Rheostats And N Scale

[peteski]

Ah, so it has a built in rectifier.  That is the only way to make it not sensitive to DC polarity. That makes is sort-of compatible with AC.  If you could open the unit and install a filter capacitor then you could power it from any AC or DC power pack.  I suspect that there is a small filter cap already installed in there.

[Rich_S]

A rheostat is nothing but a resistor. 

Yes I agree, a rheostat is a wire wound variable 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.

OK, why are you continuing to use 12 ohms in your second calculation? if the motor only draws 0.1 amp and half way down the rheostat is 6 volts, then now according to ohms law, that rheostat would be a 60 ohm resistor?

R = 6 / 0.1  = 60 ohm.

[nickelplate759]

Halfway down the rheostat (in that example) is 12 ohms.  The resistance doesn't change with voltage, only with the position of the rheostat, since
a rheostat is a variable resistor.   Therefore, following Ohm's law, if the resistance doesn't change (12 Ohm), then the voltage has to be a function of the current.

George

[peteski]

Yes I agree, a rheostat is a wire wound variable resistor.

OK, why are you continuing to use 12 ohms in your second calculation? if the motor only draws 0.1 amp and half way down the rheostat is 6 volts, then now according to ohms law, that rheostat would be a 60 ohm resistor?

R = 6 / 0.1  = 60 ohm.

Part of the problem is that Max states that the model engines draw 500mA or 100mA.  But at what voltage?  12V, 6V, 3V?  I know what he is implying: that new motors consume less current than old ones. I think that he simply tried to make the explanation more palatable for average person.

[mmagliaro]

To answer all this:

The resistance at any point on that rheostat is constant.  We figured out in the first motor example that it is 12 ohms at
half throttle.

In answer to this:
OK, why are you continuing to use 12 ohms in your second calculation? if the motor only draws 0.1 amp and half way down the rheostat is 6 volts, then now according to ohms law, that rheostat would be a 60 ohm resistor?

R = 6 / 0.1  = 60 ohm.

No, halfway down the throttle is not 6 volts.   It was only 6 volts for the first motor.  That's the whole point.
The resistance at half throttle is constant.  The voltage is not.
In other words, the "R" in your equation is not a variable.  It is 12.
The current is also not variable.  It is 0.1 amp.   The thing that varies is the voltage
that drops across the rheostat.
12 = V / 0.1   V = 1.2

And in fact, any motor with a different current draw will cause a different amount of voltage to be dropped by
the rheostat.

Peteski: Yes, I am slightly glossing over this in that motors don't draw the same current at every voltage.
But for the purposes of comparing an HO motor of "yesteryear" that draw approximately 0.5 amp, and an N Scale
motor of today that draws approximately 0.1 amp,  we can ignore that fact. 

[nkalanaga]

If you could find one of the old power packs that used a variable transformer before the rectifier, instead of a rheostat after it, you'd be set.  In the 50s those were considered the high-end models, but then transistors replaced them.

[peteski]

If you could find one of the old power packs that used a variable transformer before the rectifier, instead of a rheostat after it, you'd be set.  In the 50s those were considered the high-end models, but then transistors replaced them.

That's right!  I had one of those packs back in Poland (it was made by PIKO).  Since they were able to supply full rated current at any selected voltage, they worked quite well.   I liked the action of the speed control knob - it was off in the center, then turning it left or right gave you forward or reverse direction. Plus when turning the knob you could feel the wiper rubbing the wires in the transformer winding. That pack used a Selenium rectifier.  Ah, the old days....   

[Rich_S]

To answer all this:

The resistance at any point on that rheostat is constant.  We figured out in the first motor example that it is 12 ohms at
half throttle.

In answer to this:
OK, why are you continuing to use 12 ohms in your second calculation? if the motor only draws 0.1 amp and half way down the rheostat is 6 volts, then now according to ohms law, that rheostat would be a 60 ohm resistor?

R = 6 / 0.1  = 60 ohm.

No, halfway down the throttle is not 6 volts.   It was only 6 volts for the first motor.  That's the whole point.
The resistance at half throttle is constant.  The voltage is not.
In other words, the "R" in your equation is not a variable.  It is 12.
The current is also not variable.  It is 0.1 amp.   The thing that varies is the voltage
that drops across the rheostat.
12 = V / 0.1   V = 1.2

And in fact, any motor with a different current draw will cause a different amount of voltage to be dropped by
the rheostat.

Well actually current draw is variable. Your 0.1 amp motor might draw that when starting I don't know, but I do know that as the mass comes up to speed the current draw will fall off. After re-reading your post this morning, I understand what you are doing. I would have read the rheostat with a meter to see what it's value was at it's mid point, but it's the same difference as your example.

[randgust]

At the risk of perpetuating thread drift here.....

While I'm a DC guy, I abandoned pure rheostat control a long time ago for transistor throttles.   Way back in the 70's I discovered that the ones I had, rated for 12VDC or 16VAC as input, did create a lot of motor racket and heat.  Old news.   I'd put one on the output rheostat of the HO throttlepack instead of the 12VDC accessory, and discovered something rather amazing purely by chance.   If you throttled down the rheostat to knock down the input voltage to the throttle, the transistor throttle got much quieter and the heating problem went away.   Presumably, that was because the maximum voltage spikes were significantly lower, not jerking the motor around, and it became more of a blended power.

I changed all transistor throttles to have a rheostat-controlled DC input on the panel, so you could 'set' the power level into the throttle, very low for a single-unit small switcher (think Bachmann 44-ton), and cranked up reasonably high for an ABBA set of Kato F-units pulling a heavy train.

The real benefit phenomenon that I discovered early on was that the DC transistor throttles and the voltage pulses 'synchronized' DC motors in multiple unit situations in a highly effective manner.   Under pure DC controlled by a rheostat, the MU performance particularly under a heavy train load was awful, and also difficult to control as you crested up a grade and headed downhill; the whole ohms law thing and the resistance of the train itself changed.  With the transistor throttles, the performance of multiple units was rather amazing to watch as they'd all start and slip together, same units, same train.   You can make all the claims you want about motor damage, I've never seen it, and I have units that have been on this program now for 25 years with no ill effects, but I am knocking down the input power to the throttles so that I don't get any more than I actually need to run the train.

On the Hickory Valley, which is still pure DC, I've had similar interesting results with 3.5v pager motors and on-board 100-ohm resistors.   Loading a locomotive up a 4.5% grade to full slip with absolutely cook a small resistor to plastic-melting temperatures and is nearly uncontrollable, even with a high-quality motor like a Faulhaber.    Solution 1 was to ditch the 3.5v motors for Kato 12v motors; solution 2 was to go to 3.5v gearheads that have almost-constant draw and absurd torque for their tiny size.  Those are controllable and draw so little current the series resistor doesn't overheat.   Even on DC with a rheostat control, you can 'set it and forget it', everything stays constant and cool.

I now have a digitrax Zephyr at least for testing purposes on my custom work, and while intriguing, doesn't give me a bit

[strummer]

Interesting...so you hook up your throttle(s) to the DC out on your power source, and have had this kind of good performance?

The reason I ask (and I know I said I wouldn't post anymore about this!) is I have a Troller hand-held which gets connected to the power pack, but to the AC outputs. It is a simple little box with nothing more, it seems, than a direction switch and the dail-type throttle itself. Is it possible that, (although it works fine "as is"), it would work better if connected to the DC outputs? I think that's what you say you've been doing...?

Mark in Oregon

[randgust]

Yep.   And I also worked with those Troller throttles on another friends layouts.   Buzzy little rascals.   Never liked them.  I have my "woodsman's axe" CAMA throttle (which has virtually nothing left original equipment except the knob, I've replaced ever part including the cable and box at least once), and two PSI 550 Cabtrollers that I just love, vintage top-of-the line stuff at the time.   There's something just so cool about having to use a brake handle to stop, and to be able to power-brake against your brake setting.

You can theorize all you want, or you can just set up a test and see what it does.   I think those have on-board rectification, but if you flop the reversing switch it may only work one-way, hard to say.

I do have to admit that when I was messing around starting in N scale, it was kind of refreshing that I was the only person locally doing it, there was no Internet, and I knew I was handy enough that if I wrecked something I'd either replace it or fix it.  So free-range experimentation was the order of the day.  That's the basis for my rather divergent opinions on many issues, I was raised by wolves in the wilderness.

[mmagliaro]

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.

[randgust]

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.

[Doug G.]

I actually used my old Golden Throttlepack 500N I got in 1968 on my layout as Cab B recently while I waited for A Tech II 2500 to arrive and it worked surprisingly well. Some locos were a bit jackrabbity at minimum throttle but I just used pulse and, of course, it allowed them to crawl and without much noise.

Those packs are pretty too although they really should have been called "Coppery Throttlepack 500N".

Doug

[strummer]

Re: Troller Stuff

I have (2) Troller power packs, and the afore mentioned non-powered hand-held, and have never detected the slightest noise ("buzz" or otherwise) from any of 'em. So I dunno what to say, other than that...

Max, I wish I could send you a schematic; I do not have one.

Mark in Oregon