Electrical guru's, I need some help.

[NCBusa2001]

I'm at wits end. I've been trying to tweak something but I don't seem to be able to find the right info.

Here's the problem.  

I have rigged a 12 volt electric car radiator fan up to the vent in my enclosed trailer.  I have the fan hooked up to a 12v car battery and it works well at high speed.  

I would like to DECREASE the speed of the fan.  I got advice from the radio shack guys and followed their reccommendations.  I have experiemented with wiring resistors in line.  The resistors slow down the fan to the speed I want, BUT they get extremely hot (too hot to touch) after a few minutes.

I have looked at rheostat's and potentiometers (sp) on-line.  It seems that there is a bazillion different kinds with different ratings and I'm clueless.  I don't even know if this is the right route to go in the first place.  

A friend suggested using a 12 volt dimmer switch for a light.  I did look at one and saw it was rated for 8 amps and the fan has a rating of 20 amps.  

Does anybody have any advice?

Thanks in advance.

Chris

 

[Test_Eng]

Heat is wattage. Use several resistors in parallel equaling your required resistance. This will dissipate the wattage across all the resistors instead on just one. Also use high wattage resistors. Remember total resistance in parallel = 1/((1/R1)+(1/R2)+(1/R3)). I can draw a schmatic up if you know exactly what resistance you are looking for and the wattage of your resistors. I also need to know the specs on the radiator fan. Wattage preferably.

 

[DREW]

Yeah ! What he said !

 

[300Busa]

Heat is wattage. Use several resistors in parallel equaling your required resistance. This will dissipate the wattage across all the resistors instead on just one. Also use high wattage resistors. Remember total resistance in parallel = 1/((1/R1)+(1/R2)+(1/R3)). I can draw a schmatic up if you know exactly what resistance you are looking for and the wattage of your resistors. I also need to know the specs on the radiator fan. Wattage preferably.

 

[theemrrdkill]

+2 ..enough said

 

[Projekt]

Get a simple 12V potentiometer rated for 25amps so as to keep the adjustability. If you look into the "flexalite" series of performance electric fans commonly sold to Mustang, Camaro and other tuner car enthusiasts you will probably find a suitable control unit to meet your needs.

 

[NCBusa2001]

Heat is wattage. Use several resistors in parallel equaling your required resistance. This will dissipate the wattage across all the resistors instead on just one. Also use high wattage resistors. Remember total resistance in parallel = 1/((1/R1)+(1/R2)+(1/R3)). I can draw a schmatic up if you know exactly what resistance you are looking for and the wattage of your resistors. I also need to know the specs on the radiator fan. Wattage preferably.

WOW, thanks.

First-The fan I bought from Autozone. It is a generic "universal" 10 inch fan and came in a plain white box. I didn't see any specs regarding watts. I called Autozone and all they could tell me was it was "rated for 20 amps".

Would the watts be printed on the fan itself?

Currently I have three 100ohm reisitors in parallel. They are about 2 inches long, rectangle and off-white in color. They both have what looks like a 16ga bare alumminum wire coming out of both ends.

If I go to "the shack" and ask for "high wattage resistors" would they look at me like I'm clueless?

Again thanks for your help.
Chris

 

[omslaw]

Power (watts) = Volts x Amps

Even though the fan is rated for 20 Amps, that's probably not the continuous current draw, but even if it is, 12v x 20A = 240 watts. By adding resistors, you are changing the voltage, the current draw remains the same. So let's say you add enough resistors to drop the voltage to 6v. Your resistors will need to dissipate 120W of power.

Projekt is on track with the 25A pot. It's still gonna get warm, but that's just because of the power (heat) that will be dissipated thru it.

 

[NCBusa2001]

Get a simple 12V potentiometer rated for 25amps so as to keep the adjustability. If you look into the "flexalite" series of performance electric fans commonly sold to Mustang, Camaro and other tuner car enthusiasts you will probably find a suitable control unit to meet your needs.

I googled for 12v pots and found some resources. From what I have read in various forums (way over my head) is that the pots will burn out if you attempt to pass amperage throuh them.

I couldn't find any rated for over 8 amps.

Do you have any links or other suggestions.

CHris

 

[NCBusa2001]

Power (watts) = Volts x Amps

Even though the fan is rated for 20 Amps, that's probably not the continuous current draw, but even if it is, 12v x 20A = 240 watts.  By adding resistors, you are changing the voltage,  the current draw remains the same.  So let's say you add enough resistors to drop the voltage to 6v.  Your resistors will need to dissipate 120W of power.

Projekt is on track with the 25A pot.  It's still gonna get warm, but that's just because of the power (heat) that will be dissipated thru it.

any idea where to find a 12 amp pot?

 

[NCBusa2001]

OK I've looked at some more pots and I'm more confused.

All I seem to be able to see are rated in the single watt range (ie, 1.5 watts, 2 watts, 1 watt, etc) none seem to go as high as 240watts.

Also would I need a: conductive, wireworun, sine/cosine, carbon comp, contactless, linear or rotary.

Chris is clueless.

 

[ditchdr]

Just put a half dead battery on it lol.. 1/2 of 12 =6 right?

I use a rheostat on my hand elec grinders that are ratted for 110volts.. You can get them at

www.mondello.com

 

[300Busa]

don't burn the house down

 

[Robot]

OH Lordy

, sorry can't contain myself

. Controlling motor speed with resistance(heat), puts more load on, and then wastes it as heat. Fisher Price used/ and maybe still uses this lame 19th century resistance method, sorry some childrens cars caught on fire.

 

    Sorry motor control was my thing. Pulse Width Modulation is the way to go, minimal heat loss, about a billion times more effiecient and 20th century technology. Yes I know we are in the 21st century

Link to Module on ebay:

http://cgi.ebay.com/12V-PWM....6a32230

or google 12v PWM


Resistance is NOT the way to control motor speed(yes Detroit used it for decades on wiper motors, etc,etc.), as well as ridiculous heat generation there is a nasty effect on motor torque at lower speeds(motor will be hotter, and lifespan will decrease). Pulse Width Modulation addresses both these issues. PM me with any technical questions. Sorry to board members for my passionate rant

 

[Powers739]

Buy a different fan...

 

[Kanman]

Just throwing an idea out there... Grab an old 12v cell phone charger and strip it down for use in your project. The idea being it will already have the electronics components resistors and voltage regulator pre-wired to drop you down from 12 to five volts. Actually, just about any old 12v battery charger cord will work, most all that stuff runs off 5 volts.

 

[Mr Bogus]

may need to check the operating range on the fan itself... some are not designed to run at less than 12V (overheat)

I would just invest in a proper ventilation fan (some solar powered stuff out there that works pretty well now)

 

[djorgensen3]

Go to an RV store and look at their light switches. They may have a dimmer that will work. They may also have a better solution than those listed.

 

[Test_Eng]

OH Lordy

, sorry can't contain myself

. Controlling motor speed with resistance(heat), puts more load on, and then wastes it as heat. Fisher Price used/ and maybe still uses this lame 19th century resistance method, sorry some childrens cars caught on fire.

 

    Sorry motor control was my thing. Pulse Width Modulation is the way to go, minimal heat loss, about a billion times more effiecient and 20th century technology. Yes I know we are in the 21st century

Link to Module on ebay:

http://cgi.ebay.com/12V-PWM....6a32230

or google 12v PWM


Resistance is NOT the way to control motor speed(yes Detroit used it for decades on wiper motors, etc,etc.), as well as ridiculous heat generation there is a nasty effect on motor torque at lower speeds(motor will be hotter, and lifespan will decrease). Pulse Width Modulation addresses both these issues. PM me with any technical questions. Sorry to board members for my passionate rant

I agree PWM is the way to go when running an DC inductor or a fan. Though I also believe it should be thermostat,thermocouple, or tempsensor controlled. Though now your getting into module control and require software to properly control the fan.

 

[Robot]

OH Lordy

, sorry can't contain myself

. Controlling motor speed with resistance(heat), puts more load on, and then wastes it as heat. Fisher Price used/ and maybe still uses this lame 19th century resistance method, sorry some childrens cars caught on fire.

 

    Sorry motor control was my thing. Pulse Width Modulation is the way to go, minimal heat loss, about a billion times more effiecient and 20th century technology. Yes I know we are in the 21st century

Link to Module on ebay:

http://cgi.ebay.com/12V-PWM....6a32230

or google 12v PWM


Resistance is NOT the way to control motor speed(yes Detroit used it for decades on wiper motors, etc,etc.), as well as ridiculous heat generation there is a nasty effect on motor torque at lower speeds(motor will be hotter, and lifespan will decrease). Pulse Width Modulation addresses both these issues. PM me with any technical questions. Sorry to board members for my passionate rant

I agree PWM is the way to go when running an DC inductor or a fan. Though I also believe it should be thermostat,thermocouple, or tempsensor controlled. Though now your getting into module control and require software to properly control the fan.

OH Lordy X2

  Sorry, but trying to be helpful is getting frustrating. "DC inductor"? What is that? Induction is an AC function

 Inductive VS. Resistive load?

    The thermostat in homes for decades has been the inexpensive "tried and true" bimetal coil that rotates a mercury switch (no software,thermocouple,thermistor or PID loop controller needed). Put this in(set the temp) and let it cycle the fan on and off. If the mercury switch cant handle 20 amps, then have it trip a relay that can. Not sure if controlling fan speed would even be needed in this scenario.

    If fan speed still needs to be controlled, when I googled 12v PWM, I did see some "already built" plug and play units for sale.

    To do this "right" and safely, is not as expenisive or as complicated as it sounds.

    Some History: My father owned (2) resistor manufacturing companies, Omtronics and Tech-Ohm. As I grew up in the 70's, I saw the low tech application of resistors as energy wasters. The good news was this made our family quite a bit of money.
    There are much more sophisticated ways to control today, about the only justifed use of High Wattage resistors I see today is as a load/shut for dynamic braking with servo motors.

Sorry again if overly passionate on this subject and stepped on some toes