I've got a 12V DC motor with:

  • Output power: 3.4 watts
  • Rated speed : 170 RPM
  • Rated current: 0.9A

Currently I have a variable voltage DC power supply which can vary the voltage from the DC power supply from 0 to 30V and an LM7812 voltage regulator.

Can I slow down the motor speed by decreasing the voltage from the DC power supply? I am still unsure what speed is suitable for my motor to rotate in my final year project.

Beside that, there is a lot of things that confuse me and I need some justification.

  1. Some people say if I use the 12V L7912CV voltage regulator in my circuit, the current that passes through the voltage regulator is not high enough to activate my DC motor. This is the link of the data sheet L7912CV.

  2. Some people say if I use a resistor to control my motor speed, the resistor will easily get burned up.

  3. The goal of the DC motor is to rotate a disc which is in direct contact with a flat surface so there is friction between them. So in an ideal case, if I buy a 12V power supply and directly connect to the motor, due to the friction and the load, the rated rotating speed will decrease and is suitable for my project. So in my circuit board do I need to buy any other stuff beside a 12V power supply?

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    \$\begingroup\$ I would feel pretty bad for any 7812 with 30V input that's having more than a couple of mA drawn from it... \$\endgroup\$ Commented Apr 3, 2014 at 4:19
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    \$\begingroup\$ The L7912CV is a negative voltage regulator - you would use it if you wanted a -12 volt supply. It is not a possible replacement for the 7812 positive regulator. \$\endgroup\$ Commented Apr 3, 2014 at 6:05
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    \$\begingroup\$ I'm voting to close this question as off-topic because it was abandoned over five years ago \$\endgroup\$ Commented Oct 9, 2019 at 6:12

2 Answers 2


(source: robotroom.com)

Here is a simple example of how to drive DC motor with Transistor and PWN. The diode is to protect the circuit while the motor is free spinning and to stop fail gating the transistor. The resistor is protection for the pulse generator i.e. micro controller or 555 chip or the many other ways to make PWN signal.

  • \$\begingroup\$ The idea of using PWM in this is right, but implementation is all wrong. Don't use a bipolar transistor, use an N channel MOSFET. \$\endgroup\$ Commented Oct 9, 2019 at 6:07

The point of the voltage regulator is to regulate voltage. If you drive it with over 12V, the output on the regulator is just going to be 12V. Plus, it's going to get very hot supplying 1A. All the regulator will do in this case is act as a limiter, and also a fire hazard.

Why do you need the regulator at all? If this is just for testing, you can just connect the motor directly to the 0-30V supply. Just don't increase it above 12V if that's the maximum motor voltage.

FYI - most small motor applications use a transistor (one-way) or h-bridge (bidirectional) controlled by a PWM channel on a microcontroller or a dedicated PWM controller IC. If you need variable speed control in your final project, this is how to do it.

  • \$\begingroup\$ Thanks for your advised.I put a voltage regulator is just to ensure if i mistakenly turn the knob more than 12v the motor is still safe. For my project i actually just need one constant speed for my motor during the operation but the actual rotating speed that suitable for my project i still cant determined yet since there is a friction and load applied on it.If without voltage regulator, let say if i reduce the voltage supply by just turning the power supply from 12v to 1v to determine the speed of the motor needed for my project,can i just directly connect the power supply to the motor ? \$\endgroup\$
    – user39706
    Commented Apr 3, 2014 at 6:14
  • \$\begingroup\$ I am still of the opinion that a regulator will be more harm than help in this situation. Just don't turn the dial on the PSU past 12V. Most benchtop supplies also have a currently limiter - enable that and set it to above your maximum motor current. It will reduce or switch off the voltage if the current goes too high. Now, you mention speed control - how critical is accurate speed control to your application? \$\endgroup\$ Commented Apr 3, 2014 at 6:30
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    \$\begingroup\$ Need speed feedback and a closed loop PID control to deal with changes in load and friction while still keeping the desired rotation speed \$\endgroup\$
    – KyranF
    Commented Apr 3, 2014 at 7:41
  • \$\begingroup\$ Most (decent) motors will tolerate some over-voltage as long as you keep within certain bounds - we worked on a product driving 12v motors at 24v to get the thing moving quickly, so a brief hit of over-voltage, and it never caused problems. Ironically, the motors died when people left them running very slowly (low voltage) because the stators glazed over, we ended up sticking in a daily routine that span the motors full-speed a couple of times to clear it! Running your motor above its rated voltage/current constantly will damage it, but otherwise you should be fine. \$\endgroup\$
    – John U
    Commented Apr 3, 2014 at 7:55
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    \$\begingroup\$ @user39706: as much as I love electronic solutions to problems, sometimes they're overkill and a simple mechanical solution will do the job. In this case, why not just gear down the motor? If the torque is not too high you can do it with two differently-sized spindles and a rubber belt. \$\endgroup\$ Commented Apr 3, 2014 at 8:23

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