[eagle] Re: CAN-Do-Too! ??????????

Chuck Green greencl at mindspring.com
Tue Jul 10 09:02:34 PDT 2007


Hi Guys,

I've collected a few topics from various peoples comments.

The CAN-Do! was designed to be mounted to the front wall of the module 
box.  Since the connector could do this with the greatest simplicity, 
that's the way we went.  Part of this had to do with minimizing the 
module volume used and part with heat conduction for the module power 
switch transistor.  With the new mechanical configuration I have in 
mind, that will not be sufficient.  Mounting spacers to the module PCB 
will likely be used.  But this is all preliminary thinking and any 
specific suggestions would be appreciated.

It has been suggested that a DE-9 would be sufficient.  Technically, 
this is correct.  But it would require the elimination of five of the 
six user pins.  Maybe "require" is too strong a word.  Note that there 
are two CAN-H, two CAN-L, two power, and two ground (well, we ended up 
with three grounds).  Technically, only one of each is needed.  But two 
each were were provided to simplify the construction of the wiring 
harness (no small issue).  The six user pins (one of them is named EB) 
were for special needs a module might have.  Six may be overkill?  The 
naming of one "EB" is a legacy from P3D where the engineering beacon was 
so pervasive throughout the satellite that it was decided to actually 
name one of the user pins as such.  Part of this is also driven by 
P3E/P5A perceived requirements.  P3E has used some of the user pins to 
input structure thermistor signals to modules that do not otherwise use 
all of their analog input pins on the 40 pin connector.

The sub-miniature D connector series has served us well.  If anyone has 
*experience* with something they think might be a better choice, we'd 
love to hear about it.

The switching frequency of approximately 5 kHz observed is not the 
natural frequency of the existing supply.  It is likely the supply 
running in an idle mode since the actual power requirement of the widget 
is relatively small.  To verify this, Juan might try putting an 
additional load on it. This can be done at the 40 pin connector.  In any 
event, this points to the desirability for an experienced power supply 
designer re-designing the power supply.

It has been assumed, and enforced by the widget design, that a high 
power module would have a separate power input connector.  If switching 
such a module on/off by the IHU is desired, the widget switched power 
would be used as a control line to switch the module power.  The widget 
is capable of supplying several amps to the module but the current limit 
is specified at under an amp as a "normal" module can only dissipate a 
limited amount of heat anyway.  We also scaled the upper end of the 
current measuring circuit at the specified current limit.  And we added 
a two amp fuse.

Chuck


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