Please see below,


----- Original Message -----
From: "Ian Wilson" <[EMAIL PROTECTED]>
To: "Protel EDA Forum" <[EMAIL PROTECTED]>
Sent: Wednesday, September 03, 2003 3:49 PM
Subject: Re: [PEDA] Joining 2 different nets keeping seperate identifiers?

> On 07:40 AM 4/09/2003, JaMi Smith said:
> >Joe,
> >
> >Who says it has to be a "3mm thickness" (I think you may actually mean
> >the trace has to be 3 mm wide)?
> >
> >Is this requirement imposed on you by an Engineer, or is it some
> >found in a datasheet for some specific circuit or device?
> I would expect that the spec is wide uncalibrated current carrying traces
> with narrow current *sense* traces running off to the sense
> amplifier.  This is a pretty standard sort of interface in high or
> precision voltage or current applications.  You take a pair of sense
> to the load or sense resistor rather than using the uncalibrated high
> current traces for both current carrying and sensing.  The 3mm width
> requirement would come from the expected current, while the thinner traces
> are used for the actual sensing. Very standard stuff.
> (Many bench-top power supplies will have sense terminals that allow you to
> control the voltage at the load rather than at the supply terminals, there
> is usually weak feedback in the supply to ensure that the supply is
> controlled if the sense terminals are unconnected.)
> The requirement is perfectly reasonable to me and I would either solve it
> by setting up from-tos and appropriate rules or by the Lomax Virtual
> short.  In DXP it could be solved by a net tie component.
> Ian

The initial question is not because I want or need to know why there is a
requirement for a 3 mm trace width, but so that Joe can understand what the
problem is, and just what the requirement may apply to.

While I wouldn't necessarily state what you did in your first paragraph the
way that you stated it, it does get the point across, and is not different
than mine (although stated somewhat backwards as compared to what I stated).
If you will carefully re-read my post, and also read the follow-up post with
what I forgot to put in the original, you will see that I am trying to
explain just where the high current is and where it goes, and just exactly
what the current sense resistor does, and what the requirements may be for a
"feedback" trace from the "load" side of that current sense resistor, and
what the 3 mm requirement may apply to.

If you will also carefully re-read the original post from Joe, you will see
that this requirement has nothing to do with remote sensing, such as in your
example in your second paragraph of the power supply, but that he is
directly taking a trace from the "load" side of the current sense resistor,
and feeding it directly back into the [current sense] amplifier input.

I am sorry for my omission in my original post, but hopefully with the
supplement from my follow-up post, it will all become clear.

Yes, my original post is a bit confusing, but I think that you will see that
we are saying the pretty much same thing, with respect to your first
paragraph above, and that your second paragraph really does not apply to
this instance.

Respecting your last paragraph, I am fairly sure that when you get my
follow-up post and think the whole thing thru together with the first post,
that you would concurr with me in what I stated in my follow-up post, that
the present case that Joe is describing, that the "feedback" trace is in
fact the same "net" as the "net" connected to the "load" side of the current
sense resistor (which in fact is just exactly what you yourself describe in
your first paragraph above), and that it really should not have a different
"net" name, and that the different net name is the real problem here (but
which would not be the case in the example of your second paragraph). That
said, I think that you would additionally concurr with my stating that
anything that would introduce any "loss" or "drop" in the "feedback" trace
should be avoided (such as the loss that would almost certainly be
introduced by unnecessarily using 2 differnt net names and trying to join it
all together with a "Lomax Virtual Short" (with its intentional gap which
can allow for some small and uncontrolled amount of etching of the trace at
the point of the gap), or by using too narrow a trace which could cause too
much restivity in the trace). I would also think that whatever DXP may or
may not do here does nothing but confuse the issue (especially if there is
no reason to have 2 different nets in the first place).

Sorry for any confusion, but I think you may be trying to say the same thing
as I did with respect to the present situation, and that the case for the
isolated remote sense (as you described in your second paragraph) does not
in fact apply to this situation.

Sorry again for any confusion.


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