Re: DETECTION BLOCKS - Occupancy Detection
Vollrath, Don <dvollrath@...>
One way to detect track block occupancy is to measure current drawn by a loco when on those tracks. One can put diodes in series with the track to provide a detectable ~0.75V per diode and easily determine when a loco drawing current is on that section of track. The method you speak of probably used a full wave rectifier bridge with the AC connections in series with the track leads and the + & - DC leads shorted together. A 1.5V bulb connected across the AC-AC leads would provide an occupied/empty indication, while the nearly constant voltage drop of the diodes protected the bulb from over-voltage. The basic idea can work great for DC or DCC… If that is all you want it to do.
However, there are some limitations and drawbacks. 1) The 1.5V drop of the diodes is undesirable for DCC. In the olden days of DC one was used to constantly adjusting the throttle to maintain the desired loco speed… And there was usually plenty of voltage available from the controller to overcome the ‘lost’ 1.5 volts to maintain race-track speeds. So when the loco encountered a monitored section of track, one simply ignored a speed change or made yet another adjustment of the throttle to maintain the speed. 2) With DCC there is little extra voltage headroom to overcome the 1.5V necessary to operate the sensor. And our expectations are that the loco will be able to maintain operating speed as it enters or leaves a monitored section of track. 3) Note that for the incandescent bulb to light up, the loco must draw enough current to light up the lamp… 30-45 ma. Locos may be detected but not trailing cars with 5-10 kOhm resistor wheelsets. 4) The simple incandescent bulb will work on AC or DC and is somewhat intolerant of ‘minor’ voltage fluctuations of the diode bridge carrying current to the track. An LED is not a very good candidate to take its place because it works with only one polarity (ie – DC) and needs protection from allowing too much current to flow (10’s of milliamps) to prevent burnout. There are similar schemes using opto-couplers of today instead of incandescent lamps to yield an electrical signal isolated from track voltage.
There are other track current detection products available today using the voltage drop of a single diode plus more sensitive electronics to yield a yes/no answer of track occupancy. But these also are not necessarily isolated from the electrical circuits of the track. Measuring DCC current flow via a ‘current transformer’ such as that used by the NCE BD-20 and other similar products from other manufacturers provides an electrical signal of occupancy without any undesirable voltage drop and also electrical isolation from that of the track. These can be sensitive enough to detect trailing cars with resistor wheelsets.
From: WiringForDCC@... [mailto:WiringForDCC@...]
Sent: Tuesday, June 28, 2016 1:21 PM
Subject: [WiringForDCC] DETECTION BLOCKS - Occupancy Detection
Back in the ‘80’s I operated on a layout that had block occupancy indicators. Simplicity at its best.
A bridge rectifier with a 1.5v lamp wired across the bridge. All it was required to do was to light a bulb in the control panel. These were used in tunnels and blind track sections out of the sight of operators.
Could this be used with DCC? Would the 1.5v output light an LED?
I am building a new layout using DCC and want to install signals. I understand that this requires I set up a detection block(s) for each signal. Blocks are also set up to distribute power throughout the layout or to add reversing sections. My question involves my confusion regarding how each of these blocks fits together. I isolate power blocks by cutting each rail going into and out of the block – a total of 4 cuts per block. I feed power to the block through feeder wires connected to both rails about every 3 feet. What cuts do I make to set up a detection block? As best I can tell, I only need to isolate one rail for each detection block with a cut on either end and run the power lead to that rail thru the detection circuit with the other rail being powered by the normal track feeder. Is this correct? I also need to determine the location and length of each detection block – is there a good reference to read to help me there?