Soft Start circuit for 5V DC 1A motor

I am new here and to this topic. I want to create a simple Soft Start circuit for a small 5-6V DC motor, 1.2A rated load. The start up current is 3-5 times higher (3.6 - 6A). I plan to use a power MOSFET as a switch on low side to turn ON the motor. A logic high signal (3V) from a microcontroller connected to the Gate of MOSFET will turn it on. I read on Internet that I should put a series resistor R1 between the logic high output and Gate. Then, put a capacitor C1 from Gate pin to ground. The RC time constant will gradually increase the Gate voltage from 0V to 3V, which will fully turn ON the MOSFET. I want to know - what is suitable RC time constant for this size motor? 1 or 2 millisec, 5 ms, 10 ms or even longer - 20ms? I would appreciate help. Thanks. AB Singh

I don’t know much about what RC Time Constant would be optimal for that specific motor, however…

I just would like to point out that DigiKey has a handy Time Constant Calculator tool available Here.

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Please be aware that much of the information the internet offers on electronics topics is missing important context, over-simplified, or simply incorrect. A person does well to choose their sources carefully.

Attempting to soft-start a DC motor using a FET driven through an RC filter in the manner described sounds a lot like driving a screw with a hammer, in the sense that while it might “work” there are probably other approaches that are more advantageous. Understanding FET limitations including safe operating area and turn-on behaviors particularly would be important to making an informed device selection if one chose to take that road anyway.

One would choose that based on how “soft” one wanted the start to be. One could reason however, that one wouldn’t be “softening” much at all if the time constant weren’t made to be of a similar or longer duration than the start-up transient of the motor without it.

If a person’s speaking of using a microcontroller anyhow, it’s not a lot of extra work to use the FET in a switching (rather than linear) mode, creating a PWM waveform that ramps from 0 to 100% duty cycle over the desired soft-start period. Use of a diode in parallel to the driven load to limit voltage stresses on the switch would be recommended in such a case.

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Hello @absingh,

Yes, that may work, it’s a good thing to try to better understand the operation of MOSFETS. Note that it is difficult to drive a MOSFET with 3.3 VDC logic. May I recommend using this MOSFET as it has desirable gate drive requirements.

As for time constants, please use this tool:

Finally, if you use the PWM techniques as described by @rick_1976, you may want to switch to a Darlington pair as I described in this post:

The RC time constant idea uses the MOSFET in its linear range. This can cause the MOSFET to get very hot if the time constant is long. Also, it may or may not provide the desired soft start action. The PWM technique allows fine control.

Best wishes,

APDahlen

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Hello all tech experts. I have a lot of information sent to me, which I need to study, and then proceed further. I will let u all know when I make any progress.Thanks a lot.

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I do like the solution by rick_1976 about creating a PWM waveform using the microcontroller. I am planning to use TI’s MSPM0G3507 MCU. Can u please give more details how to create such waveform?
The only thing that comes to my mind is to write code - create a loop where I increment the ON time compared to OFF time on the output pin connected to the Gate of FET, thus increasing the duty cycle. But I am concerned that doing it with code may be too slow a process. Thanks.

Note: I am still reviewing all other info given to me by all experts.

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Hi absingh,

Sounds like you have the basic idea of what a PWM waveform looks like and how a basic MCU would create the waveform. However, to make it faster and more precise, you would do most of the work using the counter/timers of your MCU along with interrupts to increment or decrement your compare registers.

Here are some resources I have found which discuss MSP430 PWM applications. Note that these are not all for your specific part, but the concepts generally carry over.

Hello David_1528,

Thanks for ur help. I have downloaded the very 1st Tech Article by TI teaching LED dimming technique. This should be enough for me to understand usage of hardware timers in the MSPM0 MCU, instead of creating s/w timers in the code, which could be slow. The whole process will take me some time, as I have to search and select many other parts for this project. The most important is the 5.5V DC Motors - at least three. I have selected one manufacturer in Taiwan for motor so far.
I will come back to this forum as need arises. AB Singh

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