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Help me understand this - motor/amps

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nuggetz

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I'm learning electronics and built a simple circuit on a breadboard. I have a few questions that I'd like answered as I know those answers will teach me alot.

Anyway, I wired a 12volt power source thru a resistor, and to a hobby motor. Circuit is controlled by a simple on/off switch. I hit the switch, motor spins like crazy. Good.

I'm aware of OHM's law and such but I'm trying to determine what the important things are with regard to this circuit. First off, would it be correct to use a resistor to limit the current going to the motor? With the resistor in series, the circuit worked and motor spun, but I think the resistor was getting a little hot. Now, if I hook up a multimeter in series with this circuit and measure the amps, that will tell me what the motor is using right?

My main questions are about the powersupply that I have driving this thing. It's rated at 12V DC @ 500mA. What does that mean exactly? If the motor starts drawing more that 500mA, will that fry the powersupply? How do I determine what the motor actually needs and then figure out what wattage the resistor should be rated at? I'm not trying to build anything specific here or long term. I'm trying to learn electronics and have started to construct simple things with mainly resistors, led's, etc. Not caps, or any other components yet until I fully understand the above. Can anyone comment on this setup and tell me the first thing that comes to mind?
 
First of all, the resistor in series with a motor to control the current is not a good idea. At least it should be a very small motor or an effect resistor, but still not preferable. In order to control the current through the motor you should use a transistor/MOSFET. Are you trying to control the speed or the torque?

You should measure the motor current by directly supply the motor from the power supply, BUT BE AWARE that the small power supply current rating may be way to small. A motor, depending the size of course, will draw as much as 40 times the nominal current at start-up for a short time. The power supply must therefore be fitted with a slow fuse to match the inrush current.

Your power supply can supply 500 mA up to 12 VDC. If you draw more, the voltage will drop and if you draw a very higher current, the fuse will blow (if it's not just a cheap wall transformer)
 

nuggetz

New Member
Thanks for the reply. It is a cheap wall wart that I just cut the ends off of. I'm not trying to control torque nor speed at this point. I'm just learning about electronics. Thats the main goal. Since I haven't quite learned enough about other components other than resistors at this point, that's all I'm playing with.

In terms of the motor, it came out of a radioshack xmod RC car. Nothing too top end. I guess I was trying to understand the relationship between the motor and the wall wart. Motor appears to have a few caps on it already. The powersource says 9VDC output, 300mA but I measure more like 12V coming out of it.

In terms of current, I guess I need to do some more reading on why a motor would draw that much current and explore more of the technical aspects of a motor behaving in this way. Anyone know of a quick write-up somewhere that describes what happens when you hook up a motor like I'm doing? Why is a resistor not a good idea? I guess I should read up mosfets and transistors as I dont yet know what they do. I'm reading everything I can and started with the very basics. I wanted to understand the flow of electrons.This circuit was something I decided to put together real quick to note the effects but I'm not planning on taking it any furthur wihout understanding things a bit more.
 
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crutschow

Well-Known Member
Most Helpful Member
The amount of current a motor uses varies greatly with load. You can indeed measure that with an ammeter in series with the motor. If you apply a load to the motor the current will increase. If you stall the motor you will see the startup current, which as noted, will be much higher than the running current.
 

smanches

New Member
You need to match the voltage of the supply with the voltage of the motor. For current, you need to make sure you can supply at least as much as the stall current of the motor, which is much higher than the running current of the motor. If you can supply more current than the stall rating, that's fine, the motor will just use what it needs.

To determine the stall current, hold the rotor so it does not turn, then supply power to it and measure the current. Do not keep it this way for long, as it very likely will burn out the motor eventually.

It's all Ohms law in the end.
 

audioguru

Well-Known Member
Most Helpful Member
The 9V/300mA wall-wart does not have voltage regulation. Its voltage is 12V or more without a load or with a small current, its voltage is 9V when the load current is 300ma and the voltage is 6V when the load current is 600mA.
The voltage from the cheap power supply will change a lot when the motor's work is changed.
 
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