Rheostats And N Scale

[strummer]

 I recently had the chance to pick up an old (pre zip code) MRC "Ampack". I had had one of these 'way back when I first got into model railroading; it was a nice unit, and gave me years of service. It is, of course, long gone.
 This "new" one is in pristine condition, and is beautiful in its simplicity, so I thought, "why not? I can always use it for something. What could go wrong?" Well, I found out.
 I hooked this baby up to a test track and opened the throttle: ZOOM! away flew whatever engine I tried. Except, of course, the one Rapido unit I still have from the "dark ages". THAT runs really well with this unit. Everything else is either full "off" or full speed. Why? Is there a "voltage leak", or maybe the rheostat is faulty?
  I did some internet research, and found out that apparently, the newer (post '70s, I guess), engines take a whole lot less current to run, so are not compatible with this old, rheostat-controlled unit. That's kinda what I suspected was going on here.
  It didn't occur to me that the newer controllers I use have entirely different circuits and components, although it certainly makes sense.

 Just wanted to share this experience with the gang...

Mark in Oregon

[Doug G.]

Did they make both an HO and N scale version of the Ampack like with the Throttlepack? I can't remember.

Regardless, yes, it's because a Rheostat wastes more current the more current is drawn across it so newer motors, which draw less current, will cause the rheostat to not drop the voltage as much at any given setting and... your result. If you were to double head a couple of locos, you would see they would be able to go slower than with one as they would cause a greater voltage drop across the rheostat.

You can put a fixed resistor across the output terminals to draw more current and it would do the same as multiple locos.

Oh yes, modern power pack circuitry directly controls voltage to the track and, so, are not dependent on the actual current draw of any given locomotive motor.

Doug

[randgust]

Rheostats don't control voltage, really, they throw additional current in via resistance that lowers the voltage.   So you have to have enough resistance in there to get the voltage down.   Normally the motor provides a lot of that resistance, but in N, not much.

The 'simple' answer is to get some Radio Shack big honkin' 10W resistors and put them across the terminals, and make sure you put them someplace where they can safely throw off a lot of heat.   May even need a 25W on an old HO pack.    Old school, but it works.

I have an old HO Dual Loco Pack (full amp per throttle)  that powers my layout; it has banks of resistors with toggle switches to adjust things, and even a second rheostat in series so I can get the control range up where I want it to be.   For the most part, all it does anymore is to supply power to the hand-held transistor throttles.  But yeah, it throws off some heat with those additional resistors, watch where you put them.

You can sometimes find the old MRC taper-wound rheostats on Ebay, in the square gold boxes.   If you have an HO pack, put another one of these in series with the track and turn it up, you'll usually get the control range back to comfortable levels.

[strummer]

I'm guessing that, since this particular Ampack is "pre zip code", (which is to say 1963-ish), N scale probably hardly existed at that time. And the box does say "H.O.", so there's no way this will be of any use, other than using the AC side for a hand-held or whatever.

Really, my interest in buying it was to gain a non pulse power source, as I still read where using a pulse power pack might/can damage these motors, and I have been using Troller "Autopulse" packs for some time now. I fact, a lot of you may remember my asking about this very thing in the old Atlas forum...

Mark in Oregon

[mmagliaro]

Don't try to use that pack.  It's not worth it.  Doug and Randy have already pretty much covered why.
I wouild not try to add big power resistors.  That is just patching over a fundamental problem with using
resistance-based throttles in general, and the problem will just happen in other ways when you use
other types of engines, or multiple-lashups.

Here's what's wrong, and here's why you really shouldn't try to use rheostat based throttles anymore.

A rheostat is nothing but a resistor. 
That big rheostat drops voltage proportional to how much current is being drawn.
So with a loco motor that draw 0.5 amp, at half throttle, let's say it drops about 6v.   
( Those are pretty typical numbers, by the way. In the old days, it wasn't uncommon for loco motors to draw 1/4 to 1/2 amp. )

So if 12 volts is the maximum output, and the rheostat drops 6 at half throttle, in this case then there will be
6 volts remaining.  That means 6 volts across the motor and it will run at a moderate speed.

To really understand why it won't work:
This is really all about Ohm's Law, which I quietly suggest everyone become acquainted with because it is so simple and yet
goes a long way to understand throttles, LEDs, resistors, and a whole boat load of other issues that come up on model railroads  over and over.

V = I x R     
Voltage = current x resistance

If 0.5 amp through that rheostat drops 6 volts, that means:
6 = 0.5 x R

Rearranging:
R = 6 / 0.5  =  12 ohms

Okay, so at half throttle, that rheostat is a 12 ohm resistor.

Now, what happened to your newer engine?
Your new engine draws, maybe, 0.1 amp.    Engines don't use nearly as much current as they used to.

V = 0.1 x 12  = 1.2 volts

So now at half throttle, that rheostat drops only 1.2 volts.     12 volts comes into it, it drops only 1.2, so the output
is 12 - 1.2 = 10.8 volts.

There is 10.8 volts across your motor, and it will race away at practically full speed.
-------------------------------------------------------

Putting power resistors in there to knock down more voltage would "fix" it, until you run some other kind of engine
that draws more or less current.  Then it will run too fast or too slow.

-------------------------------------------------------

The major difference between old rheostat throttles and more modern "transistor" or "solid state" throttles (if those terms
can be considered "modern"  ha ha ha!)  is that the transistor throttle outputs the same voltage at a given
throttle setting, regardless of the load on it.    Rheostats control the voltage in a way, as we can see above,
that is proportional to the current passing through them, so they change their behavior based on how much load your
engine puts in them.

That's why they really are a lousy way to control a loco these days.

[strummer]

Sooo....

If I need a constant DC source for my Aristo Craft wireless remote, would this be better than using one of the Troller units, which are pulse power? Or does that even matter?

That's where I'm really going with all this....(see my post from May 31st)

Mark in Oregon

[mmagliaro]

I just saw the new post with the remark about "non pulse".

"Really, my interest in buying it was to gain a non pulse power source, as I still read where using a pulse power pack might/can damage these motors"

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!)

There was a dark time in the 1970s and 80s when some throttles were made with specially designed pulse generators
that put out square, sawtooth, and other high magnitude nasty pulses
that were brutal and could be hard on small motors (heat, melting, pitted armatures, etc).

Many of us could write a book on this.  I will leave it here and try not to.
Pulses are not evil.  They are, in fact, still necessary to make motors run their best at low speed, even today.
It's all about the *kind* of pulses you use.

One would have to put an oscilloscope on your Troller Autopulse throttle's output to see what kind of pulses
it really puts out, in order to know if it might be harmful.

Personally, I use a home-made transistor throttle from the old Peter Thorne Practical Electronic Projects for Model Railroaders.  It puts half-wave pulses on top of the DC, and it works great.  Hands-down any engine I have, including all the super-powered coreless ones, runs better with that gentle pulse turned on.  There is no heat, minimal noise,
and better low-speed running.  I have sat engines on the track and creeped them at sub 1 mph speeds for 10 minutes, 20 minutes, even more with no heat and no damage.

If your engine's motor doesn't seem abnormally hot after you run it for a while, that throttle is probably fine.  But I know if it were me, I'd want to put an oscilloscope on it to see what it's really doing.

[mmagliaro]

Sooo....

If I need a constant DC source for my Aristo Craft wireless remote, would this be better than using one of the Troller units, which are pulse power? Or does that even matter?

That's where I'm really going with all this....(see my post from May 31st)

Mark in Oregon

Now I'm completely confused.   What does this "constant DC source" do?   Do you hook it to the track?  Hook it up as the power supply to the Aristo Craft unit?  I don't have one of those.   Can you post their instruction sheets or wiring diagrams?

[strummer]

Max

You connect the "base unit" ("receiver", it's called) to the DC terminals of your power pack. Hook up two wires from it (the receiver) to the track. Turn up the power pack to "full power setting". (I usually only go up about half-way.) Then you use the hand held wireless remote ("transmitter", it's called) to run the trains.

It works really well; my only concern is that the instructions, naturally enough, do not say anything about not using a pulse-type power source, which my Troller units are. So I thought if I had a chance to obtain a non-pulse unit, I'd at least be able to compare the two types of sources, and see if one is better than the other. Obviously, the pulse power seems a bit "smoother", but I've always been concerned about the use of pulse power with N scale, be it "direct" pack to track, or with with remote unit...

I know this is all confusing, and I probably shouldn't have posted any of this in the first place.

Mark in Oregon

[peteski]

I'm so glad that I'm at home with electronic theory and practice. 

I suspect that the DC power supply he is speaking of is simply to supply DC power to whatever the Aristo Craft wireless remote thing is.  It would probably be happy with a filtered (or maybe unfiltered) 12-15V DC.

Most older throttles output non-filtered DC to the track terminal.  The Aristo Craft wireless remote manual should specify whether it needs filtered or unfiltered DC supply.  If Aristo Craft wireless remote has its own filter cap then any old throttle will do. If not, then a filtered DC source is needed.  That could be as simple as just adding a filter capacitor between positive and negative outputs of the throttle.

I guess at this point we need more info about the power requirement of the Aristo Craft wireless remote.

[strummer]

 The instructions do not mention anything regarding "filtered" or "unfiltered". It just says "connect to terminals on Power Supply source. Power supply terminal usually reads DC Track Power. Do Not Connect To Any AC Terminals". ";  "...turn to ON position DC track Power Source (and run control knob to Full power setting if applicable)."

That's all it says, as far as "power source" type stuff goes. Like I say, it works quite well; I was just wondering if using pulse is a problem; is "pulse" the same as "filtered"?

Mark in Oregon

PS:  I promise this will be the last time I ask about this kind of thing! 

[mmagliaro]

Mark,
No need to avoid the subject.  Although we grunt and groan about some of these topics, like pulse, that come up
again and again, it's good to keep hashing over them so people can learn.

From what you describe, it sounds like that Aristocraft base unit doesn't care in the least what the input
is as long as it's DC.  It probably just passes it straight through to the track.

I assume that the wireless handheld just controls what ends up on the track.  So if you put 3 engines on the track,
they will all move when you work the handheld, right?  (i.e. there's no wireless thing actually put inside the engines).

If it works, I wouldn't even bother changing it.    Something to keep in mind.  The Troller has a pulse, you say.  But does
it have a feature where the pulses are gradually reduced as you turn up the throttle?  Some pulse throttles work that way.
If it does, then by the time you turn it halfway up, it pretty much IS a "non-pulse" output anyway.
I still say, if your motors don't get hot and the remote works, I'd just leave it alone.

[strummer]

Fair enough....I've never had any problems per se, but you know how we like to fret about such things.......

The Ampack will go to the next Goodwill run, and will continue to use (and enjoy) the Troller units.

I thank everyone who took the time to "chime in".

Mark in Oregon

[peteski]

I'm surprised it doesn't mention DC polarity. Unless it has a protection diode on its DC input, it could be damaged if the the power is wired with reversed polarity,.  But since they don't mention that, I assume ii it protected from being hooked up backwards.  It probably just won't turn on until it is properly hooked up.

[strummer]

It does mention that it doesn't matter how it's hooked up, and you are correct: it will not turn on until the polarity is correct.

Good catch....

Mark in Oregon