PHOTO 3
law of physics that material’s resistance increases with an
increase in temperature. Due to the I2R power dissipation
on these FETs when current is flowing, temperature will
increase. This is why you will see current flowing at 4.8A
first, but quickly settling at something smaller.
That being said, it is imperative to realize that these H-bridges are rated to a maximum of 2.8A, but are not meant to
run at that current level for an undetermined amount of time.
You should expect to see thermal shutdown trigger eventually.
The solution is to PWM the H-bridge at some duty cycle less than
100%. This will allow larger peaks to be delivered to the load.
In theory, we can add more H-bridges in parallel and
increase the current even more, but although possible, I would
rather use a different venue. Still, I am glad the A3950
stood up to the parallel test, which means my latest robot
will soon be strolling down my kitchen and scaring the cat.
Future Revisions
After designing this little controller, I came to a series
of realizations on how it could be improved. First, the size is
about right, but there is certainly lots of room to make it even
smaller. Currently, I am working on a release which will
compress the board to 1” by 0.8” — perfect for tiny robots!
The solder pads are perfect for soldering battery and
motor wires. However, when parallelizing controllers, this is
not necessarily the best option. I am working on a way for
the boards to be assembled together into a larger unit.
Getting more current should be a clean solution, not a wire
mess which complicates things.
Conclusion
The amount of DC motor controllers on the market is
staggering. I have found the A3950 to be so versatile and
economical, I believe it should be the solution for a
great deal of small to medium sized DC motor control
applications. The device could not be easier to use. With
very little effort, you should be polarizing DC motors at all
speeds and both directions. Thanks to its increased current
capability and small size, it is very easy to group these
devices together and get more power. I definitely will be
using these little devices on most of my future DC motor
projects! SV
PHOTO 4
SERVO 04.2009
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