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Old 07-19-2003, 08:46 AM   #1
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Help! Resistor on fan switch

I am adding a 2 speed switch to my overhead vent fans that were originally single speed.

I bought SPDT - On-Off-On 35 amp chrome toggle switches that are for 12 volt DC from an auto supply store.

Pahaska recommended using a 10 ohm 10 watt resistor to slow the fan speed down. I connected the 12 volt leads to the center and to one end of the On switch. Sure enough the fan works on that side. For the other side of the switch I connected one end of the 10 ohm resistor and connected the other end to the other side On for the current to go thru the resistor (which it does as it gets warm) to the first side On.

IOW one on is full 12 volts and the other On is going thru the resistor to reduce the volts to the fan.

The result is that fan works normally on first side and not at all on the other side. I reversed sides to make sure both worked fine and they do.

The volt potential on the first side is 13.9 and 11.7 on the 2nd side thru the resistor which gets hot after 30 seconds. TT is plugged in to house current.

I exchanged wires around and got the same results.

TIA Steve
trying to get a slower fan speed.....
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Old 07-19-2003, 09:17 AM   #2
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Steve try this

12 V to the sw center pole.
From one sw end pole to one end of the resistor.
From the other end pole to the far side of the resistor.

From the far side of the resistor both wires connect together then to the fan motor.

sw 1--------------------------
sw center---- 12VDC |
sw 2---> Resistor >-------->--wire from sw1 and sw 2 to fan---------->

In the sw 1 pos you get 12V to fan
In sw 2 position you go thru resistor.

Garry
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Old 07-19-2003, 09:27 AM   #3
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Thanks for your reply! Not sure what "sw" is.

...."12 V to the sw center pole.
From one sw end pole to one end of the resistor.
From the other end pole to the far side of the resistor.

From the far side of the resistor to the fan motor. "....


I believe that is what I did. Attached the resistor to the second end pole of the switch (2nd ON position) and the other end of the resistor to the First On position which is where the other wire is that goes to the fan. That should complete the circuit on the 2nd side as it does get the resistor hot but no fan speed.

Going to granddaughters Birthday Party and will check in later.

Steve
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Old 07-19-2003, 10:45 AM   #4
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SW = switch

I suspect the 10 ohm resistor is to big.

Assuming the motor pulls 12 amps you need to go to a 1 ohm 40 watt resistor to run the fan at half speed. The resistor ohm value and wattage rating will depend on the resistance of the motor.
The smaller value resistor you use the higher the wattage rating required for the resistor.
Not knowing the resistance of the fan motor it is only a guess at the size of the resistor.
One thing I have done in the past is to wire in a 12V bulb in place of the resistor (back-up or side marker bulb) to test the effect then measure the resistance of the bulb and buy the same value resistor and bulb wattage if it works OK. If the fan dosen't turn fast enough with the bulb you need to buy a smaller value resistor.

Hope this helps !!!!!!!!

Garry
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Old 07-19-2003, 01:05 PM   #5
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Any electricians out there??

Garry,

Great idea and that is exactly what I did.

I wired the 12 volt bulb in line with the fan circuit so that the current had to go thru the bulb and then to the fan to complete the circuit.

Well, the bulb came on nicely, but the FAN didn't budge an inch! I realize the bulb has much less resistance than the motor (Electrohome 3" diameter original equipmt top vent fan), but shouldn't the motor turn with current coming to it?

The only thing that I haven't done is to cut the black wire that goes straight to the fan and insert resistance there. But that shouldn't make any difference than having it go thru the switch wire.

Still learning electricity!!!
Any electricians in the group?
Steve
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Old 07-19-2003, 02:12 PM   #6
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Get yourself this pocket size book. Ugly's Electrical-References, by George V. Hart.
This handbook has everything you will need to figure out your electrical solution. It is complete with Ohms laws and examples on how to make simple circuits using formulas, motors and what current they will draw. A good referance book for all. It takes up little drawer space in your AS and is an invaluable referance. You will wounder how you ever got along without it.
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Old 07-19-2003, 02:28 PM   #7
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Well ...

It sounds like you have it wired correctly.

The 10 ohm resistor works just fine on every fan I have tried it on in 3 trailers and I have gotten feedback from a number of other persons who have used it successfully. I have 10 ohms on both my stove vent fan and my bathroom vent fan right now.

Most 12v vent fans pull about an amp. On a 12v circuit, that means that the running resistance of the fan is about 12 ohms (I=E/R). Putting a 10 ohm resistor in series should result in a little more than 1/2 voltage to the fan motor or about 6 volts which should turn the fan handily.

I'm not quite sure why you would have 11.7 V on one switch setting; that doesn't compute. I also don't know why the resistor should get warm since there should be very little current through it. A 2 watt resistor should be adequate; the reason I specified a 10 watt resistor is that they are stocked by Radio Shack.
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Old 07-19-2003, 02:34 PM   #8
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I tried a 10 ohm resistor on mine, and it did not work either. The resistor just got hot. Motor resistance is 4.9 ohms according to my DVM. Perhaps 5 ohms would be better for this particular motor. With a 10 ohm resistor it would be drawing .84 amps. Might not be enough to start it.
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Old 07-19-2003, 03:35 PM   #9
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Some Success

Pahaska,

You were right about what they supply. I went to 3 Radio Shacks even to find the 10 ohm, 10 watt resistors.

I tried a different fan. Tried the one above the stove. It takes the 10 ohm handily and - as you say - drops the voltage and speed about 1/2. That one works great. A Keeper!

Going to put one in the bath vent also. Short wires, but it too needs slowing.

But back to the first one on the roof vent. Still doesn't work. Went back to the first switch and simply wired in the resistor in line - one lead of resistor to switch, the other lead to the fan - and still no fan rotation. Take it out and voila, it works!

So Pick, I think you are right. I'll need to go back to the store.
R.S. only has 10 and 1 ohm in the 10 watt size. May have to try several 1's. It didn't start even with a tail light bulb in the line!

Steve
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Old 07-19-2003, 03:58 PM   #10
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Talking Learning 'Lectricity

Tinsel Loaf,

Thanks for the recommendation. I've already called the book stores for a copy. I've been looking for a good ref book!!

The best I've found is "Managing 12 Volts" and I'm going to reread this one. I've checked out about 6 books from the library in the last week, but none really tell you how to change fan speeds, etc.

Pahaska was (is) my "teacher" in this respect - - as well as the vast experience on this forum. Any other forum that you know of that is good for electrical info?

I appreciate all who have responded to "share the wealth".....

Steve in Sav'h
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Old 07-19-2003, 06:35 PM   #11
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If you have it wired right and it sounds as if you do then the only thing I can assume is the fan motor requires more than 6 V to run. The light bulb will drop 6V across it leaving 6V for the fan.
However; I would have bet the fan would have run at a very low speed with the bulb in line with it !!!! Shows what I know...
If you can get a 1 ohm resistor that will leave more voltage available for the motor. However, it will need to be a higher than 10 watt . You can try it with a 10 watt as a test but it will get way to hot if left in the circut.
RS should have them in stock.

Garry
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Old 07-19-2003, 06:49 PM   #12
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If I remember it right 2 10 ohm resistors in parallel is equal to a 5 ohm one of the same wattage.

You might try that to see what the voltage output becomes if you do that.
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Old 07-19-2003, 06:58 PM   #13
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Right

Quote:
Originally posted by thenewkid64
If I remember it right 2 10 ohm resistors in parallel is equal to a 5 ohm one of the same wattage.

You might try that to see what the voltage output becomes if you do that.
You took the words out of my mouth (at least off my keyboard).

Obviously, the fan pulls more than the typical 1 amp and therefore has a running resistance of less than 12 ohms. If you parallel 2 of the 10 ohm resistors, I'll bet the fan will run.
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Old 07-19-2003, 07:33 PM   #14
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I figure the bulb at around 4 ohms that's why I assumed the fan would run at some speed ???
Same thing with a 5 ohm resistor 6V across the motor just won't start it and lead to my recommendation for a 1 ohm then work up from there !!

Garry
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