instaling a rheostate on a fan

[Boar]

how dose one pik out the size of rheostate the fan i want to contreol is a 12v 2.78A wat size would i need for that application? thanks

[merkman]

You use Ohms Law...

Ohms Law Comes in 3 "flavors"

Volts = Amps * Ohms

Amps = Volts / Ohms

Ohms = Volts / Amps

So....

Aproximations here,

With the poteniometer turned all the way down (0 Ohms added by your potentiometer) you have 12 volts 4 Ohms and that gives you a current flow of 3 AMPS. At this state the fan at top speed and the amperage is maxed.

So you need a potentiomer that will handle at least least 3 AMPs at 12 Volts DC "on the high side"

It don't matter if you "go over" on these specs over the amperage or over the voltage specs those are minimums.

To turn the fan off you need no current flow (0 AMPS). (a little more than that acutually but good enough for our conversation)

So... far as resistance goes; as long as you have a potenitomer that has enough resistance when turned all the way up it will bring the current to 0 (ie shut off the current flow) you will be fine.

The following matrix shows what happens as resistance is increased in your circit.

So you need at least ~120 ohms...

Add in the hillbilly engineering saftey factor and I would get at

12 Volt 3 Amp potentiomer with a max ohms of 200 ohms...

100 ohms may work since the resistance of the fan is not calculated in, nor is the stall voltage of the fan computed, (but hey we are not engineers here) try the 200 ohm and see if you get the full range of adjustment from your potentiomer.. if you loose adjustment you need a smaller pot, if you don't have enough resistance to shut the fan motor down then you need a larger pot..... It also makes a difference of what you have laying around and what is on the shelf at the store...

That should get you close enough to be able to tune it in from there...

Give it a shot... Radio shack or the ax man will have what you need.

Let us know how it worked.

-Merk

.

[Boar]

Really appreciate it merk, thank you. it wont stik but do appreciate it. I dont need to shut the fan down but jsut to turn it down some, so its not cooling the air flow but circulating it. Thanks again

[hydro]

Don't forget to size the rheostat/potentiometer for power dissipation as well or you may burn it up. With 12 volts and 3 Amps you would need at the very least a 36 Watt rating on that pot, and maybe a little more for safety. That would not be a typical Radio Shack buy.

A likely way to get around finding a 40 Watt Pot would be to use a couple of big power resistors and switch them in to the circuit. You would have "High" (no resistor), "Medium" (5.7 Ohms/50 Watts), and "Low" (8.2 Ohms/50 Watts). The resistance values are just SWAG on my part but the idea is there. Big power resistors are easy to find at dealers like Digikey.

[Boar]

I Forgot to mention that Im running two of those fans, each one is 12v 2.78A So how big of a pot would I need to run that just double it according to chart above?

[Whoaru99]

I suggest getting a PWM controller instead of using resistors or a rheostat.

Something like this you can mount is a small Radio Shack project box. This one is pre-assembled or you can get kits if so inclined.

http://www.amazon.com/Converter-12V-36V-Adjuster-Controller-Driver/dp/B008BGLMYS/ref=pd_sim_sbs_lg_1

[hydro]

Whoaru hit this one right on. The PWM drive is the most efficient way to control those fan motors and it is infinitely variable as well. I checked the link and it looks like the drive board is complete but may need a potentiometer to vary the modulation? This would be a good tutorial to put together showing how to put this into a box and get it running.

[Scott K]

I have just bought cheap heater blower motor resistors from a part store for ~$10 and used that. Easy and cheap/

[Whoaru99]

One of the other pictures shows the control pot so I assumed it was included, but I don't know that for sure.

[Whoaru99]

Resistors may or may not be cheaper but for sure they waste power. The PWM is considerably more efficient, prolonging battery life, and is continuously variable.

There are less expensive PWM units but this one seemed to be a good combination of favorable ratings and price, on a brief search.

[Boar]

How about a dimmer switch from a car?

[hydro]

If you have an auto dimmer switch handy you could give it a try. At worst you will get a brief flash of light and a little smoke!