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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?

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    \$\begingroup\$ You use a higher power rated resistor when you need to dissipate more power. \$\endgroup\$ – Matt Young May 28 '15 at 19:54
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    \$\begingroup\$ The 1/2 W resistor can dissipate twice as much power without getting so hot that it goes out of specification/sets on fire. If you are not using the resistors in power applications, it is unlikely to matter which you choose. \$\endgroup\$ – Oleksandr R. May 28 '15 at 19:54
  • \$\begingroup\$ Suppose you put 10 volts across a 200-ohm resistor. Your power dissipation will be P=V^2/R = (10)^2/200 = 0.5W. Your 1/2W resistor will be able to handle it, your 1/4W resistor probably won't. \$\endgroup\$ – DerStrom8 May 28 '15 at 20:13
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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!

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  • \$\begingroup\$ Now how about a zero ohm resistor (often used as a jumper in SMT designs)? If I've got a "1/4 watt" SMT resistor, how much power is OK? \$\endgroup\$ – Bryce Dec 7 '17 at 20:10
  • \$\begingroup\$ There's another question for that: electronics.stackexchange.com/questions/307728/… \$\endgroup\$ – Adam Haun Dec 8 '17 at 3:00
  • \$\begingroup\$ 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! \$\endgroup\$ – Basj Sep 13 at 19:27
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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.

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