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DigitraxPower.shtml
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DigitraxPower.shtml
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<title>JMRI Hardware Support - Measurements on power leads of
Digitrax components</title>
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<h1>Hardware Support: Measurements on power leads of Digitrax
components</h1>
<p>I wanted to understand the reasons behind Digitrax's
recommendations about separate power sources for PM4 and DS54
boards. This page describes some measurements on my layout,
and what I conclude from them.</p>
<h2>Conclusions</h2>The DCS100 and PM4 appear to use
full-wave rectifier power supplies, while the DS54 uses a
half-wave rectifier supply. These two types require different
offsets between system ground and the transformer leads,
which explains why these three types of components can't
share AC auxiliary power supplies.
<h2>Specific measurements</h2>In this section, I describe the
measurements leading to my conclusions.
<p>The following measurements were made on an operating
layout. It is powered by a single DCS100 (Chief) booster,
with PM4 units for power subdistricts and DS54 units for
various controls. Track power was on, and the DCS100 was in
"run" mode, i.e. sending packets on the rails. About 1.2 A
was being drawn from the DCS100 booster output. During the
measurements, no LocoNet® commands were sent, nor were
reversing sections being traversed.</p>
<p>Unfortunately, the camera scope photos wasn't sufficiently
sensitive, so they show a lot of blur. In all pictures, the
ground reference was centered and the vertical scale was
5V/division. The scope was line triggered. Only a single
trace is shown.</p>
<h3>Booster measurements</h3>Normally my DCS100 booster's
ground connection is connected to the AC ground, hence to
earth. I temporarily disconnected this to see what would
happen with a floating layout. I found a 60Hz offset voltage
of about 9V. A 100K resistor to ground removes this, so the
source impedance is very large. The photo was made with a
10MHz bandwidth limit; the visible spikes are about 6V with
no bandwidth limit (500MHz scope). The ungrounded layout was
probably generating a lot of RF noise...
<p><a href="DigitraxPower/images/DiscGround.jpg"><img src=
"DigitraxPower/images/DiscGround-thumb.jpg" alt=
"Booster ground signal without connection to household ground"></a></p>
<p>For the rest of the measurements, I reconnected the DCS100
ground to the power ground. Both noise and offset were then
less than 20mV.</p>
<p>With respect to system ground, the Rail A and Rail B lines
alternate between 12V and about 0.8V:<br>
<a href="DigitraxPower/images/RailA.jpg"><img src=
"DigitraxPower/images/RailA-thumb.jpg" alt=
"RailA signal relative to system gound"></a> <a href=
"DigitraxPower/images/RailB.jpg"><img src="DigitraxPower/images/RailB-thumb.jpg" alt=
"RailB signal relative to system gound"></a><br>
It appears that the DCS100 internal high-power supply is only
positive. To get a better understanding of that, I looked at
the two power in lines from the transformer:<br>
<a href="DigitraxPower/images/BoosterPower1.jpg"><img src=
"DigitraxPower/images/BoosterPower1-thumb.jpg" border="2" width="100"
height="75"></a> <a href=
"DigitraxPower/images/BoosterPower2.jpg"><img src=
"DigitraxPower/images/BoosterPower2-thumb.jpg" border="2" width="100"
height="75"></a><br>
Note they are both positive at all times, not centered on
ground. The lowest voltage is about 1.3V below ground. This
is characteristic of a full-wave rectifier supply with one
side of the output grounded. The small offset is due to the
voltage drop across the low-side diode, while the flat-top on
the waveform occurs when the transformer reaches a large
enough voltage to charge the filter capacitor.<br>
<img src="DigitraxPower/images/Fullwave.jpg"></p>
<h3>PM4 measurements</h3>My PM4 power supply is an isolated
transformer. I made no measurements with a DC auxiliary
supply. The LocoNet was connected, providing a common ground
connection.
<p>The voltages on the PM4 power connections look like:<br>
<a href="DigitraxPower/images/PM4Power1.jpg"><img src=
"DigitraxPower/images/PM4Power1-thumb.jpg" border="2" width="100" height=
"75"></a> <a href="DigitraxPower/images/PM4Power2.jpg"><img src=
"DigitraxPower/images/PM4Power2-thumb.jpg" border="2" width="100" height=
"75"></a><br>
Like the booster, this is indicative of a full-wave rectified
power supply. The load profile is very different from what's
seen on the booster supply. The ugly waveforms may be because
my PM4 supply is a cheap little Radio Shack 450mA
transformer; it seems to be clearly saturating.</p>
<h3>DS54 measurements</h3>My DS54 power supply is an isolated
transformer. I made no measurements with a DC auxiliary
supply. The LocoNet was connected, providing a common ground
connection.
<p>The voltages on the DS54 "black" and "red" power
connections look like:<br>
<a href="DigitraxPower/images/DS54PowerBlack.jpg"><img src=
"DigitraxPower/images/DS54PowerBlack-thumb.jpg" border="2" width="100"
height="75"></a> <a href=
"DigitraxPower/images/DS54PowerRed.jpg"><img src="DigitraxPower/images/DS54PowerRed-thumb.jpg"
border="2" width="100" height="75"></a><br>
This is indicative of a half-wave rectified power supply.
Note this is different from the PM4 and DCS100.<br>
<img src="DigitraxPower/images/Halfwave.jpg"><br>
Note the ripple in the ground (black) lead. Digitrax
recommends that you <b>not</b> connect the DS54 ground lead
to system ground; this offset shows why its a good idea to
listen to that recommendation. The ground in the cartoon
schematic is the DS54 black-wire "ground", but it is
<b>not</b> the same as the system (LocoNet) ground.</p>
<p>Don Crano has pointed out that the DS54 is deriving a
local ground from the rail connections so that the BD1 block
current detector can use the DS54 ground connection. The
following photos show that the offset from system ground
changes slightly as the pulse-stretching for analog operation
moves from one extreme to the other. (Note that these photos
are at 1V/division, with system ground in the center.)</p>
<table border="0">
<tr>
<td width="120" height="120" align="center"><a href=
"DigitraxPower/images/DS54ConnAna-99.jpg"><img src=
"DigitraxPower/images/DS54ConnAna-99-thumb.jpg" border="2" width=
"100" height="75">LocoNet connected, analog at
-99</a></td>
<td width="120" height="120" align="center"><a href=
"DigitraxPower/images/DS54ConnAna0.jpg"><img src=
"DigitraxPower/images/DS54ConnAna0-thumb.jpg" border="2" width="100"
height="75">LocoNet connected, analog at 0</a></td>
<td width="120" height="120" align="center"><a href=
"DigitraxPower/images/DS54ConnAna+99.jpg"><img src=
"DigitraxPower/images/DS54ConnAna+99-thumb.jpg" border="2" width=
"100" height="75">LocoNet connected, analog at
+99</a></td>
</tr>
<tr>
<td width="120" height="120" align="center"><a href=
"DigitraxPower/images/DS54DiscAna+99.jpg"><img src=
"DigitraxPower/images/DS54DiscAna+99-thumb.jpg" border="2" width=
"100" height="75">LocoNet disconnected, analog at
-99</a></td>
<td width="120" height="120" align="center"><a href=
"DigitraxPower/images/DS54DiscAna-99.jpg"><img src=
"DigitraxPower/images/DS54DiscAna-99-thumb.jpg" border="2" width=
"100" height="75">LocoNet disconnected, analog at
0</a></td>
<td width="120" height="120" align="center"><a href=
"DigitraxPower/images/DS54DiscAna0.jpg"><img src=
"DigitraxPower/images/DS54DiscAna0-thumb.jpg" border="2" width="100"
height="75">LocoNet disconnected, analog at +99</a></td>
</tr>
</table>People on the Digitrax mailing list have recommended
that all DS54 units on a layout be wired to the rails and
common power supply with the same polarity. (Mine are too.)
The offsets visible above show why you need to do this; if
two DS54 units were connected oppositely to either the common
power or the rails, they would be unable to create a
consistent ground reference. Bob Jacobsen
<p>LocoNet® is a registered trademark of <a href=
"http://www.digitrax.com">Digitrax, Inc.</a></p>
<p>Back to the <a href="Digitrax.shtml">Digitrax Hardware
main Help Page</a></p>
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