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PWM voltage dropped


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Hi,

My recent project is to design motor controller using PWM. I use AT89S52 and L293 H-Bridge as the motor driver. The DC motor is 12 V (DC motor with gear box). If I connect to DC motor, the voltage with 100% PWM dropped to 9.5 V. If I don't connect it to DC motor, the voltage is still 12 V.

Is it normal? Anybody can explain why? Do you have any suggestion on how to fix it?

Do you also know how to calculate the voltage of DC motor with PWM duty cycle 50%, 75%, 100%? For example, duty cycle 50% of VIN=12V is 6V, is this correct?

post-14947-14279142572496_thumb.gif

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