AC Throttle Circuit ?

Do you need/want to insert DC offset voltage to trigger an engine's whistle/bell?

How much power/Watts do you need/want to handle? What kind of overload/protection do you need?

To be clear, you have an incoming power source that is 20V AC??  My memory is not so good any more but I'm not aware of any 20V AC power source.

While I recommend against it because of the noise and switch-on issues, the light dimmer goes on the primary side of the step-down transformer. A better approach (but more expensive) would be to use an Autotransformer which is adjustable from 0-100% or 0-140 volts (not recommended) on the primary side of a step-down transformer. This will feed stable AC power to the track. Of course, you would need to add a horn/whistle diode matrix to the mix and appropriate circuit breakers.

It would probably be less expensive and safer to purchase an MRC PurePower AC power supply.

Hope this helps.

I have to go with Matt, why not just buy a model train transformer?  You can buy any number of post-war transformers like the ZW or KW for peanuts nowadays.

@gunrunnerjohn  I brought a 1 pound bag of peanuts to the last trainshow.   Just came home with shells and an empty bag.

Dan, you should have bought the transformer before you ate the peanuts.

25 years ago, I went down this road, using a triac as the AC power regulation device.  The driver circuit for the triac was non-trivial, because my goal was to make locomotives crawl, even with Pullmor motors.  Basically, it started out with zero-crossing detection on the incoming 24VAC supply, then dividing the frequency in half, and gating the triac drive to control the duty cycle of the triac.  As the track voltage (speed) potentiometer was raised, the conduction angle of the triac for alternate cycles would increase, until that angle reached around 270 degrees, at which point the triac would also conduct on the skipped cycle.  The cutover point for adding energy from the second cycle was determined empirically, for a reasonably smooth throttle-vs-speed relationship.  It worked quite nicely at the breadboard stage - I could run a vintage 2343 NYC F3 at a bare crawl, if desired.  And it had reversing and whistle control capabilities.  However, it didn't play quite as nicely with some newer locomotives using can motors and QSI DCRUs,, which could get confused.  The project was interrupted by divorce and relocation.  It was never re-started, because TMCC had been released by the time my life stabilized and, at that point and the idea of voltage adjustment inside the locomotive was a far better idea than at the track feed point.

Brought up old memories from about 10 years ago. My first attempt at a wired remote control was also triac based. Worked OK, but varied somewhat with different engines and a bit noisy. Finally switched to wireless RF which worked much better, so I stayed with RF.

Maybe you can find an MTH Z-controller. It does exactly what you describe.

Thanks for the replys
@ stan2004  My idea is to build a throttle that can be used with any reasonable power supply; between the supply and the track.  So figure from maybe 18VAC, 4A  up to say 24VAC 10 amp.  Should handle 300W!  That's the goal, anyway.

@AGHRMat, gunrunnerjohn,   Sorry for the confusion, the concept is like a light dimmer, but a motor control.  To be used between the transformer and the train.  Just a throttle!  As for why not just buy a transformer, this is the first part of a more ambitious project that I'll probably flesh out later for y'all.  Can't just buy a transformer.

@KarlDL, BOB WALKERThat's what I'm talking about.   I'm not that good an EE;  more of a CE.  I'm starting to think I'll have to use a microprocessor, zero crossing detector and a pair of MOSFETs.  Sort of both simpler and more complex that trying to get the wave forms that I'm thinking of with an SCR or TRIAC.    I've heard of difficulties with some, earlier, fancy electronic modern locos, but I'd like to hear more, and yes, I'd like to make them them really crawl.

The nice thing about microprocessor control, which, these days can be had for less than $3 per each, is that you could really play with the wave shape in relation to the throttle.  Imagine tiny, full height spikes of the sine wave when just opening the throttle, and different wave / pulse widths increases depending on where you are on the sine curve and the position of the throttle.  Or whatever portion, or even portions, of the wave for a given throttle position. 

Even a slowish microprocessor should be able to divide the wave into microsecond intervals, there being what, over 8,000 microseconds in 1/2 an AC cycle?  Is my math right there?

Thanks all - @KarlDL, BOB WALKER, actual circuits would be really helpful!!

Bruce

Synthesizing an AC waveform ain't quite as easy as you might imagine, been there, done that.   Generating a 300 watt pure waveform will be fairly expensive.  I also think you're being optimistic about slicing in one microsecond intervals.  The good news is, I doubt you have to have that fine a granularity to generate a perfectly usable sine wave.

Not quite clear as to your ability to construct microcontroller (e.g., Arduino) circuits and power electronics.  But if you're still doing your research/homework, I suggest you study the MTH patent for their Z-4000 transformer which synthesizes a sinewave-like waveform that many guys consider the "gold-standard" for such train transformers. It's not quite to the component level but if you're serious it should be readily find-able and has been discussed on OGR.

You didn't answer my question as to the need for whistle/bell DC offset, but if that's not an issue and you just need low-voltage variable AC you should consider home-theater audio amplifiers.  These are peanuts in price but output hundreds of Watts of low-voltage AC power in the audio frequency range (20-20,000 Hz).  Lo and behold 60 Hz is in that range!

Barn Boy, doing all of the control stuff with today's industrial controller µP boards turns the project into more of a software exercise.  It also eliminates some analog tweaks, like the phase trimmer circuit I used ahead of the zero-crossing detector, which had to be set up initially and then checked every now and then, using a 'scope.  I haven't played with any of those controllers, but that seems like a more straightforward approach than the analog/CMOS approach I took.

Decent Class D audio amplifier modules at train-compatible power levels are readily available inexpensively.  Take ordinary AC, sampled from the secondary of a transformer, run it through a resistor, potentiometer, and amplifier, and you're done.  At 60 Hz, the performance nuances are irrelevant, so you can get away with inexpensive Chinese imports if desired.  Even the Danish ICEpower modules aren't that pricey now.  Absent additional electronics, you won't have creeping, cruise, realistic braking, and all that, but the basic functionality is easy if your goal is to run "conventional".

@gunrunnerjohn Yeah, I wasn't thinking of synthesizing, just chopping, and yeah, μs slices is probably only possible in theory, too much capacitance and hysteresis, for starters.
@stan2004  For me the SW for a microcontroller is easier than tricky circuit design.  Yes, I was thinking of using a buck converter and relays to inject a DC signal for whistle, I don't know much about the Z-4000 but I do have a Z-750 I'll be looking at w O'scope.
@KarlDL @stan2004 I did think about using power transistors but not a power amplifier.  Interesting idea,  I'll have to think about that.

There used to be company called All-Trol that made a hand held tethered throttle. This was pre- command control days. I had mine hooked up to a ZW. Never had an issue with it.

Chris

LVHR

@lehighline I guess All-Trol make an AC version?  I seem to remember reading about them in an HO mag maybe in the 70's?  Although it does have a bell and whistle button so I guess it is!  Looks slick!

Found this one on e-bay for only $99 - looks like two, actually

I should add that, if anyone wants to try the class D audio amp idea, be sure to provide adequate heat sinking for the amplifier module.  Because the current draw of a locomotive is continuous and not variable like audio, significant heat can develop, absent adequate heat sinks, and ultimately damage the output switching transistors.  Merely mounting the unit in a medium-sized box, with the heat-conducting surface attached to the box, isn't enough.  The math and all that is beyond the scope of this thread - there are plenty of resources out there on how to get it right.

@KarlDL posted:

...The math and all that is beyond the scope of this thread - there are plenty of resources out there on how to get it right.

What he said.

The overwhelming majority of off-the-shelf Class-D audio amplifiers (as used in modern home theater audio amps) use what's technically referred to as bridge-tied-load amplifiers which means that neither output wire (to the track) is a "solid" ground.  Skipping the techno-speak, this has implications if trying to use the train transformer in a block system where you need to "phase" multiple transformers to maintain a common outer-rail.  Again, this is part-and-parcel of the nuances of using high-frequency chopping.

In your initial post, you presume or pre-suppose "triac" technology to chop the incoming 60 Hz AC voltage.  I would presume that the All-Trol gadget, given its vintage, uses such technology?  So if so, then are you declaring victory and case-closed?

But if as you say you're handy/comfortable with programming micro-controllers (Arduino or otherwise), then in my opinion it makes little sense to deploy such technology to control 60 Hz triac technology!  Obviously micro-controllers shine because they allow you to chop signals at kHz frequencies which have all-kinds of benefits in power-conversion circuits...specifically the ability to use smaller energy-storage (inductor/capacitor) components.  And the microcontroller has the smarts to asymmetrically chop the positive or negative half of the power signal to insert a DC offset for whistle/bell.

In addition to studying the MTH Z-4000 patent, if going the micro-controller route, do some homework into UPS (Uninterruptible Power Source) "pure-sine" inverters.  In other words, these gadgets synthesize remarkably clean 60 Hz AC power using high-frequency pulsing of power transistors.  There is an analogy here.  In the not too distant past you'd buy an inverter for your car that converted 12V DC to 120V AC and the waveform was nothing short of crappy.  Kind of like the crappy triac output waveform.  Marketing called these waveforms "pseudo-sine" or whatever but it was basically crap.

But now you can get inexpensive inverters with remarkably clean compact sine-like 60 Hz 120V AC outputs ... all thanks to high-frequency chopping and micro-controller smarts!  Same technology should get you clean synthesized 60 Hz train track voltages if your forte is in the programming realm.