I dont know how the resister works...

Discussion in 'The Projects Forum' started by Dudeliliy, Jan 26, 2009.

1. Dudeliliy Thread Starter New Member

Jan 26, 2009
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Ok. I have a 12v 55amp battery. all I want to do is send 12v and 15amp max to a motor and all the rest to something else. how would i find out what kind of resistor to use.

2. thingmaker3 Retired Moderator

May 16, 2005
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Electricity doesn't quite work that way...

I suggest using a 15 Amp fuse or circuit breaker in series with your motor.

You can use the tabs at the top of this page to access the All About Circuits textbook. It is an excellent introduction to how electricity does work. You can learn about why the motor will draw a given current based on the impedance of the motor and the applied voltage.

3. mik3 Senior Member

Feb 4, 2008
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Batteries maintain a constant voltage (actually is not constant but varies slightly with current and other factors) on their terminals but not a constant current. The current out of a battery depends on the load and not on the battery itself. It is the load which determines how much current it needs. In the case of a motor, the current will be much higher when you start it and then it will decrease. As you increase the load on the motor's shaft the current drawn from the battery increases.

4. Dudeliliy Thread Starter New Member

Jan 26, 2009
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so then how would i find out how much volts/amps to use? im not going to have a controler the motor will just run at the same speed the whole time.

5. mik3 Senior Member

Feb 4, 2008
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What is the power of the motor? Or
What is the current rating noted on the plate on its side?

6. Dudeliliy Thread Starter New Member

Jan 26, 2009
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well i dont have one yet. i am looking for one for low volts and amperes and as much power as i can. lets just say that it was a 12 volt and 5 amps.

7. Wendy Moderator

Mar 24, 2008
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2,540
Generally motors aren't quite rated like that. If you force a motor to stall it will suck huge amounts of current, until something burns out. The current a motor pulls is directly proportional to the work they are doing. Motors pull the current they have to to do the job, it is a self regulating system.

What you are asking, I think, is how to regulate the RPM on the motor. Correct?

8. mik3 Senior Member

Feb 4, 2008
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If you have a clamp meter set up the motor with the load it will drive and measure the current it draws. Don't attempt to measure the current with a multimeter because if the current is more than 20A then the multimeter is gone.

9. Dudeliliy Thread Starter New Member

Jan 26, 2009
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ok... the motor will use what ever amps it want to with the load that is givin. so if the motor has a max of say 15 amp. does that mean thats the most amp that it can take befor it will burn? if so then i will need to let no more then 15 amps through so that i wont take any more then that right?

10. Wendy Moderator

Mar 24, 2008
20,772
2,540
That's why you need either a fuse or a breaker in the circuit. I remember some of the combat robotic types with larger machines (over 100 pounds) talking about surges of 500 amps on their machines. This was stall current, as the robot accelerated from stand still to full speed, on low voltage (36V max) systems.

11. thingmaker3 Retired Moderator

May 16, 2005
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It works the other way around... The motor from your example draws 15 Amps while running at rated mechanical load. It will draw three times that, or more, when starting up. (It will only draw this for a brief moment, while coming up to speed.) If the motor becomes mechanically overloaded, it will draw more than 15 Amps - this is when you want to limit current to it! A circuit breaker won't trip quickly enough to prevent the motor from starting. A circuit breaker will trip if the motor draws too much current for too long a time.

An example would be a saw motor when the saw binds. The saw motor draws more current while fighting against the binding. If the current is not limited somehow, the motor windings will heat up and the winding insulation will begin to burn off.