Problem WIth New Power Monitor

["Ira Schneider" <ira_schneider@iname.com>]

Hi all,

I built a new power monitor for the TTAT layout.  However, there is a major problem
for which I could use some advice.

The new power monitor contains ammeters as well as voltmeters.  There is
one pair of meters for each track.  I have a single 5 VDC power supply feeding
power to all of the meters.

The problem is the internal design of the meters.  The ammeters (and, as I found
out, also the voltmeters) have the ground connection from the power supply
directly tied to one of the AC input pins.  When the power monitor only had
voltmeters, this wasn't a problem, since there is a common lead to all of the
tracks, which (by accident) I connected to the input pin which was connected
to the ground wire from the power supply.  The net result was that all of the
voltmeters, being fed from the same 5 volt supply, connected together the
common leg of the tracks, which was no problem.

However, the ammeters are a different story.  The ammeter is inserted
in series with the "hot" lead going from the transformer to the track power
controller.  Since one side of the ammeter is connected to the ground pin
from the power supply, that means that either the transformers are all
connected together or the inputs to the TPCs are all connected together
(depending on which way I wire the ammeters).
They are also connected to the common output line from the TPCs (through
the voltmeters).

This is obviously a disaster.  When I first connected the voltmeters and ammeters
to the power supply, I noticed that the voltmeters were rather dim.  I thought
the ammeters were overloading the power supply, but they weren't.  (The
combined current drawn by all of the meters is around 200 ma, and the power
supply is rated at 1800 ma.)  While I was checking voltages, etc. I saw smoke
coming from the power monitor.  I eventually found the common wire feeding
the voltmeters had burned out.  That is when I started investigating the connections
of the meters.  I think this also burned out two of the voltmeters.

So far, the only solution I see, which I think is impractical, is to have five different
5 VDC power supplies - one for the voltmeters and one for each of the ammeters.

What I really need is some way to isolate the AC input signals to the meters
from the DC power supply which powers the meters, but I don't have any
idea how to do that.

Does anyone have any expertise in designing a simple circuit to isolate a DC
power supply from the load?  Or some way to isolate the AC being measured
by the ammeter from the DC power supply?

If I need to use five separate power supplies, I will have to change the cabling to
the power monitor (there is only one spare pair of wires in the cable) and
find some place for another power strip.  If I can isolate the power supply
within the power monitor, I might be able to find room for some small circuits
inside the case.  Also, the small 6 VDC (or 4.5 VDC) power supplies which
Radio Shack has (which actually fit on a standard power strip) are only
available through their stores, and no local store has either power supply
in stock.  (I am actually powering the monitor with the 6 VDC version,
and I have one of the 4.5 VDC power supplies here.  The voltmeters
would not function on 4.5 volts, but the ammeters do.)

I will be happy for any suggestions.

Thanks.




Ira