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help : motor DC chopper .

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AbdelAzim

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hello,
this is my first time here, i need help with my graduation project.

in a part of the project,i'm building a simple DC chopper , chopping 12VDC onto a 24VDC/200mA motor .

the chopping device (switch) i used is IRF540 MOSFET.

the gate control is a simple 555 50% duty cycle variable frequency astable multivibrator.

the motor is connected between +12V and the drain of the mosfet, and the MOSFET then connects/disconnects the motor to the ground ,depending on gate signal ,

the problem i'm having is : when the motor is connected to the circuit . the 555 astable frequency drops !

help , please !
 

The 555 timer frequency depends on the supply voltage as well as timing resistors and capacitor. The problem might be due to supply voltage drop when the motor draws current from it.
I suggest you use the popular TL494 PWM IC. It simple to use, gives much improved performance than the 555, and still cheap.
If you insist on the 555 timer, you may want to add a larage capacitor near its supply terminal.
 

by measuring , i found that the voltage does drop about 2 volts when loaded by the motor,so the reason you mentioned might be it.

i'm checking the TL494 datasheet , and see if it would be better to use it here.

thank you for your time and reply.
 

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