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I have replaced the rectifier disk on a couple of transformers with diodes. One is a Lionel zw with the heavy duty stud diode rated at 40 amps and a  tw with a 6 amp diode. They work great at this seems to be what most recommend when replacing the disk. I read some old threads that said they would destroy the switch contacts in the the transformers in time. Has anybody seen this happen in the real world or is this academic banter? This is what most of the transformer repair gurus recommend.



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@dogdoc after reading many threads on the subject and asking questions here on the forum, I recently purchased 4x NTE 5991 diodes to replace the original silicon rectifier discs in my two ZWs.

After reading this post, I've searched unsuccessfully trying to find any information about these replacement rectifiers damaging the whistle switch contacts.

I'm very interested in the answer to your question. In the meanwhile, would you please post any links you may have found that discuss this issue?

It is true. I have replaced the original rectifier discs with a diode and if you watch the action with the cover off, you will see a fairly large spark when the contact is broken. This will eventually eat away the contact. The new replacement repo disc's don't have the spark problem.  I know there will be nay sayers, but this has been my experience.

I too can echo @Chuck Sartor's observations with the diodes that I install, and those that know me know that I'm pretty true to factory in my work and so I do still install the rectifier discs quite a bit of the time as well. However, I don't think diodes are imminently destroying our transformers any faster than sparks on our pickup rollers, within our mechanical switch machines (O22), or everywhere else traditional operators accept them. I think it boils down to one of those things where in theory, sure, microscopic damage is being done, but in reality, it is insignificant overall and can be minimized with use of CRC 2-26 or 846 Carbon infused Silicone grease.

Not sure where you're installing the resistor, but putting it across the contact would provide a leakage current, that probably wouldn't be good.

I was thinking a resistor in series with the diode to make up for the lack of resistance in the silicon diode (when compared to the original disc, or would this actually increase arcing due to more load?). But I did not mean for my resistor comment to overshadow the clarifying ask with regards to TVS theory and placement within the whistle control circuitry.

Last edited by bmoran4
@bmoran4 posted:

I did the experiment of adding a TVS as @gunrunnerjohn proposed and @SteveH clarified and saw no improvement with the sparking at the whistle switch.

Thanks, I was curious if that would have an effect, I guess not.   I really don't have a dog in this fight as I don't use the whistle switch on transformers, I can't even find it on mine!


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@bmoran4 thank you for conducting this experiment and reporting on the observation.  I'm wondering about two questions:

1) since the voltage is much lower (~ 5-6 volts) in this part of the circuit than what one would encounter on the track with a derailment, would a TVS diode with a significantly lower breakdown voltage work to better reduce arcing between the whistle switch's breaking contacts?

2) @gunrunnerjohn suggested adding a ~ 10-15Ω resistor in series withe the TVS.  I'm wondering what advantage this might offer?

Well, my resistor idea was just an experiment to see if that in conjunction with the TVS might make a difference in the arcing.  Typically, an RC network is employed to minimize contact arcing.  Tons of information about contact arcing, no need to reinvent the wheel.

Inductive Load Arc Suppression.pdf

Prevent Relay Arcing using RC Snubber Circuits

How to Reduce EMI, Voltage Spiking and Arcing

John thanks for this information.  I was wondering about an RC circuit for this purpose and why no one mentioned it before now, but not being an EE, this is a bit outside my comfort zone of making recommendations.  I was also concerned about the possibility of inadvertently introducing a potential oscillator and/or unintended currents from the RC network.

Do you have any specific RC recommendations for this application that wouldn't potentially lead to other issues, or would it be better to just leave well enough alone?

Update on some testing I did on this matter of whistle contact damage from use of a diode to replace original rectifier disk in a zw transformer(as was done on one of my zw). Initially with dry contacts I could see an arc when the whistle contact was released( disengaged ). I purchased some carbon black conductive grease as recommended and applied it to the contact surfaces in the whistle activation mechanism. NO VISIBLE ARC AFTER CONDUCTIVE GREASE APPLIED!! So while any concern was like minimal to begin with, it is even less now. Considering that likely hundreds of transformers have been so modified, this is a simple solution compared to TVS and resistors . The carbon grease is nasty stuff if you get it on you. I applied with a tooth pick as it will get everywhere if not careful and it is a ***** to wash off. Gloves are not a bad idea. Fabulous whistle control and sound with no slowing of train!

Last edited by dogdoc

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