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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 !!

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Old 07-19-2003, 07:45 PM   #15
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Wait a minute

Quote:
Originally posted by garry
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
If that bulb were 4 ohms, then the light would draw 3 amps (I=E/R). A typical 12v light bulb draws 1.44 amps which would make the resistance 8.3 ohms, not too far from the 10 ohm resistor.

The fact that you said the bulb lit tells me that most of the voltage was dropped across the bulb rather than the fan. The fan resistance, therefore, must be much less than 8.3 ohms.
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Old 07-19-2003, 08:08 PM   #16
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series circut
4 ohm lamp +4.7 ohm motor = 8.7 ohms=1.38 amps.
5.8 V across lamp 6.2 across motor.

Drop to a 1 ohm resistor = 4.7+1=5.7 ohms=2.1 amps
2.1V drop on resistor and 9.9 on motor.

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Old 07-24-2003, 12:44 PM   #17
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Pick & Garry

Pick,

You were right about the motor requiring more volts to start! I wired in one 1 ohm resistor which allowed it to start and then tried 2 - 1 ohm resistors in series which still allowed to start.

Garry,

The 1 ohm dropped the fan speed slightly - say 10 to 20%.
Two ohms dropped it by about 1/2 speed.

With 2 ohms I felt it was about right. You were also right about it heating up.... Too hot to touch. Need to get some higher wattage resistors and brush up on my 'Ohms Law'.

Any other recommendations for a good 12 volt book other than Ugly's mentioned above?
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Old 07-24-2003, 06:27 PM   #18
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Wish I had some of my old books.
Kids ran off with most of them years ago.
My guess is each 1 ohm resistor is disapating 9-10 watts. If you go with a single 2 ohm resistor it would have to be greater than 20 watts and still be to hot to touch.
If you go with 4 each .5 ohm 10 watt resistors in series they would disapate 4.5 to 5 watts each. Bottom line you are looking to burn off around 20 watts.

There may be an easier way once you know what voltage is required to run the motor at half speed. I "think" you can use a ziner diode but I will have to think on that, it's been a long time since I was involved with building circuts.
I am visiting in MS and don't have access to whatever books I might still have back in OK so unless someone else has the answer I won't be able read up on the problem for anothe week or so.

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Old 07-24-2003, 06:55 PM   #19
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I think it's time for you to check the current draw on your application using a current meter. Remember when you reduce the voltage you increase the current draw. Also the armature may not be wound to run at a reduced speed or it may be a faulty armature, you may need to replace the motor. Measure the current draw with and without your chosen resistor than there will be no more guessing.
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Old 12-19-2003, 10:49 AM   #20
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To have the option to change the fan speeds on the range hood in our trailer I have fitted a 3 speed controller. The fan is labelled "Electrohome 12volt". I measured the current draw as 2.5 amps, so this is a 30 watt motor with a running resistance of approximately 5 ohms. I bought from my local NAPA store, and fitted, a "Heat & Defrost Switch 12 Volt", made by NAPA Echlin, part # HC 6332, for about $15. This is rated to 10 amps, or 120 watts. After removing the slider knob and filter screen I disconnected the wire nut in the power line to the fan, and connected the wire ends to the switch terminals. The switch has its own heat sink, and on the lowest speed setting this sink reaches a surface temperature of 330 degrees Fahrenheit, so its position needs careful selection away from other wires and combustible substances. The rotary switch can either be fitted through thin sheet material, ( max. about 3/16ths of an inch), or it comes complete with a clamp to attach to anything up to about 1 inch thick. I chose to fit it through the thin plywood at the back of the overhead cabinet above the range, after discarding the clamp, and the light bulb inside the knob. The drill size for the hole is 5/8ths of an inch. Opening the cabinet door now reveals a small brown Knob which can be turned to 4 positions, namely OFF, Fast, medium, and slow. It only remains to scrape the label "HEAT" off the end of the knob, and the job is complete. Result? A quieter fan and a much happier wife! Nick.
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