greenman100
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- 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.
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.
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.
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