This question arised in the "Alzheimer" topic, but since this would be off topic I start a new one.

I have been running my braking/stop-section with just a single diode to do the negative voltage. I like this because there is no need for transition sections. I use the extra catenary contacts of 7039 for the stop section.

This works fine with all Märklin locos with 60902-decoder or newer, even with the new BR 482 with 610761-decoder(36851). The only loco that do not brake in addition to old delta locos is my V65 made by Roco.
But this puzzles me, there must be a reason why this is not commonly used. Has anyone else tried this?. I know Perz did some tests with different results (locos did not brake?)
I use the good old 6021.

Juha

Well, I have already mentioned my results in the "Alzheimer" topic: My locos (60901 and mfx) do not stop with this simple solution. If it had worked it would have been very convenient, I agree.

Hi,
I don't have any bracking effect either. With or without resistor in parallel with the diode.

Hi all,
Now I'm even more confused. Why does it work only with my locos?
I made another separate test track for testing and tried the 6021 with all Motorola formats, I&II (0100), only I (0000) and only II (1110). There was no difference, all decoder types respond to braking adjustment at the unfiltered negative voltage. I would understand if there was some random misbehaviour, but no, everything works 100%. I tried 6090x, the new 610761, one unknown in Roco's V65 and Tams LD-W-2.

This is just why I love Märklin: they just work every time.

I think I have to take home an oscilloscope from office and check the signal.

Juha

BTW,
I take it for granted that you have removed radio interference capacitators (if present) from connection track.
/Lars

Maybe you have the diode symbol backwards?[:I] Others would be getting '+' current not '-'.[:p]
Only stopping on '-' as I understand it.

As I mentioned in an other topic, I have tested:

- positive or negative stop voltage
- true or inverted digital signal
- stop voltage with or without capacitor

I tried all 8 combinations. Polarity made very little difference.
With the capacitor both locos (a 60901 and an mfx) stopped with all 4 combinations of polarity.
Without capacitor they did not stop with any combinations of polarity.

Hi Juha,

very very intersting results ! I am using a 1 N 5404 diode (the 3 amp version of the 1 N 4004) in the break circuit. It's probable that there are these positive spikes, too, as the 1 N 54xx are not the fastest diodes. So my digital decoders simply refuse communication in the break section as they don't like these spikes.

The crucial point seems to be that one somehow must "bridge" the few milliseconds where the voltage is positive and the diode is closed. Do you think a small capacitor with few nano- or micro-farads, but without polarization, would also do ?

Hans Martin

I agree with Perz. I tried with some capacitors 1nF - 100nF, didn't work.
I tested some more Shottky diodes: BYV19 and SB140. They both worked OK. No problems with any decoders.
I also tested with a high speed diode 1N4148, which also worked OK. But I don't recommend it because the max. current is only 0,5A, which might be too low for bigger locos.

So you can forget the transition sections!

Juha

A Shottky barrier diode is created if aluminium is applied on N-doped silicon. This type of diode is faster and has a lower forward voltage drop than a normal P-N silicon diode. It is generally 5-10 times more expensive than a normal diode, but 10 times almost nothing is still almost nothing. You will get one for less than 50 cent.

Lars, I know this sounds too simple. Why would Märklin have made more complicated braking module, they must be aware about this.
My tests are relatively limited, so I'm still waiting somebody to give opposite experience.

Purellum, I couldn't quite follow your proposal, maybe because I don't know how this works with 2R?

But meanwhile I continue to enjoy my simple braking sections, which just work with my locos.

Juha

Per,
The reversed diode series will become conductive when there is a positive voltage which is greater than the sum of the forward voltage drops of the diodes. At the same moment the game would be over, as there would be a positive signal.

This is my quick thinking, somebody correct me if this is not the case.

Juha

I don't think it's a good idea. The DC is like a code; when the decoder detects it, it begins to slow down to a stop. If it doesn't, it tries to keep the speed. If there isn't enough power, it will slow down yes, but different locos will probably slow down to different speeds. Try the LDT-module instead.
/Lars

I have made some tests with Delta decoders, and power resistor (Viessmann). Nonregulated decoders are supposedly better; still the power consumtion difference was very big.
/Lars

To: Forum-administrator: Could we please have this topic moved to a safe place, so that it will never be forgotten? This is, IMHO, the best solution ever made for cheap digitally braking.

Thank you. [^]

Per.

I must admit - at least the simplest, the cheapest and good enough. Still the relay approach gives you two things:
- direction sensitivity
- ability to use other brake signals (DCC).
/Lars

Thanks for the feedback Fred.

It seems that it won't work with all locos. I knew there was a reason for the capacitor, but couldn't figure it out. It would be interesting to know, which really triggers the braking function in the decoders software.

Well, so far everything works for me, but will be prepared for some changes in future. Good to know.

Juha

Hi,
I used 11DQ09, a 1A/90V shotky that has a low capacitance (in its category of shotky rectifier). In fact, the signal on the oscilloscope is quite clean. Furthermore, the same result was reached with signal diodes 1N4148 (3 in parallel for security, for they are 200mA diodes).
Anyway, thanks for your post!
Fred

Hi friends of Do-It-Yourself brake modules,

I have made some further experiments with juha's brake module. We know that it works well with loks having an fx decoder (6090x), but not with new mfx- and ESU-decoders which seem to ignore the breaking section.

Adding an RC-bridge between the anode of the Schottky diode and ground potential, however, seems to cure the problem, and at least the mfx-decoder in my GG-1 accepts the brake request now, The GG-1 which definitely disliked the original brake circuit now works fine and comes to a smooth stop, just as my 6090x equipped loks. There is some reason to believe that it may work with other mfx loks too, see my oscilloscope plots below.



The RC bridge introduced in the circuit works as follows: assume a lok is passing the brake section, so the diode will supply it with current when the digital voltage from the contol station is negative, and it cuts current of when voltage is positive. In this period the voltage across the tracks drops down almost to zero for few milliseconds.



6090x decoders can live with these "blackouts" and switch to break modus. Not so mfx decoders, they ignore the break request as the negative voltage is not continuous, and keep the lok running. This is where the RC-bridge takes effect: the capacitor (which has been charged in the negative voltage periods) now supplies the lok with negative voltaged which is reduced but clearly different from 0. Thus there is no "blackout", and mfx decoders recognize the brake request. Capacitors around 1 #956;F are sufficient, also the 1.5 k resistor lets enough current pass through.



There is no short circuit when the ski contact enters the section due to the resistor. Further, it does not matter whether a unpolar capacitor or a polar one is used, the right capacitor plate is always negative. However, it is important to use a Schottky diode and not a normal silicium one. The RC-bridge is not capable to absorb the short positive spike as shown on juha's oscilloscope images. Also loks with delta decoder will still not work with this modified module.

It would be interesting to see whether other mfx locos work with this module, too.

Hans Martin

SCIENCE!
I like that.

Timaximus

Hi friends,

Today I did some soldering: breakmodule and final circuit, everything in operation now on my digital carpet layout. Instead of the BYV 21 I have used two parallel BAT 48 for the time being, but BYV 21 is already ordered. Costs for the electronic components is around 2 EUR.

Any Schottky diode with high enough maximum current (1 A) and reverse switching time not longer than 100 ns should work.

Hans Martin

Hello and welcome to the forum Peter.

Hi all,

These are interesting findings! For the usual diode-capacitor combination I was already using fast diodes (SB160) for quite a while, assuming that that would be better given the fast protocol, but I did not know the system could do without the capacitor (I never use transition tracks, however, as digital braking is always activated by a train driving into a brake section making transition tracks redundant).

But I hope you all let me bend this findings to another sub question: how does coach lighting behave when using only a fast diode?

As a consequence of the duty cycle of the signal (which differs for the various controllers, according to some members of this forum), light bulbs might get fed infrequently and might flicker.

Taking this into account, I am thinking to modify some lighted coach sets, given the following conditions:
- The full set of cars has, for various reasons, only one pickup-shoe;
- Cars are connected by two-contact conductive couplers;
- I want to ‘ground’ several cars as I suspect that will increase reliability;
- I want to use both positive and negative phase of the signal, thereby preventing problems of an uneven duty cycle in the digital signal

Then I suggest wiring as follows:



As the schematics show, the couplers do not carry the regular ground signal and the pickup shoe signal, but an rectified DC signal instead. All cars except the pickup shoe car have 'half' rectifier bridges. Each car has its own capacitor, so the total capacity grows with the number of lights connected.

So I would appreciate input and comments to the following questions:
- how do lighted coaches of various kinds behave under the new, diode-only braking circuit?
- What do you think of my suggestion for ‘enhanced car lighting’?

Hi Rudi,

Very nice circuit, I think the capacitors will actually buffer the voltage when the period of positive voltage at the center rail is suppressed in the brake section. After having tested several types of diodes it seems indeed that using fast switching diodes is strongly recommended in digital circuits. Presently I am also planing a similar circuit, however using a one-contact conducting coupler between cars.

One difficulty of your circuit might be the large current that occurs when you switch on the voltage for the first time, as the capacitors must be charged. Perhaps some current limiting resistor could help here, included in the branch coming from the ski contact.

BTW, you actually may dispose of the mass contacts 2nd and 3rd car. The mass contact in the first car will take all the current.

Hans Martin

Hi all,

First of all, yes, you justly noticed that I made a mistake in drawing the bridge rectifier. ('Don’t do this at home! ;-) I was so busy drawing the symbols in the right place that I did not notice the mistake in their polarity. Of course, I suggest a regular full bridge rectifier.

Then, answering some of your comments:
- Indeed, when the train enters in a brake section, the pickup shoe in the coach might accelerate the loco for a short moment. However, I always activate the brake section with a switch rail that is a little further down the road. . So no problem there, at least for me. I am kind of hesitant to modify all my loco's and having to install Viessmann couplers at many more cars.

- About the current that the capacitors might draw at power-on. I think this is not really a problem. I already have some 6 or 8 braking sections, each having a diode and the regular 470 uF capacitor, and this never was a problem at startup. Maybe it will when we add these capacitors in coaches, but I think the system will manage.

- Finally, about the idea of having mass contacts in every car. I observed that feeding the power to coaches is not always perfect, and this seems to be more because of the mass contacts than the pickup shoe I suspect. I even have this issue with brand new cards, nice new connecting springs and carefully kept free of dust (and sometimes even added a bit of conductive stray in the axis, which I put using an ear stick. But at no avail, car sets having only one mass contact seem to have worse contacts than sets with mass contacts in more than one car . That's why...

Efel, I understand that you must use mass contacts AND Viessmann couplers in the same truck. Does that work well? When I was fitting some cars, this raised my doubt ( I was afraid the wires of the couplers could not move freely enough in turns because of the mass spring). So right now, my experimental sets looks like this:
- 1st car: pickup shoe AND mass connector in first truck, coupler in second truck
- 2nd car ... n-1 car couplers in both trucks
- n-th car: coupler in first truck, (additional) mass contact in mast truck

(hey, with n I do not want to suggest I have that many cars, but I try to have four coaches per train).

If you guys can convince me that having couplers and mass contacts in the same trucks works reliable, I might follow the example of connecting both poles of the Viessmann couplers. In that case I would put a full rectifier - correctly wired - in each coach.

Thanks for your interesting replies,

Rudi