When I started with command control back in the 80's mainly CTC 16 then
CTC 16/e I decided I did not want to use common rail wiring. I used the
common of the rectifier of the system. Today this is referred to as direct
home wiring with DCC. Today I know of two systems that use this type common,
Digitrax and Easy-DCC. There may be other out there, and more that can
be converted to direct home wiring. One thing that direct home wiring does
not like is a common across power districts. A common return is fine with
in districts, but there should not be any common crossing the gaps of the
separate districts. This is what lead me to opto-isolate the inputs of
my detectors. I needed to make sure there were no possible paths across
the districts. This as it turns out was a good move at the time. It solved
a few problems, and when I went to DCC in 1996, I saw Digitrax was using
direct home wiring so I now use a Digitrax Chief and my detectors were
all ready as well as the rest of my layout.
By taking my existing detectors and adding the opto-isolators to the front
end of them. At this time I was using the basic Bruce Chubb Optimized Detector.
It was fairly easy to just insert the isolators between the diodes and
the comparator input. I did this for the first group of detectors I had
and they worked fine. Later as the railroad expanded, the need for more
detectors arose. I figured that with command control there is no need for
dual polarity inputs, with analog command control the voltage is always
the same polarity, except at reverse loops. I then decided that one
LM339 is a quad pack, so why not design the detectors to use only one fourth
the chip each. Now I could get four detectors per chip. So each one of
those existing detectors would give me four in return with a addition of
a few extra components. Now I get into DCC, here the voltage is a bi-polar
AC square wave. AC Alternating Current, is the key. My new detectors would
work, but they were not as sensitive as they used to be, and also were
subject to sensitivity changes with stretched pulses on DCC. To cure this
all I needed was a better suited Opto-Isolator, I found the PS2506 from
NEC. This is an AC input, darlington output isolator with enough speed
to work with DCC just fine.
Finally we are here, my detectors as used today.
Starting with the jumper diode bridge, this is just a neat way to get the
required two diode voltage drop. As shown in the drawing you can also use
the optional diode arrangement. All that is required that they be able
to switch at the DCC frequency approx. 10kHz. Now you will notice that
the PS2506 is parallel across the diode bridge via a 150 ohm resistor.
This resistor supplies the current limiting for the internal LEDs of the
PS2506 Opto-Isolator. These are two LEDs because it is an AC input, so
as the DCC signal alternates so do the LEDS. This is how part of the sensitivity
is gained back, one or the other LED is always lit. Combine that with darlington
output and it's high gain, we have all the sensitivity we should need.
It feeds the comparator input via the emitter drive of the darlington.
The rest is just a basic comparator type circuit. A Screen clip of the
detector running under Xspice, Spice2 is a 89k
GIF.
For those who might want different type outputs here are a couple of
variations.
These give you a choice of open collector for driving a different voltage
source, such as 5 volts for TTL. The Opto-Isolation for isolation for such
things a LocoNet, computer interfacing, etc.
| Qty
1 1 1 1 1 1 1 1 6 |
Description
Opto-Isolator single 4pin dip Opto-Isolator quad 16pin dip Quad Comparator 14pin dip NPN Transistor 4amp 100v Diode Bridge 5k adjustable resistor 10uf 25v Electrolytic Capacitor 220uf 25v Electrolytic Capacitor 1/4 watt Resistors |
Part Number
PS2506-1NEC-ND PS2506-4NEC-ND LM339 2N3904 KBL01-ND 3316K-502-ND P5148-ND P5153-ND Value+QBK-ND |
Qty - 1
$0.81 $2.71 $0.42 $0.23 $1.90 $0.41 $0.22 $0.43 $0.28(5) |
Qty -10
$ 7.02 $28.08 $ 4.02 $ 2.30 $17.34 $ 3.05 $ 0.77 $ 1.85 $ 3.89(200) |
Note: Select either
the PS2506 in the single pack or the quad pack, you do not need both. The
resistors are sold in packs of 5 or 200.
You will also need some kind of board to build
them on. I use universal 44pin edge cards, and build 8 to a card. You can
use other types of boards, as the small universal one from Radio Shack
and such.
The diode bridge KBL01-ND is a 4amp bridge. They
have worked fine with my Challenger, Big Boy, and Chief.
But if you feel you need higher amperage and/or
faster diodes.
From Digi-Key Diode Bridge part # GBPC6005-ND, this is a 6 Amp Bridge from the same family as the 4amp I use. Should work with no problem...
From Mouser For those who want faster diodes. Single Diode Rectifiers, need 4 per detector..
Diode Part# 583-SF51 5 Amp Super Fast Recovery
in standard DO-201AD case, barrel with leads..
Diode Part# 583-SF81 8 Amp Super Fast Recovery
in standard TO-220A case, twin lead.
These SFR diodes are available all in #583-SF161C
16 amp and #583-SF301C 30 amp versions.
