 2 Volts drop. Now we have to calculate what the voltage would be in between lamp 1 and lamp 2. We know 12V is being applied to the positive side of lamp 1, and we know that a volt drop of 10V occurs over it. If we start off with 12V and a drop of 10V occurs we must have 2V left over. That means the voltage in between lamp 1 and lamp 2 must be 2V. This means there is 2V of electrical pressure acting upon the positive side of lamp 2. We have already calculated that a volts drop of 2V occurs over lamp 2 and so the voltage on the negative side of lamp 2 must be 0V. We already know this to be correct, due to what we have learnt from rule 2.

A thorough understanding of these four rules is essential if the diagnosis of a faulty circuit is to be fast and accurate. The rules have taught us what will occur in a good circuit, and so it will be possible for us to work out what the various voltages around that circuit should be. If we know what the voltages should be, and they are not as expected, the fault can be easily identified through effective digital multimeter use.

Diagnosis applying the rules

Example 1The circuit pictured has a fault. The lamps should be illuminated but they are not. If we test the circuit at all the points labelled we will be able to work out where the fault must be. We have tested the circuit in the order of the results below:

a = 12V

b = 12V

c = 12V

d = 12V

e = 12V

f = 0V.

So where is the fault?

Lets apply the rules:

When we measured the voltage a point a and found there to be 12V we know the battery is OK. When we measured 12V at point b we know the wire joining points a to b must be OK (as electrical pressure is managing to act at point b). If the wire were broken, the voltage here would have been 0V.

At the point of measuring 12V at point c, we know we must have an open circuit (physical break). We know this to be true because current cant be flowing through the circuit, as volts drop has not occurred over lamp 1 (reference rule 3). Only a break in a circuit will prevent current from flowing. At the point of measuring 12V at point d, we know the wire is OK in between point c and point d as 12V is able to act upon lamp 2. At the point of measuring 12V at point e we know that the filament in lamp 2 is OK as electrical pressure is able to act through it. This also tells us that the open circuit (break) must lie in between point e and point f. Just to be certain, when we measure the voltage at point f and find the voltage to be 0V, we can be sure that we have come to the correct conclusion. Now that we know where the fault lies, we know where we need to look visually in the circuit to locate the break.

Progress check!Where do you think the fault lies? Use the measured voltage values listed below to identify the problem.

The answer is given at the end of this article.

a = 12V

b = 12V

c = 12V

d = 0V

e = 0V

In summary

In the second part to this article we will be discussing other types of faults. These will include short circuits on both supply and earth sides (the latter often being referred to as parasitic drain) and also high resistance faults.

We will also be sharing with you our tricks of the trade, to enable to you accurately diagnose faults very quickly and confidently.

Answer to progress check: There must be a break in the wire between point c and point d.