On 03/25/2013 06:42 AM, Michael A. Terrell wrote:

Scott Dorsey wrote:

Michael A. Terrell wrote:
tm wrote:


Are you sure it is not already a pulse width regulator? Those transistors
look like they are TO-220 packages, not TO-3.

I've seen 20W power resistors in TO-220 packages.

It does indeed look like a TO-220 from the pin spacing (since there are no
actual transistors in the photos, just spots from which they were removed).


All those photos only show one side. I full reverse engineering
should be done to draw a full schematic but I've never had my hands on
that module. It would probably take a couple of them, because ot the
potting.

if it's just a two-layer board, maybe. assuming you get the specs on
the chips of course. but you'll need more than two units and a whole
lot of patience trying to reverse the schematic if it's 4 or more
layers. and you still don't achieve anything more than having a broken
light bulb in your hand.

what you need to do is get the operational capacities of the /working/
unit, and work with those. that the unit is a black box is completely
irrelevant.




But if it had been a PWM device, there would have been some filtering in
there, inductors and capacitors to keep the noise from getting into the
power lines. Designing clean and quiet PWM controllers is not quite as
trivial as some folks have made it out to be.

Have you looked at the National Semiconductor (Now part of T.I)
'Simple Switcher' series of controllers? Generally only one inductor
and a couple small electrolytics. A lot simpler than older designs, and
little noise because of the small footprint.

/and/ a smaller footprint. the "because" is entirely due to the soft
switching they've achieved. soft switching is still "big science" -
there are still phd's being written on how to implement and design, both
with the types of semiconductor, and the circuits in which they're used.




PWM has been around longer than SMT parts.

Maybe it is transients from the motor that are causing the failures.

This is possible, if it is the transistors that are failing. I don't see
any big protection diodes in there either.

If it's a RoHS soldering issue, though, I would not be surprised.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."




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fact check required