How to choose a face value of pullup resistor?

I recently received from a reader question about the choice of the values of pullup resistor:

Hello!
Why in the circuit with monostable actuation button you specifically connect a resistor of 10 ohms? You expect this value?

You can use resistors for many purposes. The following scheme (Fig. 1), to which the reader is referred, the resistor R2 is a pull-up resistor with a value of 10 ohms:

Resistor 10 kOhm (R2) serves to ensure that the voltage at terminal stably maintained at a high level - close to the supply voltage.

The reason that I chose a value of 10 ohms, is that in my experience I know that the pull-up resistors from 10 ohm to 100 ohm usually work fine in almost all circuits.

I have seen that it has been used many times, and I have tried it myself many times.

But if you do not know this and you need to calculate the value, then read on.

Selecting pullup resistor

There are two basic principles of selecting a value pullup resistor.

You need to value that is:

Not too small;
Not too much;

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Not too small denomination

The first principle - have the meaning which not too little .

If the value is too low, a resistor high current flows, which makes it very warm (or even hot) and can lead to its failure.

What do we mean by "too low"?

Standard resistors often have a nominal capacity of 0.25 watts (W).

If you look at the worst case scenario, all the 9 V supply (see. Fig. 1. ) Voltage will drop across the resistor.

Power dissipation in the resistor can be calculated using the following formula:

P = V * V / R

Hence:

R = V * V / P

R = 9 * 9 / 0.25 = 324 ohms

324 ohm resistor will operate at the limit of what he can endure if it falls 9B.

So you want to have a value much higher than that.

If you choose a value of at least 10-20 times greater than the calculated value, you'll be fine. For example, at least 5 ohms.

Not too big denomination

resistor value can not be too large.

If the resistor value is too high, you get too large voltage drop, so that the value at the input terminal ceases to be high enough to be called high.

What do we mean by "too much weight"?