Which components needs a resistor

Other Hardware General Electronics
1

Him,

I'm starting to play with Arduinos and starting to read about eletronics and electricity. Already know the importance of using resistors, but I still have some doubts and ask for your help and understanding.

  • wich components need a resistor when building a circuit. I know that Leds and LDR need it and if we don't use it we can damage the led.

  • i don't understand how it can damage the I/O pin. Is because the current returns to it?

  • and it's possible to damage/burn the atmega chip? How?

  • and damage the pc trough the USB Port.?

Sorry soo many questions but i wanna to be sure before starting my first projects.

Thks in advanced.

2

Resistors are used to limit and regulate the current flowing through the circuit. When the LED shines it has very low resistance. For a moment you can approximate it to a short - resistance of 0 Ohm.
So without a resistor, the pin on the Arduino is directly connected to GND and a lot of current tries to go through the pin. Since the pins have operating limits and little protection against overcurrent the pin will be damaged by excessive current.

When used with LDRs, resistors form a voltage divider.

3

The usual purpose of a resistor is to limit current. For a LED the usual safe current is about 20mA. If your working voltage is 5v, about 180 ohms will do. At 12v, 470 ohms will do.

DO NOT PUT 12V ONTO ANY ARDUINO PIN, even through a R! You will damage it.

Look up how to use a transistor to control higher voltages.

When using a transistor, 10k ohm resistor in the base is common to limit the base current.

When tying an input to 5v or gnd to use a switch, 10k is good.

The IO pin is only able to handle up to 40mA, any more can damage it.

Weedpharma

4

I know that Leds and LDR need it

Two totally different reasons and totally different ways of wiring the component to the resistor.

I think you have got the LED one except:-

  • i don't understand how it can damage the I/O pin.

You damage an output by conecting it to a circuit that draws too much current.

Is because the current returns to it?

No, that collection of words has no meaning at all in electricity.

For an LDR you can use it without a resistor without damaging it, the only draw back is that if you do it will not work. The arduino pin in this case is an input so it does not draw any current. The resistor acts as a pull up, like for switches. See:-
http://www.thebox.myzen.co.uk/Tutorial/Inputs.html

5

Thks Grumpy,

I will read it.

6

Grumpy_Mike:
For an LDR you can use it without a resistor without damaging it, the only draw back is that if you do it will not work.

Not strictly true - you connect it from the input pin to ground, and enable the internal pullup. That will work just fine.

7

But an internal resistor IS strictly a resistor.