MRC Ampack upgrade finished

HOLD ON THERE!!!
If you have your power transistor bolted directly to the case AND the case earthed to the mains/ground you have created a potential death-trap! Your controller case and one rail are at earth wire potential.
If one of your mains outlets is incorrectly wired then you could have full mains voltage on your case and rails. I know this shouldn't happen, but after I bought a mains outlet tester decades ago I tested all the sockets in the old house we were living in. I found two outlets incorrectly wired, one, which my wife used for ironing (metal cased iron with body earthed) had the phase and earth socket wires crossed. She had been ironing with an iron at 240 volts potential.

Greg; You are wise to point out this potential gremlin. However: There isn't a connection from either of the 'mains' wires to the output circuitry or case. So in this case there isn't any potential problem.
In The U.S. the common three prong outlet is hot, neutral, ground (with the 'ground' being a round prong as opposed to the other two 'spade' terminals.
If Gerard does 'ground' the case of the Ampack, it will be connecting to a (supposed) earth ground.
Those are handy, and sometimes its shocking what they reveal.
One of the 'benefits'? of backward US practice (we only get 110 VAC at that point.) Chuck D.

GP1:
Well, I haven't done it yet...
How are your receptacles wired in NZ? Here, older stuff has two wires - hot (which I presume is equivalent to your 'phase') and neutral, which is indeed connected to ground, but which carries current when the circuit is live. I wasn't talking about either of those. The neutral is formally known as the "grounded conductor", and the hot as the "ungrounded conductor", but those terms really don't have much life outside the codebooks.
I was talking about the third "equipment ground conductor", which is often bare, and provides a way to bond all metal-cased equipment to earth. This conductor does not carry current except on such occasions that some device has shorted to its case. My pack has a metal case, hence I was planning to add a 3-prong plug, and bond the case to the grounding prong.
If I don't bond the case, then I could possibly have a situation where a primary-circuit wire comes in contact with the metal case, and I could perhaps get 120v across my rails. I expect the Darlington would burn out rather quickly, but I'd still have 120v on the case, which could potentially be a problem if the case isn't grounded. That's what I am trying to protect against.
I know that if some hack wired the receptacle ground to hot or (more likely) neutral, I could get line voltage on the case, but that's a problem with any grounded appliance. I wouldn't put it past the drunken monkeys who apparently rewired this house in the 1930s, but I've finally undone their dirty work.
I have run into cross-wired outlets like the one you mention. In fact, my whole 2nd floor was that way, because some baboon mixed up color codes on the main feed to the SINGLE circuit that lit 75% of the house...sigh.
I certainly would not connect the neutral to any appliance case. I only know of one circumstance where this can be done safely - large appliances such as ranges or dryers in older houses were done this way, and it was allowed with a host of limitations. Even in this case I prefer to add a secondary bare ground to a nearby water pipe to existing installations, just in case the neutral connection should fail.
Does that make you feel a bit better? :)
Cordially yours: Gerard P. President, a box of track and a solid-state Ampack.

If the controller case is connected to the ground wire of the three wire mains lead and the power transistor case is bolted to the heat-sink whick is bolted to the case, then one rail is connected via the reversing switch to whatever the ground wire is wired to!
Sure, we call those "phase", "neutral" and "ground/earth".
True, but what is the ground pin/socket connected to??? (you don't know that unless you rewire your own house - mistakes are made)
Yup, I was shocked that my wife hadn't been shocked in years of doing the ironing!
Err, I was building a computer and discovered the powersupply case was at 110v with 230/240 volts input. It's still an appreciable belt!

All power outlets have three wires: - Phase = 240 volts. - Neutral = the centre wire of three phase 415 volts. - Ground = theoretically linked to a metal pipe driven one meter (?) into the ground.
I'm with you there - except that I've discovered two power points over the years where someone has wired the power outlets with other than the correct connections! Once with phase on the ground pin where my wife did the ironing and once with neutral on ground. Phase and neutral swapped is more common. Another fault I discovered was in an office building - the grounds from two seperate circuits were about 10 volts apart. A computer went into one and the monitor into the other - that left 10 volts to travel down the monitor cable at zero resistance!
With the controller case tied to the ground wire a transformer fault will short and take out the fuse as you say. With the plug or wall socket incorrectly wired the case becomes live - you'll spot that the hard way and begin to understand the concept of double insulation ;-)
Isolate the transistor from the case using the standard mica washer and nylon nut/bolt insulators - it's only about 20c worth and worth the effort to keep mains off the tracks.
Yes - until you take your controller to an ehibition or something and find evidence of another Micky-Mouse electrician. Extension cords are another trap!
Regards, Greg.P.

Ahhh, there you open another 'can of worms'.
ACCORDING TO CODE. The 'Ground' conductor travels back to the 'distribution' box, past the 'MAIN Breaker/ Fuse/ Disconnect/ Whatever' past the 'Meter' back out to the 'Incoming Service Connection', back to the Utility Service, they may or may not have a reasonable 'Earth' ground available, and YOU are specifficly prohbited from applying an 'Earth Ground' to the conductor.
However, all is NOT lost in the 'Can of worms' known as the 'Electrical Code'. Furthar perusal reveals that IF your building is equipped with 'Lightning Rods', and the 'ground conductor' for said 'lightning rod' pases within 5' or 6' (some not too unreasonable distance) of your 'house system wiring', you MUST bond the ground conductors together. So I have my permission, no mandate, to install a 'known good to me' "Earth Ground", and GROUND the 'ground buss' in the 'distribution panel'.
I have this thing, I want to know that that little round 'ground' hole in that outlet plate on the wall, is ACTUALLY going to a GOOD 'ground'.
Just one of my little quirks.
Yup!! Especially when it bites you unexpectedly. When you are LOOKING for problems, and checking things out, it ain't all that bad. Chuck

I always have trouble getting them back in!
Ouch!
Ok, but if you carry your power supply to an exhibition/friend's house/kindergarten or connect it via an extension cord, your little quirk isn't satiated!
I think at the time I was really not expecting 110 volts! I had this !@#$%^ concept in my head that the case was nicely grounded via that third wire, the known fixed earth wiring and a length of galvanized pipe hammered into the ground outside the window.
Regards, Greg.P.
Greg.P.

As drawn your circuit looks good to me. I see no shock hazard. Here in North America, connecting metal casework to the third (green) ground wire is good practice. Should a wiring fault short the case to power, the ground will pull enough current to pop the branch circuit breaker. I know nothing about overseas electrical practices, they may be quite different. Your transistor is running cool enough so long as you can touch it with the back of your finger for the count of ten. Worst case heating will occur around half throttle. At full throttle the transistor doesn't drop any voltage across it, at min throttle it doesn't pass much current. Back of finger (or hand) is considerably more heat sensitive than the palm. If it's running hotter than you like, make the heat sink bigger. The heat sink is more effective when the collector is directly fastened to it. The insulating washers are insulators of heat as well as of electricity. The heat sink is also somewhat more effective when the transistor is torqued down good and tight, tight as you can go short of stripping the screws.
Does they circuit develop a full 12 volts at the track at full throttle (and full wave) ? Or is it a volt and a half low from the double diode drop in the Darlington transistor? Just curious, doesn't matter much, especially as many HO locomotives will do 200 scale miles per hour at 12 volts.
David Starr

CD:
There's a little more to 120/240V 1ph US service than that. The house is fed by a center-tapped transformer , with the center-tap connected to the power-company neutral, and grounded to earth at many places, including the house. Both the "neutral" (ground/ed/ conductor) and "ground" (equipment ground/ing/ conductor) are connected to this earth ground, and this tends to confuse people - if they both go to the same thing, how are they any different?
The answer is in where the current goes. Current does, obviously, return along the neutral. At the panel, it partly or wholly cancels out with return currents from the opposite bus. Some current also returns to the transformer center-tap, and some flows to ground - just what proportion depends on the comparative resistances of the earth ground and the PoCo neutral. Break the neutral, and the current has no choice but to go through anything or anybody that happens to be in contact with it and with the ground. Touch a bare neutral, and you end up being in a parallel resistance network with the rest of it.
The ground does not normally carry current. It keeps things that might accidentally become energized from being above ground potential. If you are standing on the wet basement floor, and touch your dryer's case, which is in contact with a broken 110v conductor, you might well get a shock. If the case has a good ground, you can't, because the case is at the same potential as the ground underfoot, or only slightly different. Actually, though, even this would only happen in a few unusual cases; the heavy short-circuit current would have immediately blown the fuse or tripped the breaker as the broken wire touched grounded metal.
Even on older systems, where there is no equipment grounding conductor in the branch circuits, and the panel only has a neutral bar, that neutral bar will be connected to earth ground, so yes, this ground should be there, regardless of any lightning rods.
I hope we do realize, BTW, that the use of the words "ground", "earth", "lightning", and "code" carries heavy net.risk, for reasons that will be clear to any net.veterans of electrical-discussion groups.
Now, Greg, you're quite right about Mickey Mouse, but what am I supposed to do? I'm weighing the risk of a hardware failure vs.a meatware failure.
I can insulate the collector and eliminate some risk of 120v rails, but if I don't ground the case, and one of the 120v wires inside breaks loose, I can still get 120v on the case. Zap.
If I do insulate the case, and Mickey Mouse wires the ground to hot, then the case is live, whether the collector is insulated or not. Zap.
What I think I'll do is ground the case and then insulate the collector. That way, even if the case is energized, I'm not one unexpectedly robust transistor away from running 120v through my small-gauge feeders and setting something on fire. This would also keep the pack from functioning if plugged into Mickey's handiwork. Fair enough?
Cordially yours: Gerard P. President, a box of track and a solid-state Ampack.

DS:
Some of the discussion is leading me to consider insulating the collector. At the moment, there's no pressing need. For one thing, I wired the basement and tested it myself, and for another thing, all my 120v receptacles down there are GFCI protected. However, when and if I do insulate it, I will most likely insulate the heat sink from the case, and leave Q1 right against the aluminum.
The circuit does develop full 12V on the 10 ohm load, and about 15v when unloaded (well, loaded with the voltmeter's internal resistance, to be fair). It might be a little off 12v, since I was reading from the 0-50V scale on my cheapo analog multimeter, my good digital one being currently AWOL. It's probably within .5 V, anyway.
I measured close to 18 VAC at the transformer, so this seems about right - one silicon diode plus two more drops in Q1 would bring the output down near 15 V.
Cordially yours: Gerard P. President, a box of track and a solid-state Ampack.

Unless T1 is faulty or you have bare wires on the 120V side. You can not get any high voltage to the case in USA. The green ground were is there to ensure that the case is not hot.

That's the correct answer.
It sounds fine to me - just check the case voltage any time you plug-in the controller away from known outlets or using strange extension cords. Alternatively, mechanically isolate the trafo from the case and ground the trafo, leaving the case double insulated.
"Earth", "ground" and "neutral" are all different here in New Zealand.
Greg.P. NZ

snip
This is correct, and 'assuming' that 'all is well', tis is a safe way for things to work. Unfortunately, "Murphy" is alive and well!! and 'Doing his thing'.
Right!!!! IF things are working 'as advertised. I have been in too many homes where I could 'feel the fur' with the backs of my fingers on refrigerator 'chrome trim' (Leakage current, probably in the neighborhood of 50 microamps or therabouts. Nothing potentially fatal, but something THAT ISN'T SUPPOSED TO BE THERE.) for me to be all that confident in PoCo grounding practices.
That's the problem right there!!! It SHOULD be, but sometimes isn't, and the 'CODE' tries to prevent the 'end user' from making any provision to alleviate the problem.
so yes, this ground should be Right!!! Ain't "English" great!!, So easy to be ambiguous and convoluted.
more snip Chuck D.

CD:
Actually, the NEC doesn't prevent the end-user from doing his own work -- it makes no mention of who does the work, only that it must be acceptable to the "authority having jurisdiction" (there's another twenty-dollar term you don't see apart from the codes). Anything not specifically forbidden by NEC is allowed by NEC.
However, some shortsighted "authorities having jurisdiction" DO forbid the end-user from doing anything. Stupid. Fortunately my own city isn't one...I can wire my own home with a permit & inspection.
ObMRF: Including layout wiring. However, I'm not sure if this includes the GFCI-protected extension cord I intend to attach to my current layout, since it's basically furniture, not structure.
CD:
Convolution and ambiguity, check, but this is a very specific case. Do a quick Usenet-archive search for the four words I mentioned, and you'll see exactly why I only type those four words with fear and trepidation.
Cordially yours: Gerard P. President, a box of track and a solid-state Ampack.

The transistor is driven off the low voltage secondary of the transformer. Secondary voltage is low enough that it is safe to touch. After all you touch your track with power on and don't get a shock. That's 'cause people don't feel low voltages. In fact, if this was not true, then every kid who ever had an electric train would suffer fatal electric shock soon he put the train on the tracks with power on.
I wouldn't worry much about that fault. Someone will get a jolt off the case of something before too long and a real electrician will be called in to fix the problem.
You can, but I wouldn't bother. Your transformer is what keeps 120 VAC off the track. Long as the transformer isn't shorted primary to secondary, there is no path from the line to the track. If I really wanted to improve the safety, I'd put a fuse in the transformer primary so the unreasonable current draw from a primary to secondary short would pop the fuse. The other effective safety step would be to run the train room off a ground fault interrupter (GFI). That's a $25 dollar gadget that replaces a wall receptacle. It has a clever circuit that compares current out to current back in and if they don't match, it kills power. The theory is, current that fails to return to the receptable is going to ground some other way, possibly thru a person. Code now requires GFI protection in bathrooms, so the devices are readily obtainable.
David Starr

Good article! Thanks, i'll archive this mod for later...