Resistor Basics 1

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. A common question when encountering this concept is why would you want to affect the flow of electricity in your circuit?

Imagine our simple circuit of a bulb with a battery. A small bulb may typically operate at 1.5V, and ideally, you could provide a 1.5V battery to power it, but what if you wanted to use a 9V battery instead, perhaps some components that you want to add to the circuit need a higher voltage to operate? This is one of the most daunting dilemmas to anyone starting out in electronics, but it’s easy to solve with a little more explanation.

Bulb-with-resistorSchematic showing circuit with battery, resistor and bulb


Schematic Symbol  Description  Circuit Schematic Reference
 Symbol-Resistor-Standard Resistor – Current international standard  R
 Symbol-Resistor-Obsolete Resistor – Discontinued, but may still appear in circuit diagrams. Still in common use in USA.  

Common Resistor Applications

Lets explore some common resistor applications:

Reduce current or voltage to a component

We’ve already mentioned that resistors can be used to reduce the voltage supplied to a component such as a bulb, so that it works in a circuit that has a higher voltage for the power source. Common Microcontrollers operate at 5V DC (and sometimes 3.3V DC), so the power source is usually optimised for the microcontroller. Resistors are added between the power rail and the component to reduce the voltage supply to them to allow them to operate within their designed specification.

As a simple example of this, we’ll take our 9V battery and our 1.5V bulb, and make a calculation of what value resistor we need to drop the voltage and allow the bulb to operate within the circuit.

What we know:

 V (Battery voltage) = 9V
 R1 (Resistor) = ?Ω
 R2 (Bulb) = 1.1Ω

 - We know that the bulb is rated to 1.5V
 - We measured this with a multimeter in our Resistor basics section
 - Current of bulb = 0.26A (We measured this before in our Measuring Current section)

Note: When purchasing components, you’ll often see additional information in the listing such as their current or power in Amps or Watts, or marked on the component itself. Bulbs often have the power in watts.

Make the calculations:

Step 1: What is the difference in voltage between the bulb and the battery?

Drop voltage = Battery voltage – Bulb voltage

9V – 1.5V = 7.5V

This is the voltage that the resistor will need to drop the battery voltage by so that the circuit balances to 1.5V for the bulb to operate safely.

In a series circuit, the current is the same in all parts of the cuircuit. We know the current of the bulb is 0.26A, so the current of the resistor will also be 0.26A.

Step 2: Apply the Ohms Law formula

R = V / I

R = 7.5 / 0.26

R = 28.8Ω

So we need a resistor of around 28.8Ω to reduce the voltage in the circuit from 5V to to 1.5V for the bulb.

Putting the circuit to test, with measurements:

Bulb-with-known-resistorDiagram showing circuit with 28.8 Ω resistor

Thanks to our resistor, we can now operate a 1.5V bulb in a circuit that has a 9V battery.


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