#### greenman100

##### Disabled

- Joined
- May 18, 2003

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.

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?

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.

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.

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

7V=Imin*Rfans

Plug in Rfans, solve for Imin.

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

5V=Imin*Rrheostat

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

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.

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.

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.

**V**oltage=**I**current***R**esistanceSo, 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 ohmsRfans=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=.175AImin=.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=28.6ohmsRrheostat=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=.875wWrheostat=.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.

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