Serge NOEL
162 lines
9.3 KiB
Plaintext
162 lines
9.3 KiB
Plaintext
<!DOCTYPE html>
|
|
<html lang="en">
|
|
<head>
|
|
<meta name="generator" content="HTML Tidy for HTML5 for Apple macOS version 5.8.0">
|
|
<title>JMRI Hardware Support - Measurements on power leads of Digitrax components</title>
|
|
<!--#include virtual="/help/en/parts/Style.shtml" -->
|
|
</head>
|
|
<body>
|
|
<!--#include virtual="/help/en/parts/Header.shtml" -->
|
|
|
|
<div id="mBody">
|
|
<!--#include virtual="Sidebar.shtml" -->
|
|
|
|
<div id="mainContent">
|
|
<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="images/DigitraxPower/DiscGround.jpg"><img src=
|
|
"images/DigitraxPower/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="images/DigitraxPower/RailA.jpg"><img src="images/DigitraxPower/RailA-thumb.jpg" alt=
|
|
"RailA signal relative to system gound"></a> <a href=
|
|
"images/DigitraxPower/RailB.jpg"><img src="images/DigitraxPower/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="images/DigitraxPower/BoosterPower1.jpg"><img src=
|
|
"images/DigitraxPower/BoosterPower1-thumb.jpg" width="100" height="75" alt="power 1"></a>
|
|
<a href="images/DigitraxPower/BoosterPower2.jpg"><img src=
|
|
"images/DigitraxPower/BoosterPower2-thumb.jpg" width="100" height="75" alt="power 2"></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="images/DigitraxPower/Fullwave.jpg" alt="full wave diagram"></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="images/DigitraxPower/PM4Power1.jpg"><img src=
|
|
"images/DigitraxPower/PM4Power1-thumb.jpg" width="100" height="75" alt="pm4 power 1"></a>
|
|
<a href="images/DigitraxPower/PM4Power2.jpg"><img src=
|
|
"images/DigitraxPower/PM4Power2-thumb.jpg" width="100" height="75" alt="pm4 power 2"></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="images/DigitraxPower/DS54PowerBlack.jpg"><img src=
|
|
"images/DigitraxPower/DS54PowerBlack-thumb.jpg" width="100" height="75" alt=
|
|
"ds54 power black"></a> <a href="images/DigitraxPower/DS54PowerRed.jpg"><img src=
|
|
"images/DigitraxPower/DS54PowerRed-thumb.jpg" width="100" height="75" alt=
|
|
"ds54 power red"></a><br>
|
|
This is indicative of a half-wave rectified power supply. Note this is different from the PM4
|
|
and DCS100.<br>
|
|
<img src="images/DigitraxPower/Halfwave.jpg" alt="half wave diagram"><br>
|
|
Note the ripple in the ground (black) lead. Digitrax recommends that you <strong>not</strong>
|
|
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 <strong>not</strong> 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 style="width: 120px">
|
|
<a href="images/DigitraxPower/DS54ConnAna-99.jpg"><img src=
|
|
"images/DigitraxPower/DS54ConnAna-99-thumb.jpg" width="100" height="75" alt=
|
|
"connected -99">LocoNet connected, analog at -99</a>
|
|
</td>
|
|
<td style="width: 120px">
|
|
<a class="td-image" href="images/DigitraxPower/DS54ConnAna0.jpg"><img src=
|
|
"images/DigitraxPower/DS54ConnAna0-thumb.jpg" width="100" height="75" alt=
|
|
"connected zero">LocoNet connected, analog at 0</a>
|
|
</td>
|
|
<td style="width: 120px">
|
|
<a class="td-image" href="images/DigitraxPower/DS54ConnAna+99.jpg"><img src=
|
|
"images/DigitraxPower/DS54ConnAna+99-thumb.jpg" width="100" height="75" alt=
|
|
"connected +99">LocoNet connected, analog at +99</a>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td style="width: 120px">
|
|
<a href="images/DigitraxPower/DS54DiscAna-99.jpg"><img src=
|
|
"images/DigitraxPower/DS54DiscAna-99-thumb.jpg" width="100" height="75" alt=
|
|
"disconn -99">LocoNet disconnected, analog at -99</a>
|
|
</td>
|
|
<td style="width: 120px">
|
|
<a href="images/DigitraxPower/DS54DiscAna0.jpg"><img src=
|
|
"images/DigitraxPower/DS54DiscAna0-thumb.jpg" width="100" height="75" alt=
|
|
"disconn zero">LocoNet disconnected, analog at 0</a>
|
|
</td>
|
|
<td style="width: 120px">
|
|
<a href="images/DigitraxPower/DS54DiscAna+99.jpg"><img src=
|
|
"images/DigitraxPower/DS54DiscAna+99-thumb.jpg" width="100" height="75" alt=
|
|
"disconn +99">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="https://www.digitrax.com">Digitrax,
|
|
Inc.</a></p>
|
|
|
|
<p>Back to the <a href="Digitrax.shtml">Digitrax Hardware main Help Page</a></p>
|
|
<!--#include virtual="/help/en/parts/Footer.shtml" -->
|
|
</div>
|
|
<!-- closes #mainContent-->
|
|
</div>
|
|
<!-- closes #mBody-->
|
|
<script src="/js/help.js"></script>
|
|
</body>
|
|
</html>
|