Resistors

Owned by Kevin Li
Last updated: 2022-06-22 by Julian Yam
4 min read

What’s the symbol for a resistor

 

What is ohm’s law?

Ohm’s law describes the relationship between voltage, current and resistance. It can be expressed as this equation.

V=IR

Max heat through a resistor?

  • The heat produced by a resistor can also be regarded as the power loss by the resistor. The power loss can be calculated by ohm’s law

P=I^2R

  • The job of a resistor is to reduce the current in circuits to protect the circuit components from an overdraw of current. It does so by dissipating the kinetic energy of electrons (current) into heat

Pull up and pull down the resistor

Pull up and pull down resistors are used in digital logic gates. The pull up and pull down resistors are used to ensure that the inputs of the digital gates are set to the correct state (on or off).

In order for a logic gate to work the inputs and outputs need to be set correctly. They need to be set either high or low (on or off).

The high or low states of a pin is determined by the voltage. For a high state there will be a higher voltage (ex. 5V) for a low state there will be a low voltage (ex. 0V).

However, sometimes the inputs to a logic gate is not within range (ex. In the above examples, the voltage is between 0V and 5V.). In this case, the circuit might false trigger because the circuit will not recognize the correct input value.

For example:

 

There are two inputs to this digital logic gate:

  • input A

    • When switch A is on (switch is closed), input A is connected to ground or (LOW voltage)

    • When switch A is off (switch is opened), the value of the voltage (HIGH or LOW) is undetermined. From the diagram it can be assumed that voltage is HIGH because it is not shorted to ground but this also might not be the case. The input in the open condition is unconnected to a defined high or low condition so it can float in between (0 and 5V)

  • input B

    • When switch B is on (switch is closed), input B is connected to ground or LOW voltage

    • When switch B is off (switch is opened), same scenario as switch A

Pull up and Pull down resistors are used to make sure there are no false triggers in the circuit. They set the input pins to be defaulted to off or on.

Pull up resistors:

  • To ensure the state of a digital logic gate you can connect the unused pins directly to ground or direct to VCC. In the figure below the A and B are defaulted to 5V because they are directly connected to 5V. When switches are closed A and B will be connected to ground just as before.

  • Therefore the default state when switch A and switch B is open is 5V. However, when you directly connect A and B to 5V there will be a short circuit between 5V and ground which will result in excess current flow. This will damage the circuit. This is where pull-up resistors are used. See figure below

  •  

  • By putting a resistor between A and 5V there is less input current into the logic gate, thus protecting the circuit and defaulting the state of the switch

Pull- Down Resistors:

  • Pull Down resistors work the same way as the pull up resistors except the switches are defaulted to ground (0V) instead of power (5V)

  • If a pull down resistor was used in the figure above, the resistors would be connected to ground instead of 5V

Digital logic gates:

  • A digital logic gate is a circuit that makes logical decisions based on input digital signals.

  • These inputs can be seen as pins in the circuit

What is a voltage divider?

  • A voltage divider is a simple circuit that turns a large voltage into a smaller one.

  • A voltage divider applies a voltage source across a series of two resistors. The voltage drop is the voltage across the second resistor.

The figure below are examples of voltage dividers in different forms.

 

Calculating voltage drop in a voltage divider. Use this equation

 

How can you sense current in a circuit using a resistor?

Method 1: Ohm’s Law

  • Use a multimeter to take the voltage between the resistor. If the voltage is 0 that means there is no current going through the resistor.

Method 2: Current Clamp:

  • Loop the current clamp in series with the resistor and see if the reading has a current or not.

Sources:

Heat Dissipated by Resistors | Brilliant Math & Science Wiki

Pull-up Resistor and Pull-down Resistor Explained

Digital Logic Gate Tutorial - Basic Logic Gates