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I am using a 7.2 Ampere-hour Sealed Lead Acid Battery to power a 12V DC motor which is driving a fan, and I need to understand how to control the fan speed.

When I power a 12V DC LED strip directly from the battery there are no issues, so the battery works fine. But when I instead connect a DC motor or a DC Fan directly to the battery, the speed is way too fast and I am concerned about possibly damaging the motor.

The motor takes about 500-700mA current. I have seen in many answers that 500mA capacity would mean the device would draw only 500mA, even if higher amperage is available. Is this also true of DC motors?

Can someone guide me on how I can regulate the current supplied by the battery, to control the speed of the motor? Since I am just starting out, I want to understand how to implement both a fixed-speed control solution and a variable-speed control.

Would simply adding a resistor in series with the load reduce the supply current and thus control the fan speed? If so, how would I select the component and would there be any other issues? Or is there a better solution?

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  • \$\begingroup\$ DC motor control shuld be included issues like minimum to maximum speed adjustment (linear or logarithmic), constant torque (especially in low speeds), avoid motor stall, reverse rotation option. \$\endgroup\$ – GR Tech Apr 22 '15 at 4:31
  • \$\begingroup\$ Yes, adding a resistor will slow down the motor. But the resistor will have to dissipate power. I believe your typical DC motor is speed controlled using PWM of the power. In other words, the power is turned on and off very quickly with transistor switches. \$\endgroup\$ – mkeith Apr 22 '15 at 15:08
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Do a Google search for "12 Volt DC PWM Speed Control". There are many products that you can buy (eBay will sell you something for less than $10) and even more that you can build.

Is this something that you want to build yourself or are you just trying to solve this particular problem?

A 555 timer driving a N-channel MOSFET is pretty easy to build.

schematic

simulate this circuit – Schematic created using CircuitLab

You will want to use a CMOS 555 timer such as the TLC555.

Adjust the value of C1 for your desired PWM frequency.

Ratio of R1 to R3 sets the minimum & maximum PWM duty cycle. The values shown allow the PWM to be varied from about 1% to 99%.

I would set the PWM frequency to anywhere from 100 Hz to 500 Hz. If you go above that, change D3 to a faster diode such as a UF4002 or similar.

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  • \$\begingroup\$ Thank you, will try this out. but what about a fixed speed solution, would it be simpler ? In fact, i would be first checking on ebay if the PWM controller is available then will attempt building one. thanks for the help when i tried to find a DC DC adapter, couldnt find proper solutions to suit my requirement. \$\endgroup\$ – Rajesh V R Apr 22 '15 at 8:16

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