Jump to content

12v current limiter to slow 12v fan

real_vibes

By real_vibes
in Boat Equipment

 Share

Featured Posts

Señor Chris Proficient

Señor Chris

Posted
Posted (edited)

 

This sort of thing should do:

 

http://www.ebay.co.uk/itm/BLACK-2A-MULTI-VOLTAGE-CAR-DC-DC-CONVERTER-/270959020720

 

How much do you want to slow it down, and what's it going to be used for, some sort of eco fan replacement?

 

 

I've just bought a CPU fan for exactly that purpose. I've not tried it yet so don't know if it will need slowing down but if it does, would the car adapter work ? The fan is rated 6-15v and 2.2W at 12v.

 

Edit: Just noticed it consumes 2A which defeats the object of using a low power fan. I'll think again.

Edited by Señor Chris
Link to comment
Share on other sites
Ian Mac Proficient

Ian Mac

Posted
Posted

You need a PWM device to control the speed otherwise you will burn the motor out, the reason a motor works is that it generates a back voltage because it is rotating in a magnetic field. This back emf stops the forward voltage and thus reduces the current being excessive, and thus cooking the motor. So if you run a motor at say half the voltage, as achieved by putting a resistor in the circuit the back emf may be different and so there may be a the larger current flow so the motor overheats. Using a PWM system like the maplin one stops this. The thing you need to understand is the load placed upon the motor at a given speed by the fan, then you can work out if you can get away with a resistor. Nothing in life is simple. by £12 fixed this one.

Link to comment
Share on other sites
bottle Veteran II

bottle

Posted
Posted

 

I've just bought a CPU fan for exactly that purpose. I've not tried it yet so don't know if it will need slowing down but if it does, would the car adapter work ? The fan is rated 6-15v and 2.2W at 12v.

 

Edit: Just noticed it consumes 2A which defeats the object of using a low power fan. I'll think again.

 

I am not doubting you but check again, I have two computer fans both 12v and they are 0.2 amps and 0.18 amps

 

2.2/12 = 0.18 amps that decimal point is vital. wink.png

Link to comment
Share on other sites
Señor Chris Proficient

Señor Chris

Posted
Posted

 

I am not doubting you but check again, I have two computer fans both 12v and they are 0.2 amps and 0.18 amps

 

2.2/12 = 0.18 amps that decimal point is vital. wink.png

 

Sorry, I didn't make myself clear - I was referring to the car adapter when I said "it" consumes 2A which as your figures show, is about 10 times more than the fan.

Link to comment
Share on other sites
smileypete Veteran II

smileypete

Posted
Posted (edited)

 

I've just bought a CPU fan for exactly that purpose. I've not tried it yet so don't know if it will need slowing down but if it does, would the car adapter work ? The fan is rated 6-15v and 2.2W at 12v.

 

Edit: Just noticed it consumes 2A which defeats the object of using a low power fan. I'll think again.

 

The 2A is the maximum output current capability, how much it draws from the boat supply will depend on what load is connected. For a PC fan running on reduced voltage it'll be a mere smidgen of power.

 

But a 2A output is waaaayyy above what a PC fan should use, a lower current output should be fine (unless you might want to re-use it with more power hungry stuff) eg:

 

http://www.ebay.co.uk/itm/800-mA-REGULATED-DC-DC-MULTI-VOLTAGE-IN-CAR-POWER-SUPPLY-ADAPTOR-CHARGER-/121087755373

 

By the sound of it you have a 120mm fan, but 200mm ones are around and should do about 100cfm, they sometimes go for as little as a fiver on Ebay.

 

Whatever you do make sure the fan is securely mounted and sufficient distance from the stove or flue, as plastic and hot stoves don't mix. help.gif

 

cheers, Pete.

~smpt~

Edited by smileypete
Link to comment
Share on other sites
Sir Nibble Veteran II

Sir Nibble

Posted
Posted

You need a PWM device to control the speed otherwise you will burn the motor out, the reason a motor works is that it generates a back voltage because it is rotating in a magnetic field. This back emf stops the forward voltage and thus reduces the current being excessive, and thus cooking the motor. So if you run a motor at say half the voltage, as achieved by putting a resistor in the circuit the back emf may be different and so there may be a the larger current flow so the motor overheats. Using a PWM system like the maplin one stops this. The thing you need to understand is the load placed upon the motor at a given speed by the fan, then you can work out if you can get away with a resistor. Nothing in life is simple. by £12 fixed this one.

Nice to see someone else with a nice functional understanding of the effect of back EMF. Of course in a series circuit the volt drop across each resistor is proportional to the resistance and the resistance of a stationary motor is very low so until she gathers a bit of speed and back EMF does it's stuff the motor has a very low voltage to work with which of course knackers the low speed torque. This is why wiper motors no longer use resistors for two speed operation, they tried it and it just didn't work because the motor just didn't have the torque to drag the blades across the screen unless it was really running with water. Fans on the other hand offer load proportional to their speed so a resistor is entirely feasible and is the preferred option for all car fans heater or cooling. I would personally be inclined to have a damned good look first to see if I could identify the noise and see about dealing with that. Is this a two speed fan?
Link to comment
Share on other sites
Señor Chris Proficient

Señor Chris

Posted
Posted

 

The 2A is the maximum output current capability, how much it draws from the boat supply will depend on what load is connected. For a PC fan running on reduced voltage it'll be a mere smidgen of power.

 

OK - it was the "Continuous Current" bit that threw me.

 

 

By the sound of it you have a 120mm fan, but 200mm ones are around and should do about 100cfm, they sometimes go for as little as a fiver on Ebay.

 

It's 80mm which delivers 30cfm so I'm hoping to avoid the need to slow it down. I don't actually want a strong blast of air - just a gentle waft of warm air over me toes will do nicely.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.