Resistor Basics 1

[Banner showing selection of different resistor types]

In our Resistance and Ohms Law Basics Tutorial, we discussed how current flowing around a circuit can be restricted. Resistors are a type of component used for deliberately causing resistance in a circuit. Why? A bulb may be glowing too brightly for the battery used. It may not be practical to change the battery to one of a lower voltage, so dimming the bulb by controlling the amount of current that the bulb is allowed to use can be achieved by adding a resistor. Subsequently, as we block current using a resistor, we also drop voltage.

[DIAGRAM SHOWING BULB WITH RESISTOR]

Schematic Symbol Description Circuit Schematic Reference
Standard Resistor Symbol Resistor - Standard Symbol R
Obsolete Resistor Symbol Resistor - Discontinued, but may still appear in circuit diagrams. R

 

Common Resistor Applications

When designing a circuit using resistors, considerations need to be made about how much current the resistors will need to drop (disperse), and therefore whether they are suitable rated in terms of power consumption, as they can overheat and catch fire in a worst case.

1. Reduce current to a component

Circuit Protecton: Some components can burn out if they’re exposed to too much current. Adding a suitable resistor can provide circuit protection by restricting the available current to that component preventing them from being damaged.

Delay: Limiting current to capacitors using a resistor creates a specific delay in how long the capacitor takes to charge, creating a simple timing circuit. We will cover capacitors at a later stage.

Power Saving: Adding a resistor to a bulb may make it shine slightly dimmer, but the effect may not be too noticable, however, the power saving could be significant.

2. Create a voltage divider

A voltage divider can be made with a chain of resistors in series, where different voltages can be tapped off (divided) in between the resistors. If the resistor values are all equal, the voltage drop is divided equally between number of resistors in the chain. If we use different values for the resistors, the voltage drops are fractional proportional to the resistor value differences.

Same [2 resistors] [3 resistors]

Different [2 resistors] [3 resistors]

We need to remember that we may need a higher voltage supply to ensure that there is enough voltage for the circuit as it’s being divided up by the resistors.

Using a voltage divider in this way also allows us to introduce a negative voltage into the circuit. Our circuits so far have used a battery, which has positive and negative. As an example, we use two 3V batteries in series, giving us 3V supply at the positive and 0V at the negative.

By tapping off the centre of two resistors in series, we create positive, 0V and negative. Here, we refer to the 0V as Ground, and the positive and negative is tapped off in respect to ground. We need to remember that we are dividing up the available voltage, so in this example we would end up with 1.5V, Ground and -1.5V. If we used a single 1.5V battery as a voltage divider, our result would be only 0.75V, Ground and -0.75V. Having a larger power source is often necessary if it is being split up in this way.

Voltage dividers are ideal for low current applications, but inefficient for high current applications. A lot of power will be wasted as resistors heat up to dissipate current.

Further Reading:

See Voltage Label Designators for more information on labels that you may see in circuit schematics that have a split power supply.

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