Why do my 100 ohm resistors keep Burning up?

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orig:I have a 120 V 1.2 amp electric fan motor, im using 100 ohm resistors and they keep burning up. What am I doing wrong? Thank you Bonobo and Rowlfe! Do you know what part i ...show more
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Excellent! you did many researches, very fun and enjoyable.
according to ur description, I wonder where is that resistor in the circuit? In parallel or in series?
ur calculation is simply applicable if the resistor is in series with the other components.

and u are right, if it is installed in series with the other components, it must be a 144W 100Ohm resistor. Let me tell u the truth, this resistor will be very expensive and big in size. for electronics application, most of the resistors are in 1/4W, and some are in 1/2W, the 1W one is not very often.

To make a higher power resistor, u can apply the parallel resistor equation:

1 / R_TTL = 1/R_1 + 1/R_2 + 1/R_3 + ... and the final wattage should be raised.

For a 1/2W 100 Ohm resistor, maximum current flow cannot exceed 5mA.
If ur fan current is 1.2A, then 1.2 / 5e-3 = 240 pcs of 100ohm resistors.

So that, go to any electronics surplus store to buy 240 pieces 24K Ohm resistors, put them all in parallel, then u've built a 144W 100 Ohm resistor. It can support up to 1.2A current flow when 120V is applied.

DON'T DO THIS, it causes too much and I pretty sure, the broken component is not a resistor. If you replace the resistor with a fuse holder, and install it with a 2A fuse. I pretty sure the fan can run!

I guess that piece of animal is a thermal fuse, it is used to protect the fan circuit from over heated.
It occurs when the fan gets older, and there is some short circuit inside its coil, the current flow and temperature will rise and it has fire hazard. So that, the thermal fuse is necessary instead of a general 2A current fuse.

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5 out of 5
Thank you, very informative, solder'ed it up and the motor has been running for hours! THANK YOU!
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  • Technobuff answered 3 years ago
    I would suggest the part you "broke" was a thermal fuse of some type, either a self- resetting or single- shot device.
    Try wiring a fuse of about 3A. in place of your resistor. If that blows, you have a worse fault, and you can ditch the fan.
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  • jcherry_99 answered 3 years ago
    Actually you should have a resistor that is at least 2 or 3 times the wattage you calculate. It will always get hot. You can put a bunch of these things in series and parallel to make 100 ohms. You might also try light bulbs in series with the fan. They're made for high wattages. They could handle what you are trying to do and the "resistor" would only cost you the price of a base and a scrap piece of wood to mount it on.
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