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Afternoon GRJ, and anyone else on the forum that may be able to shed some light on my problem.

After all the talk about circuit board failures in the MTH loco's, I finally went and got some TVS's that I was assured would be right for my application.

I installed one across the "wire nuts" of my MTH RailKing SD45 loco while the body was off (getting re-painted in the Norfolk Southern colours), and when I put the chassis on the track and fired it up, the horn started blasting continuously (even in neutral), and I couldn't get it to switch-off using the horn button on my transformer. I run this particular loco (and most of my other MTH loco's) in conventional mode using my Lionel RS1 transformer, as I don't have WIFI or any of those other remote things (I'm a dinosaur!!!).

When I remove the TVS, everything goes back to normal, and I can control all of the features again from the transformer.

What am I doing wrong.....have I been sold "a pup".... the wrong type of TVS?  The part number for the TVS's I purchased is:  SA24A+ TVS DIODE, 500W, 24V, UNIDIR, DO-204AC - Tariff No: CN 85411000.

I have added a couple of photos to help you see what I have.....hope this helps.

I dearly want to install a TVS to help protect the circuit board, but this just doesn't seem to be working for me.

Peter in sunny Queensland, Australia

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Last edited by OGR CEO-PUBLISHER
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@Buco posted:

... got some TVS's that I was assured would be right for my application.

... part number for the TVS's I purchased is:  SA24A+ TVS DIODE, 500W, 24V, UNIDIR, DO-204AC

Where did you get the advice on component selection?

1) You need a bi-directional (not UNIDIR) TVS.  Unidirectional is generally for DC operation.

2) 24V is too low of a clamping voltage for a RS1 18.5V AC output.  The peak voltage of an 18.5V AC output is over 25V.  So a 24V TVS would be engaged on each AC line cycle in "normal" operation.

See this OGR post for some suitable bi-directional TVS parts.

I'm surprised you didn't see the dreaded magic smoke wafting from the TVS. 

Last edited by stan2004

Stan has the answer, just follow the posted link. Actually 33 and 36 v TVS's are recommended. Even though Lionel wants you to run their trains at 18 volts, many transformers can put out much more. Z4000s put out 22v+, and some others can output 24v. Multiply those numbers by 1.4 to get the minimum voltage rating of the bipolar TVS needed.

Pete

Last edited by Norton
@Buco posted:




... Peter in sunny Queensland, Australia

Shipping charges from DigiKey in the US might be a factor.  If you bought these at a local store that carries a limited selection of TVS diodes, find the clerk that assured you the SA24A+ would work and swap them for the SA33CA+ .   That's what "assured" means to me!

The markings on bidirectional parts should contain the letters "CA" (not just "A") and there should NOT be a silver band on one end. 

Out of curiosity, did said individual tell you which way to install the banded TVS - whether the banded side goes to center or outer-rail wire nut?  You can install the bidirectional TVS either way.

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If by end result you mean abject failure.

Actually, if installed the other way, I'd think the Bell might have turned on (rather than the Horn).  That is, a Uni-directional TVS will clamp at 1-diode drop or, say, 1 Volt on each half-cycle.  I believe this is what created the DC offset as seen by the engine - thereby triggering the Horn.  Of course, this would generate massive current, heat, and rapid component failure. 

So, yes, same end result but I was curious about the intermediate result.

Thank you one and all for your replies and assistance!!!

Stan2004:  I didn't see any "magic smoke" coming from the TVS when I tried it a couple of times on the track, before I gave up and finally removed it from the wire nuts. When I did this, everything went back to normal.

And yes, the sales assistant at the electronic store did say that the "banded" end must be fitted to the centre rail hot wire, that's why the TVS's were sold in strips with red tape and white tape at each end. The red tape indicates this is the end to go onto the + (hot) wire, and the white tape end goes to the - (ground) wire.

Guess where I am off to this morning (Monday down-under here) with a strip of TVS's in my hand!!

Just a final comment......Stan2004 and Gunrunner John's comments made me smile (laugh out loud actually), and added a bit of levity to something that absolutely frightens me to death - modern electronics. Thank you both!!!

Peter (Buco Australia)

Buco, I hope that you are able to get your hands on the TVS diodes that are recommended.  They are a bit of insurance that will pay for themselves.  It's nice that folks Down Under still have electronics stores they can walk into and pick up supplies.  I'd  like to know where there's one in the Chicago, Illinois area.  Really doubt there is one.

Phil

Just an update on the TVS saga.

Well I went back to the electronic's store (Jaycar) with the errant TVS'S, and was promptly told they did not have the Bi SA33CA+ in stock. Told them what had happened when I installed the SA24A+ TVS and they didn't seem to understand what I was going on about.

At this point I just gave-up (they are definitely not model train people) and asked them to refund me the original purchase (minus one TVS), and to order me the correct ones. They said they will have to place an order with Elemant14, their supplier in Sydney (1,000km away, and it may take a week or two to get them up here.

Good news, the correct TVS's have arrived, and I have fitted them to four of my MTH RailKing diesel loco's, with perfect results!!!!  Everything works on each loco, just as it should.

Thank you guys for all of your help. I have another four MTH loco's to go, and the full fleet will now be protected.

Just a quick aside:

How will I know if the TVS has done its job and protected the circuit board?

After a "spike" will the TVS be rendered useless, and in need of replacing?

How will I know if the TVS needs to be replaced.....does it get that "burnt component" appearance?

Peter......in sunny (well its finally raining!!!) Queensland, Australia

@Buco posted:

How will I know if the TVS has done its job and protected the circuit board?

The engines keep running.



After a "spike" will the TVS be rendered useless, and in need of replacing?

No, the TVS diode is good for pretty much infinite spike suppression as long as it's overall ratings aren't exceeded.



How will I know if the TVS needs to be replaced.....does it get that "burnt component" appearance?

You won't know if they fail open.  However, the most common failure mode is shorted, that you will notice!



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