The bogobox booster

Table of Contents

• What is the bogobox?
• Important notes and warnings
• The bogobox schematic
• How to use
• Input connections

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:

• transformer built into the booster to avoid additional wiring
• transformer with two secondary windings for optimum electrical performance, i. e. minimized voltage ripple thanks to full wave rectification
• generous filter capacitors to reduce voltage ripple and voltage drop under high load
• symmetric bipolar output driver stage in common emitter configuration for high efficiency with minimum heat dissipation
• protection through current limitation and automatic turn-off after short circuit
• output signal is off if input signal is not present to avoid unintended autonomous operation.
• external push button inputs to turn on and off the booster, i. e. an "emergency stop button"
• serial computer interface via optocouplers
• feedback of operational state via serial computer interface

The bogobox can be used in the following configurations

• booster to a computer/software-based control unit, like DDL or DDW.
• booster for any digital control unit that generates a bipolar voltage signal, e. g. Märklin 6021, Märklin Delta Controls, Intellibox, etc.
• since the booster produces a zero output for a zero volt input, it might even be suitable for three-level digital signals, like selectrix or FMZ. However, I am not familiar at all with these systems, so you may go ahead at your own risk and knowledge. Feedback is welcome.
• the booster does not support any bidirectional communication such as Märklin mfx or DCC CV reading or Railcom communication

Important Notes and Warnings

A serious word of warning: Only 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

Diese Seite als Dokumentation meiner Arbeit zu verstehen, 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.

The bogobox schematic

Here is a schematic of my bogobox booster:

The schematic is also available as PDF-File.

Some notes on the schematic:

• Connector K1 goes to the primary side of the transformer. F1 is a melting fuse (rated at approximately double the nominal transformer primary current, medium to slow characteristic). I use a transformer with two secondary windings. This has the advantage that both positive DC voltage (at C1's positive side) and negative DC voltage (at C2's negative side) can be rectified in full wave rectifier bridges (D1-D4) which reduces the voltage ripple.
• The transformer secondaries are protected by semiconductor PTC fuses, F2, F3, known also under the brand "multifuse", or "polyswitch".
• The booster output current is limited to prevent any immediate damage to the output driving transistors, T5, T6, and other components due to overcurrents. The output current is sensed by resistors R9, R10. If their voltage drop exceeds approximately 0.7 V, T3 or T4 become conductive and cut off the driving transistor's base current.
• The booster output could directly be taken from the collectors of T5 and T6, but here it is switched by a relay Rel51. Once the relay has been activated manually by a push botton switch connected to K4, it holds itself from the booster's own voltage output. But if the voltage drops (due to high load and current limiting operation) or collapses completely (caused either by a short circuit on the rails, or by a missing voltage input from the computer) the relay switches off. The relay is activated by a manually operated push button switch connected to K4. The relay can be manually deactivated by push button "emergency" switch connected to K5. All components of the undervoltage turn-off part have reference numbers of 50 and above.
• Expected voltage drop at maximum current: Assume that 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. 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.
• Maximum output current: 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.
• 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. However, at a short circuit situation still 3 A output current flow, but the full voltage is applied across T5 and T6. At a short a transistor produces heat of approximately 15 V * 3 A = 45 W !
• Thermal design of T5 and T6: 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 K / 21 W = 5.95 K / W; The transistor's thermal resistance is 1.78 K / W, so the heatsink must have thermal resistance of 5.95 - 1.78 = 4.17 K / W or less.
• In an overload or short circuit condition, the current limitation is in effect. Then the output voltage drops and the voltage across T5 and T6 rises. To avoid thermal damage 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).
• If the bogobox should be used as a booster to digital command station (like märklin's 6020 or 6021, märklin's delta control 6604, 66045, 6607, the Intellibox, or something equivalent) but not to a PC, the optocouplers IC1 and IC2 are not required, and the input signal (track output signal of the command station) is connected to (the optocoupler's end of) R3.
• Feedback of operational state to PC: The operational state (that is the state of Relais Rel51) is signaled to the PC on the DSR status line of the serial port. The DSR line state is switched by a contact of the relais. In the normal operational state, the DSR line is connected with the DTR line. If the PC sets DTR to high, the DSR = high means normal operation. In the power off state, the DSR line is tied to GND via R14. Although strictly speaking, GND is not a valid voltage for RS232, a PC's serial port usually interpretes this reliably as a DSR = low.

How to use

When the bogobox is initially powered on, the "GO" button at K4 has to be pressed to activate 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.

In an emergency, the "STOP" button at K5 has to be pressed to de activate the relay contact and disconnect power from the rails.

The PC must output the digital signal on the TxD line of the serial port.

The PC may detect the operational state of the bogobox booster from the DSR line of the serial port.

External connections

Connections at the rear of the box comprise

• a power cord to the mains (from schematic K1)
• a red and brown plug to the track (K2 and K3)
• a 9 pin D-Sub male connector, to be connected to a PC by a 1:1 cable (K6)
• two green plugs to a normally open momentary switch (K4) for a "GO" button
• two red plugs to a normally open momentary switch (K5) for a "STOP" button