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DC motor speed control (lemmy.ca)

submitted 3 months ago by [email protected] to c/[email protected]

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I'm trying to achieve variable speed control on two brushed DC motors powered by a 3s or 4s LiPo battery (~12V or 15V). This is for a nerf blaster I'm modifying, which is why I'm not using a pre-made speed control ie I want control over the shape/layout. I'd like to vary projectile speed with a thumb knob.

I just finished watching ElectricMonkeyBrain's YouTube video on the TL494 PWM chip.

I was initially planning to vary the duty cycle with a potentiometer on the chip's control pin, to get a PWM signal and feed that into a MOSFET. But in the video he mentions that the chip has an integrated over current protection function. Ie the chip will

monitor the voltage across a sense resistor in series with the load

and will

kill the output if the sensed voltage/current goes above a reference voltage

It occured to me that I could actually adjust the reference voltage as a way to control the motor speed.

Would this be a better way to achieve speed control and protect my motors/battery? Or is it a terrible idea altogether.

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[–] [email protected] 3 points 3 months ago (1 children)

From a cursory read of the datasheet, using the "dead time control" pin seems to be the way to go. Basically, this pin is used to set the voltage, while the error amplifier inputs (that's the closest function to "over current protection" this chip has) are used to adjust the output according to the load. For your application, you probably don't need to use them at all.

My instinct would be to disable the error amplifiers by connecting pins 1,2,15 and 16 to GND. You can then connect the wiper pin of the potentiometer to the deadtime control input, with the other pins of the potentiometer connected to GND and 3.3 V.

I haven't worked with this chip before, so take this with a grain of salt. You should probably use a simulation tool to check the circuit before you start destroying chips.

[–] [email protected] 1 points 3 months ago

Thanks. Yeah you're recommending what I was initially thinking.

You should probably use a simulation tool to check the circuit before you start destroying chips.

Fair!