Motor Drive Blows Fuse
The following was in response to a dot matrix printer
blowing the power fuse whenever the paper advance motor was driven.
A 74LS273 was getting hot as well:
(From: Tony Duell (ard@p850ug1.demon.co.uk).)
I'm going to guess (based on what I've seen in
other printers) that there's a set of power transistors (either
H-bridge drivers or 1 per coil, depending on the motor) that drive
the stepper. These transistors are driven from the printer's microcontroller
via an output port - in this case the '273.
Now, if a TTL chips is getting very hot, then either
something is drawing too much current from it, or something is overvoltaging
one of the pins. A particularly unpleasant failure mode is when
a PNP power transistor, with the emitter tied to the V++ rail (the
20V + rail that supplies the motors) decides to short and apply
said voltage to the output of whatever device is driving it. If
you're unlucky, the next stage is that the output port device breaks
down as well, and the CPU data bus gets 20V or more on it. The result
is blown chips all over the printer. Please don't ask how I found
that out ;-)
My guess is that there's at least one shorted transistor
in the stepper motor drive circuit. If the system uses an H-bridge
driver (an equal number of NPN and PNP transistors) then if one
transistor shorts, its companion is connected across the power rails.
When it turns on, the supply is effectively shorted.
I think you'll have to trace out the driver circuit
for the stepper motor. Figure out what drives what. Test the transistors,
and then replace the defective ones _and_ that '273, which is probably
now blown.
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