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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1826
Loc: Central Texas
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I'm an electronics dim wit, so please speak slowly and clearly.
I have installed 2 parallel wired fans in my SCT to cool the mirror and would like to put a controller on it.
The fans pull .08 amps (.96 watts) ea .
Can you recommend the proper potentiometer ?
Do they typically have an off position?
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David
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3463
Loc: Doylestown, PA
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I'll assume the fans are powered from 12V DC.
So using Ohms law, 12V/0.08A = 150 Ohm equivalent resistance.
By adding a pot (or fixed resistor in series) you are essentially creating a voltage divider. So if the pot were 150 Ohms the voltage at the fan is 6V.
Vout = Vin * Rfan/(Rpot + Rfan). So a 500 Ohm pot would 2.7V minimum to the fan and 12V maximum. That should be give a workable speed range.
Since each fan uses 0.96 Watts max, a 2 Watt pot is minimum, a 4 Watt pot would be better.
Typically pots don't have an off position. However, the ones used as volume controls on old radios have a switch built in that can be wired in series with the pot and fan to have an off position. Audio volume pots are not linear, they have a logarithmic response, but his shouldn't be a problem.
The other option is a separate switch to turn the fans off. This has the benefit that the speed setting isn't changed by turning off the fans.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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grendel
sage
Reged: 04/12/09
Posts: 250
Loc: Canterbury, Kent, UK
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I have one of these speed regulators for a motor, not sure how well it would control your fans though.
http://www.mfacomodrills.com/speed_regulators/voltage_regulators.html
Edited by grendel (10/09/09 04:52 PM)
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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That looks like it'd do very well, PWM is almost always better than a regulated voltage. Only concern is they use a 1200Hz switching frequency so you might hear a whine out of it or the motors... which could drive you nuts.
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5029
Loc: Mobile, Alabama, USA
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Why not use a separate switch ahead of a variable resistor to control your fan? That is exactly how I wired up the cooling fan for my 10-inch Dob and the 15-inch Dob I am building right now. As a bonus, you can connect to the output side of the switch a 680 Ohm or greater resistor in series with a red LED to indicate when the fan is switched on or off. When something is connected in series, it means there's only one path for electrical current to flow through. It will not be excessively bright but you need the resistor to prevent the LED from being burned out. Resistors not only function as voltage dividers, they also limit electrical current. Always install a fuse too in the circuit, between the power switch and the battery. This way if there is a short circuit, it will prevent damage to your equipment or your telescope. Batteries can leak or even explode because a short circuit. Since you already know how much current the fans will draw, you already know what rating the fuse should be.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1826
Loc: Central Texas
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"PWM"?, "regulated voltage"?, "switching frequency" ?
Yes, a motor whine would drive me nuts.
I jealously guard my silence under the stars.
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David
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jasonharris
sage
Reged: 09/16/06
Posts: 253
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You could probably find something at a computer shop. Guys that 'overclock' their computers are religious about cooling, i.e. fans etc. There are little fan control units that will do the speed and cost next to nothing.
Jason
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Ian Robinson
Pooh-Bah
Reged: 01/29/09
Posts: 1166
Loc: Gateshead.NSW Nth Coast,Austra...
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Yes a pwm to generate a square wave DC voltage and current signal with the appropriate duty cycle and a control circuit.
Edited by Ian Robinson (10/11/09 06:31 AM)
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arpruss
scholastic sledgehammer
Reged: 05/23/08
Posts: 858
Loc: Waco, TX
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I use RadioShack 4 AA battery case which has a built-in switch and two wires coming out. This goes to a 150 ohm (I think) pot and two 12V Sunon maglev fans (I think in parallel) which are attached with velcro. The 12V fans run just fine on 5-6V. They are very quiet (I can't tell if they're running, unless maybe I put my ear to the mirror box), especially at that voltage, and I have not noticed any vibration (maglev fans have no contact between the motor and the shaft, and velcro is a good damper).
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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer
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