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Topic: What Resistor capacity should I chose (learning!) (Read 1 time) previous topic - next topic

Grumpy_Mike

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from 0 Ohm (wonder what such a resistance exists..

These are called zero ohm links and are used quite a lot by professional engineers. Mainly when laying out a PCB they often have alternatives they would like to try, like the choice between an internal oscillator and internal. They lay it out and then put zero ohm links round it so they can swap between one configuration and another just by removing and placing these links.

To the others:-
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I thought there was a 30 mA limit on Arduino I/O pins?

We are talking about inputs not outputs here.

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5v/10k ohm = 0.5 miliamps

Yes but that is only the current going through the button when pressed, the arduio input pin takes some but that is very tiny.

MarkT

This use of a resistor is called a pull-up or pull-down - its purpose is to hold a signal at logic HIGH (or LOW) until the button is pressed.

The value is extremely uncritical, so long as it is 10 times the switch's on-resistance and less than a tenth of the pin's input resistance it will work.

However to be practical we want to keep the current low to avoid wasting power, but we also want to keep the resistance low to reduce the risk of picking up noise from nearby signals via capacitive pick-up.

So in practice pull up and pull-down resistors tend to be in the range 1k to 100k.   Using a very low value resistor (1ohm) would mean wasting 25 watts of heat in it.  Using 10M ohm resistor would be sensitive to pickup and would only switch slowly (not a problem with a push button).

Often in digital electronics resistor values are uncritical in this way - however in analog electronics most resistors have to be the right value for the circuit to function.
[ I won't respond to messages, use the forum please ]

HighSeraphim

The concept of a pull-up/pull-down resistor can be understood mathematically with ohms law.

If you have a pull up resistor from a button to 5vdc and the button wires to ground when pressed then the reading of the voltage at the button can be found.

5VDC -----(RES)------- || ---------GND
                             Button

When this button is pressed, you have the following circuit
5vdc ----- (RES) ------- GND

When it is open you have an open circuit
V=IR
if I = 0 then V = 0
This means the drop across the resistor is 0VDC. No drop means that the voltage at the resistor is 5vdc

When you push the button, current flows and you will get a drop across the resistor equal to I*R which will be 5VDC. IN other words, the value at the button will be 0VDC.
Always Think!

xberg

Fantastic! Thanks so much for the clarity of your responses. I love this forum!

Xberg

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