Hello all,

I want to slow down trains using zener diodes. Purpose is to lower the speed so that even my longest train stops in the proper part of the shadow station, and not on the turnout...

I was wondering: does any of yoyu have experience using zener diodes (in anti serial configuration) for slowing down trains?

Or perphaps ordinary diodes (many of them, in anti parallel configuration)?

Thanks,
Rene

Hi,

Using zener diodes for reducing track voltage allows, in all cases, to reduce the MAXIMUM loco speed. The effect on the speed of locos when the throttle is in low/medium position depends on the loco decoder, and is very small with "load regulated" decoders. There may be a little "jump" when the loco reaches again the standard track voltage. Then, it's not very "aesthetic".
But I think its use in a shadow station, in order to reduce the maximum stopping distance (if the loco is full throttle) is very effective, and mandatory in small layout.
The zener diode to be used should be carefully chosen: a too large value may lead some locos (driven at first step speed) to a stop (specially not regulated delta locos). A too small value is not very effective. It also depends on the track voltage: 19V peak with a MS feeded with the 18VA "transformer"; 22V peak when using 16V transformer + MS or CU6021.
In my case, I use 6.2V zeners (2W), in a "antiserial" configuration without any problem.

You'll find some measurements of loco currents at: https://www.marklin-user...ault.aspx?g=posts&t=2348 .

Hope this can help

Fred

Hi Jorge,

What happens when the loco shoe shorts the 2 sections?
In case of zeners, the zeners are simply shorted, no problem.
In case of a underfeeded booster, I agree that its track voltage is in phase with the main track voltage, but nevertheless the voltage difference is about 7V, or more if the booster is largely underfeeded. And the output impedance of the CU, MS, or booster is low. Then I guess that a large current may flow between the 2 generators, leading to sprinkle on the track, or short protection activation if the shoe stays a long time between the 2 sections, or damage to the booster whose output transistors are reverse-biased (although I think they are protected?).
I have not checked that solution, and may be I 'm wrong on those points.
In any case, I must say that your proposal is quite elegant (not cheaper than zeners: 2 x 0.2 Euros per section).

Fred