Resistors, the E24 Code and Voltage Dividers

Choosing the right resistor

A resistor limits the current in a part of a circuit and, paired with another resistor, sets the voltage at a point. This module is about choosing resistor values and using resistors to build sensing circuits. It works through four ideas: - Reading a value: the coloured bands and the E24 set of preferred values tell you a resistor value, its tolerance and how much power it can handle. - Combining resistors: connecting them in series or in parallel changes the total resistance. - The voltage divider: two resistors in series share out a supply voltage, giving a smaller output. - Sensing and protecting: swapping a resistor for a sensor makes a divider that responds to light or heat, and a resistor in series with an LED keeps it safe. Ohm law and power belong to an earlier module. Here the focus is resistor values and the divider.

Words for resistors

Five terms you need before you use them. Learn what each one means.

Match each resistor idea to its meaning

Resistors in series against parallel

Two resistors can be joined in two ways, and the way changes the total resistance. Knowing which is which is a common exam point.

Read the resistor

A resistor is marked as one kilohm with a tolerance of five per cent. Which statement is correct?

• Its real value should lie within five per cent above or below one kilohm
• Its value is exactly one kilohm with no variation at all
• Its power rating must be five watts
• Five per cent means it can only be used five times

Tap the two input sensors

Tap the TWO components that act as input sensors in a circuit.

• A light-dependent resistor whose resistance drops in bright light
• A thermistor whose resistance falls as it warms up
• A resistor placed in series to limit the current to an LED
• Two fixed resistors joined to raise the total resistance

The voltage divider

Two resistors in series across a supply do something very useful: they share the supply voltage between them, and the point in the middle gives a smaller output. The size of that output depends on the ratio of the two resistors. The larger the share of the total that belongs to the output-side resistor, the larger the output voltage. Now swap one of the fixed resistors for a sensor, such as a light-dependent resistor or a thermistor. As the light or the temperature changes, the sensor resistance changes, its share of the supply changes with it, and the output voltage rises or falls. That is how a handful of components becomes a circuit that senses the world.

Pick the true resistor facts

Select the TWO statements that are true.

• Putting resistors in series increases the total resistance
• A thermistor in a divider lets the output voltage sense temperature
• Resistors in parallel always give a larger total than either one
• Tolerance tells you how much power a resistor can handle

Order how to work out a divider output

Put the steps for finding the output voltage of a two-resistor voltage divider in order.

• Note the supply voltage across the two resistors
• Note the value of each of the two resistors
• Add the two resistor values to get the total resistance
• Divide the output-side resistor by that total to get the fraction
• Multiply the fraction by the supply voltage to get the output

A porch light that knows when it is dark

Here is a simple light-sensing circuit, to show how a resistor and a sensor work together.\n\nThe parts: a fixed resistor and a light sensor are connected one above the other across the supply, and a wire from the point between them carries the output on to the rest of the circuit. In daylight: the light sensor has a low resistance, so only a small share of the supply appears at the output, the output voltage stays low, and the porch light is off. At dusk: as the light fades the sensor resistance climbs, its share of the supply grows, the output voltage rises, and once it is high enough the porch light switches on. Nothing in the circuit is really watching the sky. It is only the changing share of the supply voltage between two resistors that does the sensing.

Complete the resistor facts

Joining resistors end to end so their values add is a _____ connection. A standard set of twenty four preferred values is the _____ code. Two resistors sharing a supply to give a smaller output voltage form a _____. A resistor whose resistance changes with temperature is a _____.

Build a resistor point

Choose the words that complete this statement about the voltage divider.

Work out the divider output voltage

A voltage divider is connected across a supply of 10 volts. The upper resistor is 4 kilohm and the lower resistor is 1 kilohm, and the output is taken across the lower resistor. Add the two resistor values to get the total, divide the lower resistor by that total to get the fraction, then multiply the fraction by the supply. What is the output voltage in volts?

Choose the right resistor

Read each case and choose the best answer, then think about why.

• You need a total resistance larger than any single resistor you have to hand. How should you connect two resistors?
• You want a circuit whose output voltage rises as a room grows darker. Which component goes in the divider?
• You are connecting an LED to a supply that would push far too much current through it. What do you add?

Explain resistors and voltage dividers

A student in the year below does not understand how resistors are chosen and used. Explain it using what this module has covered.

• Explain how resistors combine in series and how parallel differs
• Explain what the E24 code and tolerance each tell you about a resistor
• Explain how a voltage divider produces a smaller output voltage
• Explain how a light-dependent resistor or a thermistor turns a divider into a sensing circuit
• Finish by explaining why an LED needs a current-limiting resistor