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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.

Garry
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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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Old 12-19-2003, 03:57 PM   #21
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Talking

Nick,
If the heat sink is getting to 330 deg F.,
then the label "HEAT" seems appropriate. You might want to find a way to disable the position that gets that hot.
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Old 12-19-2003, 06:01 PM   #22
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NAPA variable rheostat

I started this thread some time ago.

Just yesterday on the Escapees discussion forum I saw a similar discussion with the recommendation to use a heater fan switch for older cars

Try NAPA # HC 6144 Variable rheostat. I haven't tried it yet, but may do that. I understand it doesn't have the 'heat' label.

Let me know if anyone does try it.

Steve
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Old 12-20-2003, 09:01 AM   #23
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Sander17, the high temperature is localized, and at the back of the switch away from combustibles. The dissipated power is probably no more than 15 watts at the highest, and the device is rated to 120. I would imagine it glows red hot at that rate! Any traditional rheostat will dissipate heat in this way. An extra point is that the heat sink is in the air flow of the fan, and is cooled by that. It operates happily for an hour with no localized excess heat. You make a good point, though. I made the mistake of holding the heatsink while testing the circuitry! Nick
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