slowing a 12v dc fan motor

Thread Starter

scobar

Joined Jul 6, 2010
10
this DC fan motor pulls 19amps at full speed. It's too fast. I have a switch with on/off/on and would like one position to slow the fan say to half speed. I would I do this?
 

t06afre

Joined May 11, 2009
5,934
A fan in a car cooling system can draw that current or more. Scobar What kind of fan is it. Do you have any details like a type or serial number. Is 19 ampere the fuse size. Or have you data saying 19 ampere is the current it draw.
 

CDRIVE

Joined Jul 1, 2008
2,219
Good grief, what automotive fan draws 19Amps? I'm not saying it isn't so. I just want to know so I never buy a vehicle that has one! :eek:
 

THE_RB

Joined Feb 11, 2008
5,438
Good grief, what automotive fan draws 19Amps? I'm not saying it isn't so. I just want to know so I never buy a vehicle that has one! :eek:
It's quite likely your car already has one. ;)

Just pop the hood, look in front of the motor and see if it has an electric radiator fan or a fan driven by a vbelt. With modern cars it's probably a 50/50 chance of either type.
 

RFactor

Joined May 1, 2009
33
Like Russpatterson says, you could use an N-Channel MOSFET and pulse width modulate it with a 555. The IRF530 shown in the diagram is only good for 14A so you might need one that will handle more current.

The RB: I know that many radiator fans are electric motor powered, I was surprised at the 19 amps but I guess that is not too far out of line. However, my 2.4 liter Toyota engine's electric radiator fan is rated at 3.4 amps. Perhaps a larger vehicle could have a 19 amp current draw. But...we don't even know if this is a radiator cooling fan, Scobar didn't mention the motor's application.
 

ifixit

Joined Nov 20, 2008
652
The fan speed in many car heaters is controlled by putting a power resistor in series with the motor to slow it down. Mount the resistor, which is going to get very hot, in the air flow to keep it cool. A 60W, 0.6Ω resistor would be a good place to start.

Regards,
Ifixit
 

CDRIVE

Joined Jul 1, 2008
2,219
Alright, you've got us kicking conjecture all over the place. Are you going to tell us what the application of this fan is?
 

THE_RB

Joined Feb 11, 2008
5,438
...

The RB: I know that many radiator fans are electric motor powered, I was surprised at the 19 amps but I guess that is not too far out of line. However, my 2.4 liter Toyota engine's electric radiator fan is rated at 3.4 amps.
...
Wow. I'm as surprised at how low that amps rating is as people were surprised at the high 19 A figure!
 

CDRIVE

Joined Jul 1, 2008
2,219
I have a 120VAC box fan in my wood shop. I draws 1.3 Amps and moves a heck of a lot of air. If you scale that to 14V you get about 11 Amps and I've never seen a passenger car electric radiator fan that moved this much air..

I still find 19 Amps a bit much;... maybe a Kenworth! :D
 

Thread Starter

scobar

Joined Jul 6, 2010
10
Alright, you've got us kicking conjecture all over the place. Are you going to tell us what the application of this fan is?
it is out of a race car that crashed. I am using the fan in my farm tractor. It is a Flex-a-lite 580 dual motor fan. When it's on (had only one speed) it is moving some air. I have the on/off/on switch so that is why I want twop speeds.....sorry for the delay in returning, you guys are quick. I am hoping for a simple fix.
 

Thread Starter

scobar

Joined Jul 6, 2010
10
U will need a MOSFET to do this....I guess our Sgt will be up for the task
please do help with this MOSFET switch. I was doing some google but still need help.

Could I also use the resistor, high wattage that simply goes in series from sw to fan? There was metion of a 60W .6 ohm. Is the a radio shack purchase for either?
 

PerunaPete

Joined Mar 17, 2011
22
please do help with this MOSFET switch. I was doing some google but still need help.

Could I also use the resistor, high wattage that simply goes in series from sw to fan? There was metion of a 60W .6 ohm. Is the a radio shack purchase for either?
A MOSFET is basically an electric switch. Putting a voltage between the gate and source allows current to flow through the transistor (it's more complicated than that, but it should give you an idea).

A great way to control a motor is by using PWM, which basically turns the transistor on for a certain amount of time, then turns it off, then back on..repeatedly (and quickly) to vary the amount of current going to the motor. This is what the 555 timer is for.

You can get the 555 timer, and most components for the circuit from radioshack, but you'll need to get the transistor online (probably).

Using a resistor is very inefficient, as all of the energy that isn't going to the motor is just being dissipated as heat.
 

Thread Starter

scobar

Joined Jul 6, 2010
10
A MOSFET is basically an electric switch. Putting a voltage between the gate and source allows current to flow through the transistor (it's more complicated than that, but it should give you an idea).

A great way to control a motor is by using PWM, which basically turns the transistor on for a certain amount of time, then turns it off, then back on..repeatedly (and quickly) to vary the amount of current going to the motor. This is what the 555 timer is for.

You can get the 555 timer, and most components for the circuit from radioshack, but you'll need to get the transistor online (probably).

Using a resistor is very inefficient, as all of the energy that isn't going to the motor is just being dissipated as heat.

So this looks like it's a field fabricated switch. I was hoping to buy it "pre-built" where all I have to do is install it. Remember this is a farm tractor, outside 100%, so I am hoping for a weather tight setup.
 

PerunaPete

Joined Mar 17, 2011
22
So this looks like it's a field fabricated switch. I was hoping to buy it "pre-built" where all I have to do is install it. Remember this is a farm tractor, outside 100%, so I am hoping for a weather tight setup.
Well, then it's not much of a circuits project.

You might try checking an automotive forum, I don't know of many fan controllers that can handle 19A off the top of my head.
 
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