Help me vary the speed of a DC 500mA motor

will it work ??

Guys i want to vary the speed of a DC 500mA motor, now tell me am i right ??
am i doing in the right way if not then tell me what the proper way to do so please . . .

Re: will it work ??

You need a driver for a brushless DC motor. Some of them has got integrated driver, which needs an analog input for speed adjustment.

If you want to design your own BLDC motor driver (which is not an easy task), the speed control is done by using PWM. PWM is also used for brushed DC motors.

Re: will it work ??

basically its a 12V FAN, Brushless Driver is inside the body,
just tell me is it a right way to vary the speed of the FAN ???
will it work??
what r the pros & cons of using this type of speed control circuit ??

anyone please help me

will it work ??

What are you trying to do with this circuit.
This is a switch which works in saturation or cut off.
The moment the base of the transistor is biased and will goto saturation and your motor will run, other wise its in cutoff mode.
I dont think you can achieve a speed control with this.

You could vary the 12V supply and do the speed control.

Re: will it work ??

MY aim is to Vary the speed of the motor,
i have a fixed 12v 3A supply & a 50mA 6V variable voltage source,
i 'm not allowed to use other things with it
sir i'm applying variable voltage at the base of the transistor via 10K resistor,
will it work or not ??
if not then what should i do to make it work ???

Re: will it work ??

Hi FMradio,

You may use the ckt given below. Don't forget to add a capacitor at pin5 and use a transistor of required rating instead of 2N2222.

Re: will it work ??

sir i'm not allowed to use PWM signal to vary the motor speed,
i want any analog signal

Re: will it work ??

Hi FMradio,

Then it comes down to: 'You could vary the 12V supply and do the speed control'
as said by cameo_2007.

Re: will it work ??

FMradio said:

sir i'm not allowed to use PWM signal to vary the motor speed,
i want any analog signal

Click to expand...

Use LM317 and vary the volatge which is fed to the motor. One 3 terminal regulator will do it all..Rerfer the datasheet for more details.Cheers

Re: will it work ??

FMradio,
The circuit will work. However there are some severe practical problems. By varying the input voltage source, you are controlling the base current. The collector (motor) current will depend on the hFE of the transistor. The hFE is a function of temperature; it increases with increasing temperature. The junction temperature will rise, resulting in an increased hFE and a consequent increase in collector current for a given base current. Lets assume you are not using a heatsink. The data sheet that I found for the D313 does not give the junction - to ambient thermal resistance. However, a typical value is around 60 degC/watt for a TO220 package. Worst case power dissipation will occur at 1/2 the supply voltage (6V). therefore the power transistor dissipation would be 3W. This results in a temperature rise of 60 X 3 = 180 Deg C, which is unnacceptable, so you must use a heatsink. You must calculate the temperature rise using the junction to case thermal resistance (typically around 2 degC/watt), and the case to ambinet thermal resistance of the heat sink. After you have done that, you can calculate the junction temperature rise, and determine if the variation of motor current vs temperature (time) is acceptable. What will happen is that you will have to constantly vary the input current to compensate for changes in junction temperature with time. As a result, this is not a practical circuit.
Regards,
Kral

Re: will it work ??

Good analysis. Appreciate it.

will it work ??

Hello,

You can make the control somewhat more predictable by using the "current mirror" principle, with some emitter resistance. You lose about 0.5...1V (because of the emitter resistors).

In addition to a heavy transistor with heatsink, you need two emitter resistors and another NPN smalls signal transistor.

will it work ??

The maximum base current is less than a mA.
For a collector current the hfe required exceeds 500.
Data sheet shows for the transistor in question this value is less than 250.
If the series resistance to base could be manipulated this would allow higher currents desired and perhaps allow for speed control as well. A much lower value or variable could be considered!