# Reduce voltage of a DC throttle?



## Never Get Old (Apr 16, 2016)

I know how to solder and I know not to electrocute myself. I know what a resistor and capacitor and diode are and ohm's law, etc. However, I can't say that I really know anything about circuit design and modification.

In N scale, I find that DC throttles always seem to seriously limit the usable range on the knob/pot, sometimes down to just a few degrees between too fast and too slow. I have had four different MRC throttles, all with basically the same result over the years.

My question is, can I use a resistor in series or in parallel with the DC track output to get more usable range on the knob/pot, or is it not that simple? I want finer slow speed control. I never run over scale 30-40 mph on my little coffee table layout and never more than 2 locos together. Even that speed looks fast on my small layout.

The throttle I would test with is an old MRC Tech II although I also have a Tech 7 and a Railpower 1300, all of which are about the same in this regard. Ideas?

-Never Get Old


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## gregc (Apr 25, 2015)

Never Get Old said:


> In N scale, I find that DC throttles always seem to seriously limit the usable range on the knob/pot, sometimes down to just a few degrees between too fast and too slow. I have had four different MRC throttles, all with basically the same result over the years.


what is the track voltage are the ends of the range ("a few degrees") of the knob?

is it the throttle or is the locomotive that sensitive to voltage


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## Never Get Old (Apr 16, 2016)

It's the same problem across all of my locomotives, which includes eight or nine new Atlas, two new Kato, and a few older Life-Like SW9's.

As for the voltage, I'll have to get out my voltmeter tonight. I should test with a locomotive running at my desired speed, right? Should I also measure the maximum output (no locomotive because it would fly off the track).

-Never Get Old


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## gregc (Apr 25, 2015)

how about measuring the max voltage without a locomotive on the track.

and if you're willing, with the locomotive blocked from moving, wheels spinning.


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## Lemonhawk (Sep 24, 2013)

The Tech II has a pulse switch on it. Did it help slow speeds? That was the intent of the pulsing. Maybe it doesn't work too good on N scale. Other than that its - DCC where you can play with the CV's to really fine tune the speed.


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## DonR (Oct 18, 2012)

NGO

The figure you want that is related to running a loco
slowly is the lowest voltage that your power pack
provides. Obviously, the lower voltage the slower
the speed.

You could try a few different 1 or 2 watt resistors in series with
one leg of the track power wires. A resistor would reduce the
voltage to the track but you would have to experiment to
find which resistance does what you want. 

Don


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## gregc (Apr 25, 2015)

adding a resistor will certainly reduce the voltage, but it will reduce voltage proportional to the current being drawn. Things will really slow down going up a grade and speed up going down a grade.


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## tjcruiser (Jan 10, 2010)

Couldn't he add a pair of diodes, wired in parallel, in opposing directions? The diode would offer a voltage drop more-or-less independent of current, and the two in opposing directions would allow current flow in either forward or reverse directions.

If the voltage drop isn't enough, double up the diodes ... two in series forward, wired in parallel to two in series reverse.

Etc.

TJ


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## MichaelE (Mar 7, 2018)

The potentiometers used in those throttle controls might be swapped for another linear taper pot rather than (what I guess is) a pot that is too short of a spread from full on to full off. This might give you the control you need.

It's not a matter of resistors, diodes, or any other component. It's the taper of the pot that is supplying voltage to the locomotive. It sounds as though you need something with a finer taper.

And how do I know this you ask?

It's from designing from scratch a Boeing 727 throttle quadrant to be used with a full sized flight simulator. The taper had to be correct for the amount of throw the actual throttle provided for power to the engines.

I would start with replacing that pot with a 100K and go from there. You might have to experiment with different values to get the results you desire.


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## wvgca (Jan 21, 2013)

just to try it, a group of 1N4000 series diodes would work, [as TJ mentioned],each set or pair would give 0.7V drop, so a group of five [each way] would drop the final 3.5V


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## gregc (Apr 25, 2015)

here's a schematic which i assume is typical of the 3 throttles mentioned. D1 determines the voltage range at the output can be replaced with one with a lower voltage if that's all that's desired.

the circuit shows that some of the unrectified 60Hz transformer voltage is coupled to the output transistor so that there are pulses to help break motor friction and improve low speed performance.

but the OP still hasn't reported the voltage range that the locomotive seems to operate over. So it's not clear in my mind what the problem is.


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## Never Get Old (Apr 16, 2016)

I haven't given up. I have had too many things pop up since I posted to get out my voltmeter and test my two Tech II's. I'll get to it soon I hope. 

-Never Get Old


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## Never Get Old (Apr 16, 2016)

MichaelE said:


> The potentiometers ... It sounds as though you need something with a finer taper.


I'm afraid you might be right. New pots would be a pain in the neck though, plus new knobs that fit them because the originals probably would not.

My goal is that I want to be able to use more than just a small angle range on the knob between what I guesstimate to be 15 scale mph and 45 scale mph. Right now it seems to be about 30 degrees to go from 14 to 45, which is precious little of the roughly 300 degrees available. It would be nice if I could use, say, 3/4 of the available range on the knob.

I'll get out the voltmeter soon and test the voltage at my minimum desired speed and at my maximum desired speed and also test the maximum that the Tech II's put out. After that maybe I'll crack one open and see what's inside. One weighs more than the other, so there must be different transformers inside. That surprised me. I guess I'll have to test both of them.

-Never Get Old


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## Ace (Mar 30, 2016)

tjcruiser said:


> Couldn't he add a pair of diodes, wired in parallel, in opposing directions? The diode would offer a voltage drop more-or-less independent of current, and the two in opposing directions would allow current flow in either forward or reverse directions.
> 
> If the voltage drop isn't enough, double up the diodes ... two in series forward, wired in parallel to two in series reverse.
> 
> ...


That is certainly the easiest simplest best way to modify the existing throttle. Better than using resistors.

One time I had a dual-cab N-scale throttle with a burned-out transformer in one half. I wired the two rheostats in series to give more range of adjustment for one throttle, to avoid jackrabbit starts of some locos.

I've made my own solid-state throttles for HO and N scale locos with an LM317 voltage regulator chip. They give much better control than the old-style rheostat throttles with a relatively simple circuit.


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## Dennis461 (Jan 5, 2018)

*making a silk purse out of a sow's ear*



tjcruiser said:


> Couldn't he add a pair of diodes, wired in parallel, in opposing directions? The diode would offer a voltage drop more-or-less independent of current, and the two in opposing directions would allow current flow in either forward or reverse directions.
> 
> If the voltage drop isn't enough, double up the diodes ... two in series forward, wired in parallel to two in series reverse.
> 
> ...


I do not think diodes will help. Tech 7 is designed using pulse (from 60hz) at low voltage, which fade out at higher voltage. If you force the output to start at a higher potentiometer setting, it will have less pulse action, and the loco would be starting like a non pulse throttle.

I would try looking at the voltage with a scope. It's possible he has a defective throttle, no pulse or dead spot in pot.

A simple PWM throttle will most likely work better than the Tech 7 on N scale, right guys?


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## bluenavigator (Aug 30, 2015)

Dennis461 said:


> A simple PWM throttle will most likely work better than the Tech 7 on N scale, right guys?


Yes and no. PWM is same as pulse mode on the power pack. It is same as analog but with digital means but power packs are not digital. PWM has the pulses that are being changed in the width of the voltage (in square waveform), being off and on per period cycle.

Yes, I agree with the solid state circuit for power drive. It will give out precision flow of the current, which does drive the motor. I can understand why voltage is the other key that drive the motor, which is true, too. As the rheostat changes, its voltage drop changes, too.

When the rheostat is set to zero throttle, the rheostat is set to its full resistance, which causes the voltage drop to increase to almost whole voltage taken, leaving very little voltage drop over the motor, which would cause lowest current flow across both rheostat and motor. 

When the rheostat is set to full throttle, the rheostat is set to its minimum or zero resistance, there is no voltage drop across the rheostat and the motor has the whole voltage drop, which cause full current flow allowed from the power pack.

Hope this makes sense.

The purpose of the LM317 voltage regulator chip is to control the voltage across the motor, eliminating the rheostat. The voltage limiting is being controlled by simple low current potentiometer.


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## bgennette (Jan 1, 2019)

*Re-scaling throttle output suggestions*

It looks like the MRC Tech xxx throttles use a simple potentiometer (Pot) to supply a variable voltage signal to control a separate Power Output circuit. This means that the Power output can be easily changed by just changing the pot. :smilie_daumenpos: As it is before the power section you only need low power components such as one-quarter or one-eighth Watt resistors. 3 resistors and 3 cheap slide switches should cost less than $5.









This diagram shows 3 simple modifications that may be applied to alter the output of a Pot.
R1 halves the value of the Pot; (new R value) = 1/(1/Pot + 1/R1). Used alone it makes no real difference to the operation.
R2 adds a fixed amount of resistance below the Pot resistance; Total R = R2 + Pot. This makes the full anticlockwise position of the Pot become what the output was at about one-quarter of a turn. With R1 also ON output starts from about what was one-third of a turn.
R3 does the same as R2, but on the maximum speed (or full clockwise) part of the Pot control. R3 alone limits top speed to around three quarters of original max, R3 + R1 limits to about two thirds. R2 + R3 gives only the middle of the range while R1 + R2 + R3 narrows the middle range.
Fine tuning of R3 will allow exact 'creep start' voltage setting.

You can 'fine tune' the ranges by changing the values of R2 and R3 from the suggested values . Note that there may (probably will) be an R2 already fitted to set the lowest output level, adding a little more will just move this 'starting' point.

I don't have one of these throttles to experiment on. So please, if someone does have a go please do some Pot resistance and Output Voltage measurements BEFORE and AFTER at fixed throttle positions.

Edit - Ok, thanks to http://www.sumidacrossing.org/Musings/files/140126_MRC_Tech_7_780.php I now know how the throttle works. And its even simpler than I thought!
The DC voltage from the Pot directly controls a large power transistor. There is also a low power AC voltage applied to the transistor to generate pulses, but this is swamped and has little effect after about a quarter turn of the Pot. Earlier versions (in the 1990s) needed to pass the AC pulses through a pre-amplification stage to drive the power transistors that were available then, now days the more robust and easier to drive transistors just need a resistor to absorb extra voltage.

Here's the output at low, mid and full turn of the Pot.









bye.


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## bluenavigator (Aug 30, 2015)

I do have MRC Tech4 280... Dunno if it is worth the trouble to do this. Maybe one day... will see about that, unless someone do have similar Tech series.


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## gregc (Apr 25, 2015)

bgennette said:


> It looks like the MRC Tech xxx throttles use a simple potentiometer (Pot) to supply a variable voltage signal to control a separate Power Output circuit. This means that the Power output can be easily changed by just changing the pot.


your schematic suggests that the throttle uses an emitter follower circuit to provide a relatively constant voltage output. (looks like your using a non-standard symbol for a transistor). Because there is a single diode drop between the base and emitter of the transistor, the transistor passes as much current as needed to maintain the voltage at the emitter at the base voltage - 0.7V. This is the starting point for Linn Westcott's TAT.

the base voltage can be controlled with a pot connected between the voltage source and ground. Under higher loads the supply voltage will vary which is why a the pot may be across a zener.

but i believe your point is to add 2 variable resistors between the pot and ground an the pot and the voltage source to to adjust the minimum and maximum voltage to the pot. These would need to be adjusted for each locomotive.



isn't learning about electronics fun


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## DonR (Oct 18, 2012)

There is no doubt that what you guys are suggesting
would be the preferable way to go.

But, I still say try a couple 1 or 2 watt resistors
in one leg of the track bus. He's running
N scale trains...no big load. He just wants
to have ability for his train to go slow. Resistors
are cheap and easily available.

Don


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