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How to Choose a Rheostat/Variable Resistor

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greenman100

Disabled
Joined
May 18, 2003
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trying to keep all the magic smoke in
I've seen this come up a lot lately, so I thought I'd make a short guide. All bumps appreciated.

Guide to Choosing a Rheostat

Variable resistors work like valves, limiting current flow. However, as current flow is limited, the energy must go somewhere, so it is converted into heat. This post will cover the basics of choosing a rheostat that meets your needs.

Examples are in italics.

Step One, Identifying Needs

Identify your needs...What fan(s) will you run on this rheostat? What range are you looking for? What are the specifications of the fan(s) you are running? We will assume one fan, total current draw at 12v of .3A

Step Two, Calculations

Get down and dirty with pencil and paper or a calculator. Add up the current draw total for each rheostat. You may choose to run one fan per rheostat, or more. Keep in mind the more fans per rheostat, the bigger the rheostat must be. The total current draw will be stated in amps, and called Imax for our purposes. Most fans will not run at less than 7v, so we will keep this as a bottom floor for our calculations.

Now, we refer to Ohm's law.

Voltage=Icurrent*Resistance
So, at 12Vdc, the following is true of the fans:

12V=Imax*Rfans
Plug in Imax, get Rfans. Remember resistance is fairly constant no matter what the voltage is. Resistance actually depends on load on the motor, and therefore RPMs. Foru our purposes, we will consider it constant.
12V=.3A*Rfans
Rfans=40 ohms



Now, we must calculate the current draw of the fans at 7v:

7V=Imin*Rfans
Plug in Rfans, solve for Imin.
7V=Imin*40
Imin=.175A


Now, we calculate what the rated resistance of the rheostat must be. The rheostat is dropping 5V at Imin amps, so...

5V=Imin*Rrheostat
5V=.175*Rrheostat
Rrheostat=28.6ohms


Rrheostat is the approximate minimum resistance of your rheostat, expressed in ohms. It is important that you don't exceed this number by too much, or you will have a rheostat with only a half-usuable turning range. Up to double the calculated values is safe.

The last thing to calculate is the wattage of the rheostat. Remember, a rheostat must convert the energy into heat, as energy cannot be created or destroyed. The wattage indicates how much heat the rheostat can dissipate safely.

W=V*I

So since the rheostat is dissipating 5 volts and Imin amps...

Wrheostat=5*Imin
Wrheostat=5*.175
Wrheostat=.875w


Your rheostat must be rated for Wrheostat watts or more. More is better. It is a good idea to double this rating or more, to be safe.


Step Three, Finding a Rheostat That Meets Your Needs

Good places to look:

Radio Shack, expensive but easy
www.mouser.com
www.digikey.com
www.mpja.com
www.newark.com

I'm sure more people will add later....

Hope this helps.

Revision 1.2 Clarified constant resistance
Revision 1.1 Added example.
 
Last edited:

Electron Chaser

Senior Delta Fanatic
Joined
May 6, 2004
Location
I live by a Delta
Great post!!! I would like to see it made a Sticky

For some people this may be new stuff for them and may be a bit confused :confused:

Especially for those that do not even understand Ohm's law. Perhaps you could add an actual example. That would be nice. I mean even DOS had an example /? :D
 
Last edited:

stan03

Member
Joined
Mar 30, 2003
would it be a good idea to add "good" fan contollers to this also? there are quite a few posts about what rheobus should i get etc. although maybe its too off topic? idk up to you.
 

matttheniceguy

Member
Joined
Apr 1, 2004
Location
Vancouver Canada
good explanation, it's nice to have a place to point people to rather than typing all that out every time.

I don't mean to be a pain, but the resistance of a fan isn't actually constant. It is partially realated to the RPM of the fan. As the fan spins it creats an EMF that opposes the voltage through the fan. Essentially the fan will become a voltage source in the circuit, but it's voltage opposes that of the power supply. This is why electric engines will rev up to a certian speed and stop. They are at an equlibrium state where the voltage through them minus the voltage they are producing will provide enough power to overcome whatever mechanical resistance the engine is facing.

Usually assuming the resistance of the fan to be constant will provide a close enough value that everything will work fine. I just like to add in that these calculations are just approximate and are technically wrong so people don't get the wrong idea.
 
OP
greenman100

greenman100

Disabled
Joined
May 18, 2003
Location
trying to keep all the magic smoke in
matttheniceguy said:
good explanation, it's nice to have a place to point people to rather than typing all that out every time.

I don't mean to be a pain, but the resistance of a fan isn't actually constant. It is partially realated to the RPM of the fan. As the fan spins it creats an EMF that opposes the voltage through the fan. Essentially the fan will become a voltage source in the circuit, but it's voltage opposes that of the power supply. This is why electric engines will rev up to a certian speed and stop. They are at an equlibrium state where the voltage through them minus the voltage they are producing will provide enough power to overcome whatever mechanical resistance the engine is facing.

Usually assuming the resistance of the fan to be constant will provide a close enough value that everything will work fine. I just like to add in that these calculations are just approximate and are technically wrong so people don't get the wrong idea.

You are correct, I just didn't want to confuse anyone by explaining that..

The resistance of a fan will vary as the load on it changes, also.

Will update original post
 

enduro

Member
Joined
Mar 31, 2004
Location
Montgomery Alabama Yall
Awesome job. I was just about to ask this question about a week ago. And then I remembered my favorite saying. " the Search button is there for a reason " I vote for Sticky too. It's a question that many newbies will have, and even for those that have taken physics, after a couple of summers, you forget everything you've ever learned. I should have been able to do that in my head, but thanks for the review. I was always forgetting to divide 5V by 1.75A to get the needed R. Two thumbs.
 

Overload

Member
Joined
Dec 18, 2000
Maybe this will be a useful tool. (standard disclosures apply) It has not been tested so use at your own risk and let me know if there are any errors or improvements that could be added.
 

Attachments

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bwanaaa

Member
Joined
Sep 24, 2003
thank you for the clear guide. The theory is nicely explained but the practical results are a little disconcerting-a single rheostat to control 4 fans costs $20-30. i guess these are not hiigh volume items.