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What is the bogobox?
The bogobox is my selfmade transformer-booster combination, which I use for my märklin digital layout together with DDL, the software-based digital control unit. The principles of digital control units and software-based units are described in the next two chapters, for those readers who are interested. The bogobox, as a booster, takes the digital signal to provide power to the tracks, or a section of tracks. The features of the bogobox are:
The bogobox can be used in the following configurations
A serious word of warning: Please consider the information provided here as a documentation of my work, not as instructions how to build such a booster yourself. The circuit is connected to the mains which has lethal voltage levels! If you have decided to build such a booster yourself, you are fully responsible to comply with the applicable rules and regulations concerning electrical safety and radio interference. I strongly recommend to seek professional advice.
According to German VDE regulation, and equivalent European Norms, electric toys (including model railways) must be operated on so-called "safety extra low voltage" (SELV). AC voltage must be below 24 V. Hence, the later proposed operation of the booster from two 16 V transformers of opposite phase is, strictly speaking, not allowed, since the voltage between two poles can have 32 V AC. I therefore suggest to build transformer and booster into one closed case.
To produce SELV, only safety transformers complying with VDE 0551 are allowed. Such transformers separate the SELV from the line voltage by double insulation.
The voltage-carrying parts of the SELV circuit must not have connection to protective ground or earth potential. Several SELV circits may only be connected if SELV voltage limits are not exceeded. This has a severe consequence if the booster shall be used together with a computer acting as central unit (e.g. running DDL, or DDW). The computer runs with "protective extra low voltage" (PELV) which is connected to protective ground from the power line. Connection with SELV is not allowed and therefore optocouplers must be used for operation with a computer.
Due to its importance, the same again in Deutsch
Betrachten sie diese Seite als Dokumentation meiner Arbeit, nicht als Anleitung zum Nachbau eines Boosters. Das Gerät ist mit Netzspannung (230 V) verbunden, es treten also tödliche Spannungen auf! Wenn sie so einen Booster bauen, sind sie voll dafür verantwortlich, alle Regeln und Normen bezüglich elektrischer Sicherheit und Störstrahlung einzuhalten! Wenden sie sich bitte an eine Fachkraft.
Gemäß VDE und entsprechenden Europäischen Normen darf elektromotorisches Spielzeug nur an Anlagen mit max. 24 V Schutzkleinspannung (safety extra low voltage, SELV) angeschlossen werden. Der nachstehend empfohlene Betrieb des Boosters mit zwei 16-V-Transformatoren, deren Ausgangsspannung gegenphasig ist, entspricht damit nicht dieser Forderung, da zwischen zwei Leitungen eine Spannung von 32 V auftreten kann. Ich empfehle daher, Transformator und Booster in ein gemeinsames geschlossenes Gehäuse einzubauen.
Zur Erzeugung von Schutzkleinspannung sind nur Sicherheitstransformatoren nach VDE 0551 zulässig. Solche Transformatoren trennen die Schutzkleinspannung von der berührungsgefährlichen Niederspannung durch eine doppelte Isolierung. Dies geschieht z. B. mittels einer Zwei-Kammer-Wicklung.
Die aktiven, d. h. spannungsführenden Teile des Schutzkleinspannungs-Stromkreises dürfen weder mit Erde noch mit aktiven Teilen anderer Stromkreise verbunden werden. Schutzkleinspannungs-Stromkreise dürfen untereinander nur dann verbunden werden, wenn dadurch die oben genannten Spannungsgrenzen nicht überschritten werden. Dies hat Konsequenzen, wenn der Booster an einem Computer betrieben werden soll, wo der Computer die Rolle der Zentraleinheit übernimmt (z. B. mit Software wie DDL oder DDW). Der Computer wird mit Funktionskleinspannung (protective extra low voltage, PELV) betrieben und ist mit dem Schutzleiter der Netzzuleitung verbunden. Eine Verbindung mit der Schutzkleinspannung der Modelleisenbahn ist nicht zulässig. Es müssen daher Optokoppler beim Betrieb mit Computer verwendet werden.
As many other märklin model railway fans, I like to run my layout digitally. Usually, this requires a "digital control center", like märklin's control unit 6021 maybe extended by 6040 keyboards and a 6051 computer interface, or the Intellibox by Uhlenbrock and Modeltreno. However none of these devices is particularly economical to someone's wallet. From an electrical point of view their sole purpose is to deliver positive and negative voltages of constant amplitude to the rails, can't this be made cheaper?
The necessary elements are some kind of intelligence that generates the positive
and negative voltage steps in appropriate time intervals, and a power
amplifier to boost these voltage steps to high currents. The voltage signal can
be generated by a computer, which is always available in nowadays homes.
But since a computer can't produce high output power some
external amplification is necessary, and this can be done perfectly by the bogobox.
Background Info - The computer as a control unit
The idea to use an ordinary PC computer as a control unit was put into reality by Dr. Konrad Froitzheim in the project DirecTrain. He wrote a DOS-based PASCAL program that let the computer generate the voltage signal and he built a simple power amplifier from a few components.
Later, Thorsten Vogt was inspired by Froitzheim's ideas and software, and he improved the software by some useful additional features. He calles this project XDirecTrain. However, Thorsten realised that a DOS based software has some serious constraints and so he started to write software for Linux. By now he has created an exceptional package of software known under the acronym DDL, which stands for Digital Direct for Linux. The main difference is that the software package consists of several elements. Like the märklin equipment can be divided into several functional units (central control, control 80, keyboard, computer interface), so is the software separated into a "core" process (called ERDDCD) running permanently in the background and generating the rail signal, and application software which connects to the core process. There is only one core process running, but many application processes can connect to it. So the core process is the equivalent to a control unit, the application softwares correspond to control 80s, keyboards, switchboards, et cetera. The communication between core and applications is handled via a port address, I. e. the core process is addressed by the IP-number of the machine on which it is running, and its port number. Therefore if the railway PC is within a network, application software can run on PCs other than the one with the core process. The most basic communication with the core process can be done via telnet, but there exist also graphical user interfaces, or a script language to control a railway automatically from a script. This software can generate Märklin/Motorola signals and DCC signals, it has similar capabilities as the Intellibox.
Any of above software generates the electrical signal on the serial port.
However, a computer's serial port cannot drive a model railway directly but
requires a power amplifier, also known as "booster" among model railway
people. Suitable boosters are commercial boosters, like the one from märklin,
also märklin's delta control unit 6604. In addition,
Dr. Froitzheim
has presented a
very simple self-made booster,
Dr. König
has published a voltage-regulated
high-end booster, mail-order companies, like
Conrad.de
also have booster kits in their catalogue.
The bogobox schematic
Here is a schematic of my bogobox booster:
The schematic is also available as PDF-File.
Description of function
In the beginning I was inspired by Froitzheim's design, which seemed to be quite simple but did its job. I took it as a starting point for my own booster, the "bogobox", but improved it at important details, especially safety issues.
When the bogobox is initially powered on, the "GO" button has to be pressed to close
the relay contact and power the rails. If there is no voltage on the serial
input which is the case when the computer is off or not connected, no output
voltage will be produced and the relay cannot hold itself activated.
Electrical characteristics
Often occurs the question "Do we need a stabilized output?" Consider the user of a märklin 32 VA transformer with a 6604 delta control. If there is one loco on the rail it has a very high maximum speed, if there are four locos and maybe cars with lights, the possible maximum speed is much lower. The reasons are: transformer output voltage at idle and nominal load condition, and the filter capacitors after rectification.
Of course, a stabilized output voltage would be nice, but usually means higher heat dissipation. The unwanted effects mentioned before can be minimized quite well by (1) using a transformer of higher power rating, because the higher the power the smaller is the voltage difference between idle and load; (2) by using full-wave rectification to recharge the filter capacitors every 10 ms instead of only every 20 ms, which requires a dual AC supply (transformer with two secondary windings); and (3) by choosing filter capacitors of sufficient capacity.
Let's assume a worst case scenario: We draw a constant DC current of Imax = 3.0 A from the output, the filter capacitor (C1 or C2 in the schematic) shall have a capacity of C = 10000 uF, and we use full-wave rectification. The resulting voltage ripple then is dU = Imax * dT / C = 3.0 A * 10 ms / 10000 uF = 3.0 V. This means, the voltage of C1 or C2 drops by 3.0 V until it is recharged. For half wave rectification we have dT = 20 ms and the voltage drop becomes dU = 6.0 V.
The maximum output current is set with the sensing resistors R9 and R10. Choose R = 0.6 V / Imax = 0.6 V / 3.0 A = 0.20 Ohm, which has been rounded down to 0.18 Ohm. The resistor must withstand a power of P = 0.6 V * Imax = 0.6 V * 3.0 A = 1.8 W. If the output current should be higher than 3 A, I would also decrease R6 and R7 to something around 220 Ohm.
In usual operation, the voltage drop across T5 and T6 is less than 2 V. At 3 A output current this makes 2 V * 3 A = 6 W heat dissipation, which seems not be critical at all. Now assume that because of overload or short circuit conditions at the output, a much higher current would be drawn than the 3 A if there was no current limitation. But the consequence of limiting the current through T5 and T6 to 3 A is that the collector voltage level decreases (in the worst case down to zero), and thus the voltage drop across the transistor increases, at still 3 A of current. At a short the transistor produces heat of approximately 15 V * 3 A = 45 W, which is quite tough to get rid of!
To avoid thermal death under overload conditions, which are characterised
by a decreasing output voltage, the output voltage collapse is sensed and
the output is switched off by a relais contact. The undervoltage threshold
is set with R52 and depends also on the relais's winding resistance. With
a greater R52 the threshold rises (because the voltage at the relais reduces).
Thermal characteristics
Let us assume that the undervoltage turn-off is set at 5 V below the
usual output level. Then the voltage drop across T5 and T6 may be up to 7 V instead of
the usual 2 V. With 3 A of current this makes 7 V * 3 A = 21 W of heat.
Therefore we need a thermal resistance of our total cooling means of
Rthermal = 125 °C / 21 W = 5.95 °C / W;
The transistor's thermal resistance is 1.78 °C / W, so our heatsink must
have thermal resistance of 5.95 - 1.78 = 4.17 °C / W or less.
Input connections
Connections at the rear of the box comprise
9pin serial | bogobox
port |
7 RTS ------o-.
|
8 CTS ------o-'
1 DCD ------o-.
|
4 DTR ------o-*
|
6 DSR ------o-'
9 RI ------o not connected
2 RxD ------o not connected
3 TxD ------o--- to P3, TxD
5 GND ------o--- to P4, GND
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last update: 2003-02-14;
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