Calculating resistors wattages when schematics don't state it

[VSV_electron]

I've noticed that quite a few random schematics floating around the Internet don't specify the wattage of resistors used.
What's the correct basic "rule of thumb" approach to calculating resistor wattages in such a situation?

If the schematic was just 2 resistors in series or parallel that would be easy but when the schematics includes couple dozen of elements with quite a few passive and semiconductor elements in series/parallel and intricate interconnections how to approach the task?

As a random example I was looking at 2n2222 schematics just for curiosity and the following schematic popped up:
http://www.electroniq.net/radio-frequency/shortwave-radio-reciver-using-2n2222.html

It is not too complicated by your hardcore electronics standards, I'm sure but nevertheless that would be a good example to consider, wouldn't it?

As far as I understand it the cumulative resistance of the circuit needs to be calculated first to know the total current drawn and then it would be possible to calculate currents through each branch based on the known power supply. However that would be easier for the passive elements but when there are transistors along the path it somewhat complicates the matter. Besides there are branches running in parallel to the power supply.

[M0HZH]

When resistor model / type / power dissipation is not specified it's assumed to be irellevant, so the lowest spec / most common value will do just fine.

For simple circuits like these, 0.25W (THT or 1206 SMD) is considered the most common.

[ledtester]

As far as I understand it the cumulative resistance of the circuit needs to be calculated first to know the total current drawn and then it would be possible to calculate currents through each branch based on the known power supply. However that would be easier for the passive elements but when there are transistors along the path it somewhat complicates the matter. Besides there are branches running in parallel to the power supply.

You could go through all of that analysis, but there are simpler approaches to get an idea of what wattage resistor you need.

For instance, in the vast majority of circuits the positive supply voltage is the highest voltage that you'll see in a circuit and thus it will be the highest voltage drop that can occur across a resistor. Using the formula P = V^2/R you can easily test if a 1/4-watt resistor is sufficient, i.e. check if 0.25 > V^2/R or equivalently if R > 4*V^2. I.e. for V=9 volts, any 1/4-watt resistor over 320 ohms is safe.

Here's a small chart of resistor values and their 1/4-watt limit voltage:

Code: [Select]

    100R       5 V
    1K        16 V
    4.7K      34 V
    10K       50 V
    100K     158 V

Another approach is based on the total power used by the circuit. For instance, if you know a circuit powered by a 9V battery only uses 10 milliamps, then the power dissipated by all of the components is 90 milliwatts, i.e. 0.09 W so any 1/4-watt resistor is safe.

[coromonadalix]

lots of manuals in the past  assumed an 0.5watt resistor unless specified, or sometimes you had or have a legend before the schematics who explain tolerances / wattages  etc ...

[EPAIII]

In the days of tube circuitry 1/2W and even 1W resistors were the default value. Then transistors came along and that became 1/4W to 1/2W. With ICs it went down again to 1/8W to 1/4W. And even to lower levels in modern circuits.

Why?

Well, tubes tended to need Voltages measured in the hundreds in order to operate. 150V, 200V, 250V or even more. Power is calculated as Voltage x Current or Volts x Amps. So tube circuit that operated with a 200V power supply could, in theory, have a current passing from that supply to ground with only the resistance of the resistor to limit the current. 200V and 10K Ohms gives you 20 mA and a power of 4 Watts! Tube circuits had to be carefully designed to never allow that much current to pass through most of the resistors. Or if it could, then they needed to have higher resistances to limit the current or higher power ratings. At 200 V, 100K Ohms would pass 2 mA and the power level would be 0.4W. So a 1/2W resistor would suffice, BUT BARELY. Not much safety margin. I was once told to calculate the power expected and then double it and then select the next larger sized resistor. That is a good rule. That 0.4W calculated power would be multiplied by 2 for 0.8W so a 1W resistor would be used.

Transistors worked with supply Voltages from 9 to 24 or so. So the power expected to be dissipated in the resistors was around ten times smaller. I guess at that time in the history of electronics there was no real price incentive to go to resistors smaller than 1/4W and the automated assembly equipment probably came into play so the 1/4W size was common.

IC circuits started with 10 or 12 Volt supplies and quickly went to lower Voltages. And miniaturization was a desirable objective. Now the 1/8W or 1/10W looked like a good idea.

All of this is based on the idea that the power supply is the highest Voltage that can be encountered in a given circuit and most of the resistors will be of high enough values to limit the current to a sensible level. BUT, even with transistors, ICs, and low Voltage power supplies, it is still possible for a low value resistor to conduct enough current to operate at higher Wattage levels and generate greater amounts of heat. And some circuits can multiply the Voltage or current levels. So some resistors need to have their power levels carefully calculated. A good design engineer will usually know when these situations are likely. And, of course, circuits should be bread-boarded and tested or simulated in software. Sometimes both are needed to fully know the power ratings required.