Subject:
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Re: motor port problem .
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Fri, 20 Nov 1998 14:55:20 GMT
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Original-From:
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FThompson9@aol*spamcake*.com
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Viewed:
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1338 times
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Philip
In a message dated 98-11-19 21:05:04 EST, you write:
<< hi,
I have a big problem. I followed the method of yours that just lift up pin
8 of one L293
and wirewrapped it. The positive supply is wirewrapped to pin 8 . How about
the negative supply of the batteries should insert or wirewrapped? I
inserted the negative supply in the bottom row of digital input as ground
together. >>
The negative supply attaches to ground and the pins on the digital I/O port
are fine, looks like nice thick traces to me.
<< Suddenly. after a short while, the L293 turn so hot. It burnt my hand.
Besides, the socket on the HD mounted with pin 4,5,12,13 of L293 has melted
in that particular area. Simply it is a plastic part. What is really
happens ? >>
[[Attached from the previous mail, I have already solved the problem listed
below. It is the 7404 is damaged .What causes it? Any ideas? ]]
I don't know why your 7404 went bad, but I can see why that might cause
trouble for the L293. I also don't have spec sheet on the 293. Anybody know
what happens to a 293 if both inputs are turned on at the same time? (Just
turns on both outputs so no motor motion, right?)
Sounds like you are drawing too much current. The 293 chips can be piggy
backed to switch more current. The pins you mention are the ground pins for
the chip. Most motor driver chips use the middle pins for grounding and LOW
LEVEL cooling. Inside the plastic, the silicon is mounted on a wide piece of
metal, the same width as these pins. This metal strip is cut to the shape of
the pins outside the plastic body. The idea is if this is soldered to a big
fat ground plane on a printed circuit board, it can disapate some heat.
Probably ehough heat to keep the part from distroying itself at the rated
current. As socket does not conduct heat like a solder joint, and even if it
did the Handy Board does not have a ground plane to spread the heat out. In
the near term I strongly suggest that you glue some metal to the top of these
chips. I've seen penneys used, slugs from electrical socket boxes. If you
use glue, you might think of getting some heat sink compound in between the
metal and the top of the chip. They make special heat sinks that clip around
chips of this size you might look for.
<<Should I also wirewrapped the negative supply to one of the grounding point
pin of L293 ?By the way, If I cut the trace on HD, is it only cut the
trace to the positive supply only . How about the negative supply trace ?
should I cut? BEsides, If I cut, it there any way to mend it back to
originally ?>>
On the component side to the left of the word START is a wide trace.
This is the trace that elads from the switch (batter), to the motor supply
pins. Cut this trace and plug the other motor supply at the + and - connector
(which is above and to the right of the STOP swtich). If you cut exactly in
the middle of exposed trace with a small gap (any gap is enough), you could
repair the trace by scraping the green solder mask from the remaining trace
and bridging the gap with solder and a little wire. See page 56 of the
Handy Board Technical REference to see this trace easily. If you are really
thinking of repairing this modification, I would really just consider using
the lifted pin. It is much simpler to impliment and recover from. I would
not go back and forth too often with either method. Folding the pin up and
down will make it fall off, heating the trace too often will cause it to
delaminate (come off the board).
Hope this helps
Pherd
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