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Author Topic: Will my Motor Driver Design 'splode?  (Read 432 times)

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Offline BigKLaxerTopic starter

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Will my Motor Driver Design 'splode?
« on: February 13, 2012, 08:58:10 AM »
So I've been using two relay H-Bridge motor controllers I built for my 4WD Experimental Robotics Platform with one controller per side, but the motors are very unhappy at being instantaneously turned on and off, so I'm attempting to design a PWM H Bridge I can use for softer starts and stops.  This is my first attempt at designing a PCB so I'm looking for constructive criticism about how I can make this design better.  The pics of the schematic and board are below.  Let me know what you guys think!  Thanks!
Experimental Mobile Robotics Platform: http://www.udrobot.blogspot.com

Offline Soeren

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Re: Will my Motor Driver Design 'splode?
« Reply #1 on: February 13, 2012, 12:06:31 PM »
Hi,

So I've been using two relay H-Bridge motor controllers I built for my 4WD Experimental Robotics Platform with one controller per side, but the motors are very unhappy at being instantaneously turned on and off,
Snubbers over the contacts (and a delay when changing direction) would help.


so I'm attempting to design a PWM H Bridge I can use for softer starts and stops. 
Unless you use PWM, it won't change a thing.
You could use PWM with a single MOSFET and a relay to achieve this as well.


This is my first attempt at designing a PCB so I'm looking for constructive criticism about how I can make this design better.  The pics of the schematic and board are below.  Let me know what you guys think!  Thanks!
The PCB would indeed "explode" the very first time you energized it  :)
You need wide, really wide, traces for the motor current and it's almost always a good idea to keep traces as wide as space allow.

If you post the .sch and .brd files I can clean it up for you, unless you prefer to do it yourself :)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline BigKLaxerTopic starter

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Re: Will my Motor Driver Design 'splode?
« Reply #2 on: February 13, 2012, 12:53:24 PM »
Quote
Snubbers over the contacts (and a delay when changing direction) would help.
I'd rather just upgrade to MOSFETs entirely since relays have a shorter life and take more energy.

Quote
If you post the .sch and .brd files I can clean it up for you, unless you prefer to do it yourself
I learn better by example  ;D but if you could, can you inform on the process so I know for next time?

Experimental Mobile Robotics Platform: http://www.udrobot.blogspot.com

Offline Soeren

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Re: Will my Motor Driver Design 'splode?
« Reply #3 on: February 13, 2012, 05:10:48 PM »
Hi,

I learn better by example  ;D but if you could, can you inform on the process so I know for next time?
Seems I promised more than I could keep. Tried under Eagle 5.4, 5.7, 5.11 and 6.1, but they all break (close completely) as soon as i try to route the traces :(

Things to note, apart from the trace width (plenty of trace width calculators on the web, to find the minimum required width for a given current), is that you need basis resistors for Q3 qnd Q4. You need fast (reverse biased) diodes over each MOSFET and you need a large buffer cap on the motor supply.
If you want to use SMD caps over the motor terminals and the logics supply, I'd recommend a larger size, as it's too easy to short the small ones (due to the small pad distance).
The cap on the motor terminals will have much more effect if mounted on the terminals of the motor.

Unless you prefer a double sided PCB, either use through hole components or change the libraries to place the SMD's on the bottom layer.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

 


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