Why do my 100 ohm resistors keep Burning up?

Question

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 should be using if not a resistor?

Well let me explain my whole scenario... The exhaust fan in my bathroom stop working, so i took it down and disassembled it. it revealed a 120v/60hz 1.2A electric motor(A shaded pole AC motor - i discoverd after some reasearch.) I began ripping off the cover of the coils to see if there was a broken connection of some kind, in doing so i crushed something that looked like a resistor(without any signafiying color bands). It was in too many pieces to tell what it was or said on it. So i did some research, discovered ohms law, did the math and realized i needed a 100ohm resistor to achieve 1.2 amps from 120v. I got to radio shack and picked up 1/4 and 1/2 watt 100 ohm resistors. I quicky learned about the power of watts. (they set on fire pretty quickly.) After further research I realized i would need a resistor with a 144 watt power rating and 100 ohms. and they just don't make those (actually they do, but i'd have to order it oversea's for $10+shipping when a brand new motor is $20) Went back to radio shack and got their highest rated resistor, 5 watts. Solder'ed it up and the motor ran great! Except the resistor got HOT. I nearly burned off my fingerprint after a less-than-a-minute run. I figured this wasn't safe. And i have not ran the power supply directly to the coil in fear of what might happen.

SO HERE I AM. you mentioned i should probably be using something other than a resistor. What would one suggest. What was that part I broke? Thank You! (By the way, i have become obsessed with electromagnetic motors)

Answer 1

Best Answer - Chosen by Asker

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.

Asker's Rating:

Asker's Comment:

Thank you, very informative, solder'ed it up and the motor has been running for hours! THANK YOU!

Answer 2

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.

Answer 3

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.