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I have a 4v motor with no current rating. I want to drive it tusing a 12V 7amph battery. I made a voltage regulator circuit using lm317 ic but it gets very hot within seconds of operation.
Plz help, its urgent.
What type of LM317 pakage u used here.Since LM317 is a linear regulator If you use TO220 package means then the thermal resistance is 50C/W.The voltage drop of the regulator is 8V and if we assume the drive current of the motor is 100mA then the power dissipation is 0.8W.So temperature rise in regulator is 90'C but the maximum supporting junction temperature is 125C.So up to LM317 will support up to 0.2A driving current.By using the multimeter you just measure the current by putting in between LM317 O/P and motor I/P.If the meter reads more than 200mA better replace TO220 package to TO3 package.
This will solve your problem.
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It would be much better to not use a linear regulator at all and simply use PWM modulation to limit the current to the motor. Regulating from 12 down to 4 volts is throwing away over 70% of the power that's coming from the battery.
I am a noob to this field so if you could give me the circuit diagram or a reference, because I can make a astable multi vibrator using a NE555 ic but don't if I could use the same for driving a DC motor.
You could use a multivibrator operating at about 1KHz with a 4/12 = 33% duty-cycle (waveform high 1/3 of the time) driving a power N-MOSFET. Connect the +12V to one motor terminal and the other terminal to the transistor drain. Connect the gate to the multivibrator output and the source to ground (battery minus).
You also need a power diode across the motor terminals (cathode to plus) to protect the transistor and provide a path for the motor current when the transistor is off.