Do-It-Yourself Booster for the Märklin Motorola and DCC Format
The following considerations made me design my own booster:
All these points are fulfilled at full extent with the booster presented here.
Further considerationsMany of the available booster kits can not be connected to the booster interface of the control unit, instead they must be attached to the rails of an other circuit or to the rails output of the CU (brown and red). Here the same signal is available, only the level (and naturally the output current) is higher. This signal is used as the input for these boosters. This system has however a serious disadvantage: If it comes to a brief short circuit in the electrical circuit of the control unit, also the signal for all further boosters is interrupted in this time. The consequence: every small short-circuit affects the entire layout. The booster interface of the Control Unit on the other hand would still have been supplying the digital signal, in fact the signal is available even when the stop key of the CU has been pressed. The second disadvantage not using the booster interface is that the booster can not switch off the CU in case of a short circuit in its own circuit (feedback).
To clarify this, the different ways of connecting the boosters are shown schematically:
The connections at the booster interface of the cu 6021 and Intellibox are as follows:
To ensure the compatibility these functions must be used in our booster.A stabilization of the output voltage has not been implemented, after a series of intensive tests and measurements. Why? In order to stabilize an output voltage, the input voltage must be significantly higher. Transferred to our application this means that the input voltage would have to be larger than 18V, i.e. at least approx. 20V. Thus meaning that all standard model railway transformers are omitted. The alternative is to make the output voltage smaller. This naturally has different negative effects, like increased contact problems, general under voltage for decoders and magnetic devices etc..
According to my opinion it's better beginning with a generous dimensioning of the transformers and boosters, as well as - very important - the sufficient cross sections of the wiring.
The output stageFor the output stage I selected high current Darlington transistors : TIP140 (T6, NPN) and TIP145 (T5, PNP). I choose an arrangement, which makes the direct assembly of both transistors on a heat sink without isolation possible. This is done by connecting the two collectors of these transistors. The control of the output stage happens with the two transistors T1 and T2. The output current is limited by the transistors T3 and T4. These receive the base voltage from the resistances R7 and resp. R8. If the voltage drop over these resistors exceeds a value of approximately 0.6 Volt, they become conductive thus "taking away" the input signal of the Darlingtons. By changing the value of R7 and R8, the maximum output current be adjusted:
Table 1: Parts depending on the desired output current.
The current supply
A commercial model railway transformer is used for the 18V AC supply for the booster. This must be able to supply the desired output current. For the maximum current of 6 ampere a transformer of at least 108VA is necessary. For lower output current both resistors R7 and R8 must be adapted according to table 1.
The voltage is rectified by Q1, using a simple half-wave rectification. It becomes thus the positive half wave for the plus voltage, and uses the negative half wave for the minus voltage. This method has the advantage that only one secondary winding (one transformer) is needed. However the expenditure for the smoothing becomes more highly than during a full-wave rectification. Therefore the circuit is laid out for capacities up to 20'000 μF for each for DC voltage (total of 40'000 μF!).
The remainder of the circuit is for the control, input and output signals etc.. A signal quality is ensured by using comparators (Q3).
For the circuit I developed a printed circuit board with the dimensions 160 x 100 mm, which is euro-format. This size PCB easily available are as photo lacquer coated, one side copper-covered GRP plates in Europe. All parts are accommodated on this print. The design is based on standard parts, not using any SMD devices. It should therefore be no problem for DIY.
Both Darlington transistors are screwed on as large as possible heat sink. Hereunder applies the following to consider: From a technical view it's not necessary to insulate the transistors to the heat sink. However, the output voltage of the booster is then on the entire heat sink. In that case the heat sink may not have any connection to other live parts or to ground! Safer is therefore the insulated assembly of the transistors.
The circuit looks more complicated at first sight, than it is. Many parts serve the control and feedback for the control unit 6021 or the Intellibox. The booster can be called rightfully Plug and Play equipment. If soldering knowledge is present, the building of this boosters is no problem at all.
Oscilloscope pictures of the output signalThe output signal was measured under practice close conditions and compared with the output of a 6021. Click here to view the results.
The boosters come with a five pin interface connection, that must be wired 1:1 to the booster interface of the Control Unit 6021, the Intellibox, the Fleischmann Twin Center or the TAMS Easy Control. The boosters you can buy in the shop come with a 14 inch cable.
Connect brown and yellow to the output of your transformer and brown and red to the tracks (brown = ground).
Notice regarding the Intellibox or the Twin Center
If you plan to operate the Intellibox with attached booster also in the DCC mode, then
the special option 901 must be set to 3, otherwise the boosters send a short circuit message.
Do you need additional information? Then do not hesitate to contact me by using my feedback form. I will then mail you all plans needed.
Parts and built modules
In my shop you may order single pcbs with or without all the parts needed or complete and tested devices.