Rheostats And N Scale

[peteski]

The buzz is probably coming from rectified but unfiltered DC from the throttle feeding the Troller.  Installing a filter cap would most likely quiet it down.

Most older throttles have no filter caps on the DC output - only pulsing DC.  That results in a 120Hz ripple.  If the throttle is capable of operating in pulse mode, then one half of the bridge rectifier is disconnected, resulting in a 60Hz pulsing DC.

[nkalanaga]

One possible problem with running a throttle designed for AC inout off DC is that some circuits require the voltage to go completely to zero in order to work right.  These often use "half-wave" rectified AC, and a maybe a filter capacitor at the output end, to make sure it does.  If the DC input has any filtering, it won't go to zero, and the results could be unpredictable.  In some cases the throttle circuit refuses to turn off.  The ones I remember reading about, years ago, used something called an "SCR", which seems to have had some quirks.

[strummer]

At the risk of opening up a hideous can of worms, here we go.
(This whole ugly topic just went through 15 rounds over at Trainboard!)

Max

I just now got around to reading that thread; Jeez, it did indeed get "ugly" at times. I'm so glad everyone here is so polite...

Where did the whole "pulse vs. momentum" thing come from? H*ll, I don't know anything, but I do know there's a difference. One of my Trollers has the momentum feature: it has an on-off switch, so you can use it only if you want. Yet I'll bet the "pulse" is (more or less) "always" on.

In retrospect, this topic garnished a lot more interest than I would have thought....

Mark in Oregon

[Rich_S]

One of my Trollers has the momentum feature: it has an on-off switch, so you can use it only if you want. Yet I'll bet the "pulse" is (more or less) "always" on.

Mark in Oregon

Mark, I'm going by memory and lately I suffer from CRS  but I believe the pulse is only used for slow speed and as the voltage is increased the pulse is eliminated by the internal circuits.

[Doug G.]

The only relationship between pulse power and momentum is that, when momentum is engaged, the momentum circuitry has to enable pulses at low speeds as the voltage is increased gradually and then let the pulses reduce as voltage is further increased.

Doug

[strummer]

Rich
 I think you're correct; My late father-in-law always claimed he also suffered from "CRS". I miss him.

Doug 
 That's what I've always thought, too...

Mark in Oregon

[peteski]

If I do a 16VAC input into my old CAMA, it's every bit as buzzy as one of the Troller packs.    I never run it at full power.   Much below 5V, something happens somewhere where it simply doesn't work at all, but above that, it's nice and quiet.

It's particularly sensitive on the Kato 11-105 chassis with those pager-size 12v motors; those will almost hop up and down on higher power settings they are vibrating on the pulses, I have to turn the power way, way, down to run those, but when I do, they are nice and quiet where all you hear is the railjoints clicking.

I built my TAT V not too long after the construction article came out in April 1988 MR magazine.  I didn't utilize the walkaround feature or the brake lever. I stuffed the electronics into a gutted MRC throttle case.







To me this throttle has all I will ever need for running DC models.  MIN and MAX voltage adjustment, AC/DC compensation and fully adjustable pulse injection at low speeds (where you control both the frequency and width of the pulses).  That way you can fine-tune the throttle to pretty much any motor.

A good portion of the original construction article (describing the throttle's full functionality) is available here .

[mmagliaro]

I think the old Heathkit RP-1065 model railroad throttle was an implementation of the
TAT IV, but with update components that are easier to get.  At least, it appears to have all the same features.

I actually built and used an RP-1065 for a few years.   It did work great.  But I found that
fussing around with starting voltage, pulse wave forms, and pulse cut-out point, etc, was more fussing
than I wanted to do.  It did make it possible to tune the throttle precisely to any engine and make it
run amazingly well.

Here's the patent for it, complete with schematics and a parts list.  Have at it!

http://www.google.com/patents/US3994237

The MPSA20 can supposedly be replaced by the ibiquitous 2N3904
The MJE-181 = NTE184
2N4121 = NTE159

The LM3900 quad op amp can still be purchased.

So there ya go.   Everything in the Heathkit version can be had.

[peteski]

TAT V (as the name implies) is a bit more advanced than TAT IV.  The circuit is slightly simpler and it uses a FET output transistor instead of bipolar.

[strummer]

I think the reason it helps to put a transistor throttle after the DC outputs of a pack and then turn the pack's output down,
is simply that you lower everything.  The maximum DC output voltage is lower.  Any spikes or pulses generated by
the transistor throttle (which might be "full throttle DC pulses") will be of a lower voltage, and of course, you get
better control range on your throttle knob because full throttle is a lower speed.

Mark, you have nothing to lose by trying that Troller hand-held on the DC outputs from a power pack.  The worst thing
that could happen is that it just won't work.  If it is designed to be connected to AC terminals, then it has to
have a bridge rectifier on its input, or at least a diode (that would be really cheap, though!)   

The buzzy thing Randy mentions gives me pause.  I am not a fan of any throttle that makes a lot of buzzing in the motors.
Gentle, half-wave rectified pulses make for a little humming at really low speeds, but that's really all you need with a decent
motor.   Buzzing comes from motor vibration caused by the pulses.

I sure wish I could see a schematic for that Troller.   Now I'm curious.

OK, I just tried connecting the hand-held (Troller TransCab 2000) to the DC outputs, and sure enough, it worked just fine. The instructions that came with the Troller pack(s) I have say to hook this unit up to the AC outputs, but it seems to work just fine on DC. As you said, Max, because I can control the amount of output "current" with the power pack itself, I can fine tune and limit the available juice that will ultimately go to the track. But how is this possible? I mean, this little unit can rectify AC current if it needs to, but then allow DC to pass through it "on command", if you will?

Oh, and I'm not detecting any "buzz", either, and I tried several different engines.

Mark in Oregon

[mmagliaro]

Mark,
If you run DC into a bridge rectifier, you just get the same DC out the other side, minus about 1.4 volts because of the fixed drop in the rectifier diodes.   But otherwise, it doesn't matter.  And in fact, if you hook up the DC on the opposite terminals, it still doesn't matter.  The rectifier will flip the polarity around and the
correct DC will come out the other side.

You can see this if you look at a diagram of a standard 4-diode bridge rectifier and trace the paths from
the input "+" and "-" to the output.

And if you hook up AC, the rectifier actually rectifies and you get DC out the other side.

In other words, putting a full-wave bridge rectifier on the input of any DC circuit is a cheap, easy way to protect it and allow it to work on any AC or DC input, and to even allow the input leads to be hooked up either way.  Like I said, the only thing you lose is a little voltage drop through the diodes, but there is usually way more than we need anyway.

Oh... and you are not limiting the outpu tcurrent with that power pack.  You are limiting the voltage.
The only thing that controls the current is your loco's motor,  or possibly the Troller throttle if it has some current limiting designed into it.  But usually transistor throttles are voltage regulating devices.  They provide a steady output voltage to the motor, regardless of the load, which is why they are better at keeping the engine speed steady.  Throttles like the TAT IV/V go one step further.  They have compensation circuits in them to actually increase the output voltage when the load goes up, so that the engine doesn't slow down as much, for example, on a hill.

[strummer]

I suppose the reason why it was specified to use the AC side was to allow for the DC side to be used at the same time, making for, if you will, 2 control units in one.
Thank you for the explanation, and clarification. I've been messing around with this stuff since the late '60's, but have not been clear on this  aspect of the hobby. It just goes to show that, regardless how much you think you may know about a given subject, there's always something new to be learned. I appreciate all the input, from everyone.

Mark in Oregon

[randgust]

It's only taken like six years here, and finally somebody actually tried this!  Thank YOU.

[peteski]

If someone wants to send me their Troller unit I will gladly reverse-engineer it for you and provide a schematic diagram to all the interested parties. Unless of course the circuit is encapsulated in potting compound. 

[strummer]

It's only taken like six years here, and finally somebody actually tried this!  Thank YOU.

No, thank you! After all, it was your suggestion. (See how polite we all are).

 I never would have thought this would work at all, much less, in some ways, actually work better...I love to learn something new!

Mark in Oregon