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I have never understood the difference between 1/4 & 1/2 watt resistors, and in all of my EE lab courses I have always just used 1/4 Watt resistors. When does one use 1/2 Watt resistors and what is the difference between 1/4 & 1/2 Watt resistors?
Would using 2- 20 ohm 1/4 watt resistors in parallel, be the same as using 1 - 10 ohm 1/2 watt resistor?
The watt rating tells you how much current you can run through the resistor before it overheats and burns up. Power is given by:
$$P = I^2R$$
So the maximum current through a 1/4 watt resistor is:
$$I_{1/4W} = \sqrt{\frac {1} {4R}}$$
while the maximum current through a 1/2 watt resistor is:
$$I_{1/2W} = \sqrt{\frac {1} {2R}}$$
You can also use voltage instead of current:
$$P = \frac {V^2} {R}$$
$$V_{1/4W} = \sqrt{\frac R 4}$$
$$V_{1/2W} = \sqrt{\frac R 2}$$
For example, if you have a 100-ohm resistor:
$$I_{1/4W} = \sqrt{\frac 1 {4 \cdot 100\ \Omega}} = 50\ \mathrm{mA}$$
$$V_{1/4W} = \sqrt{\frac {100\ \Omega} {4}} = 5\ \mathrm V$$
Try putting 10 volts across a 100 ohm 1/4 watt resistor. You'll quickly see it start to smoke. The smell is very distinctive. Just don't hold the resistor in your hand while you do this!
Try putting 10 volts across a 100 ohm 1/4 watt resistor. You'll quickly see it start to smoke. The smell is very distinctive. Just don't hold the resistor in your hand while you do this! I have done this accidentally, and I confirm the smell and heat!
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Would using 2- 20 ohm 1/4 watt resistors in parallel, be the same as using 1 - 10 ohm 1/2 watt resistor?
Two parallel resistors of (say) 10 ohms value makes a 5 ohm resistor and the overall power that can be taken is double. So is using 2 series resistors of 2.5 ohms. They'll add up to 5 ohms and have exactly the same power rating.
If you try to mix and match different ohmic values then you won't get double the power rating of course but it will be certainly better than 1 resistor.
