# secondary voltage control



## mickey (Jan 28, 2009)

I currently run RC with the DC power supply set to a constant 18v. It is inside to protect it from the elements (central Texas humidity, heat, etc.) with a run of of 12ga solid to the track. However, sometimes, I want to run it traditional DC to test new engines, etc. But since the rheostat is back in the closet inside on the power supple, it's an issue. I am wanting to eventually create a control station for the switches where I have a junction box for all wires coming from inside to outside. By adding this rheostat outside, I can just leave the power supply at the 18v and run it thru either a switch and rheostat or just the rheostat by itself. But first question is which rheostat. Second question is opinions of switch/rheostat combination vs rheostat by itself where I would just leave it all the way up when doing RC.


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## Greg Elmassian (Jan 3, 2008)

Probably one of the Aristo controllers, but no idea on the used marked availability.

Yes, you could leave it full blast, but I think I'd make it so I could switch it out of the circuit.

Greg


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## Dan Pierce (Jan 2, 2008)

The LGB hut at 5 amps can be used outdoors.
Just use a DPDT switch to pick straight through power or power from the hut.

I live in Massachusetts and have a train engineer (27mhz) in a mail box that looks like a barn and it stays outside year round. (over 10 years now).


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## mickey (Jan 28, 2009)

I guess I didn't make it very clear. I'm not adding a power supply, just adding an in line rheostat. You know volume control if you will.


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## krs (Feb 29, 2008)

Depending on which R/C system you're running, you may be able to just buy another receiver, mount it in the house and connect the motor outputs to the track for the analog (traditional DC) locos. If the receiver motor drive is PWM which is most likely the case, you may need to convert that to linear for some locos.
Control would be with your regular R/C throttle.

knut


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## krs (Feb 29, 2008)

mickey said:


> I guess I didn't make it very clear. I'm not adding a power supply, just adding an in line rheostat. You know volume control if you will.


I thought you were clear and all the suggestions so far essentially take the constant 18 VDC as the input and provide a variable 0 to 18 VDC of either polarity at the output to run a DC loco.
There are many ways to accomplish that.

Knut


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## Totalwrecker (Feb 26, 2009)

1 DPDT switch to route power to track or controller, plus 1 DPDT switch to control polarity between controller and track.
It's an 'either or' set up, can't do both at the same time. A SPST switch can isolate a spur track to park a track powered loco while you use R/C.
John


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## Dan Pierce (Jan 2, 2008)

The LGB hut is a speed control only, it is not a power supply, and is more than just a rheostat as input can be DC or AC.


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## toddalin (Jan 4, 2008)

This one would do it. These generate a lot of heat so leave it well vented.

http://www.ebay.com/itm/Ohmite-0452...301?pt=LH_DefaultDomain_0&hash=item51c48b250d


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## Pete Thornton (Jan 2, 2008)

mickey said:


> I guess I didn't make it very clear. I'm not adding a power supply, just adding an in line rheostat. You know volume control if you will.


Rheostat's reduce voltage with resistance, like an electric heater. As Todd says, it will produce heat, and it's a very clumsy way to do it in today's electronic world.

The Aristo Train Engineer is designed to do exactly what you spec'd. It fits between the 18v power and the track. A switch can take it out of curcuit when you don't need it. Crest now sells the system.

Interestingly, my pal here in FL had a similar problem. His big transformer/speed control is in his garage out of the weather, but he likes to sit by the track and run trains. So he designed and built his own remote speed control - a simplified version of the Aristo/Crest TE. I think he plans to market them when he gets time to package and ship!


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## dbodnar (Jan 2, 2008)

Pete Thornton said:


> Rheostat's reduce voltage with resistance, like an electric heater. As Todd says, it will produce heat, and it's a very clumsy way to do it in today's electronic world.
> 
> The Aristo Train Engineer is designed to do exactly what you spec'd. It fits between the 18v power and the track. A switch can take it out of curcuit when you don't need it. Crest now sells the system.
> 
> Interestingly, my pal here in FL had a similar problem. His big transformer/speed control is in his garage out of the weather, but he likes to sit by the track and run trains. So he designed and built his own remote speed control - a simplified version of the Aristo/Crest TE. I think he plans to market them when he gets time to package and ship!


Something like this makes more sense:
http://www.tmart.com/DC-10-30V-120W...ntroller-with-Fuses-Green-Silver_p299945.html
The only thing you have to add is a DPDT toggle switch to change direction.
dave


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## Pete Thornton (Jan 2, 2008)

> Something like this makes more sense:
> http://www.tmart.com/DC-10-30V-120W-...r_p299945.html


Nice one Dave. Certainly better than a rheostat.


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## Greg Elmassian (Jan 3, 2008)

You guys need to understand Todd, he thinks of ways to do things differently than others. He understands logic, circuitry, and has made thing work.

So his suggestion, yes, makes heat, but it's an elegantly simple solution too.

We have not heard the budget for this venture yet, but I'll bet Todd's suggestion is the lowest cost.

Regards, Greg


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## mickey (Jan 28, 2009)

Cost isn't the big issue. I'm just looking for something simple that can be left outdoors (covered but central Texas heat and humidity) to use very sparingly when I get a new unit not yet converted but want to test run it or whatever.


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## Mark_s (Jun 24, 2013)

One way to do it would be to use the Bridgewerks Mag-Mate : http://www.bridgewerks.com/#!mag-mate-100/c722
You would plug this into your 18V supply, and then take the output of the Mag-Mate to the track. This will give you control over voltage to the track, forward/reverse, and has meters to show the voltage and current.

The UR-15 is similar, but controlled via a wireless remote rather than a manual control,

If you do want to just use a rheostat - you will want to make sure that you get the appropriate resistance (which will depend on how much current your locomotives draw), and sufficient power - I would look for a 200W rating or more. It would be best to mount it to a large heat-sink and/or have a fan blowing across it.

Mark Sauerwald
Bridgewerks


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## Jerry Barnes (Jan 2, 2008)

Del Taprio just came out with a track unit that works with his Rail Boss radio control unit. Pretty reasonable price. G Scale Graphics, probably advertises on here.


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## mickey (Jan 28, 2009)

So in reading up on this, a rheostat is used for controlling current and a potentiometer is for controlling voltage, as best I can figure. So with trains are we not controlling voltage to speled up or slow down?


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## riderdan (Jan 2, 2014)

A potentiometer is really a voltage splitter (that's why there are three terminals) where a rheostat is a variable resistor. There's a sort of functional equivalence at low power levels, but a pot works by adjusting the voltage split point, where a rheostat works by varying the resistance. Because you're looking to run trains, you want a rheostat, because I've never seen a potentiometer rated for train-level output.

Probably someone with a detailed understanding of this (Dave B.?) will be along to explain more clearly


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## East Broad Top (Dec 29, 2007)

When I was in HO scale, I built a very simple circuit using a potentiometer and a variable voltage regulator to use as my throttle. I've seen the circuit diagram on line here and there, and Radio Shack's packaging for the variable voltage regulator used to (may still) have the circuit printed on the back.

Later,

K


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## toddalin (Jan 4, 2008)

mickey said:


> So in reading up on this, a rheostat is used for controlling current and a potentiometer is for controlling voltage, as best I can figure. So with trains are we not controlling voltage to speled up or slow down?


Motors, including our trains, run on neither just voltage nor current, but power, which is voltage x current. To control the speed of the trains, we control the power going to them.


In this case the OP specified 18 volts. A typical train draws about 2 amps at 18 volts. Sure there is variation, but this is well within the range of our equipment and serves for our demonstration purposes.


18 volts x 2 amps = 36 watts.


So, the original 100 watt rheostat I proposed could easily accommodate two trains.


In this case, the impedance of the motor would be 9 ohms, and again, this is well within the range of our equipment.


18 volts / 9 ohms = 2 amps.


I proposed a 200 ohm rheostat.


200 ohms + 9 ohms = 209 ohms


18 volts / 209 ohms = 0.086 amps (86 milliamps)


18 volts x 0.086 amps = 1.55 watts


All things being equal, the speed of train is directly proportional to the power. Therefore at 1.55 watts, the train is going at 1/23rd of its potential top speed. So if it tops out at 80 scale mph, at the minimum throttle it would move 3.4 scale mph, assuming there is enough power to overcome all of the internal frictions of the trains.


Why didn’t I suggest more than 200 ohms letting the trains move slower? The next jump I saw in 100 watt rheostats at reasonable cost were at 1,000 ohms and the trains would have no movement for most of the throttle span.


And what are the pitfalls?


At 200 ohms, the minimum current is 0.09 amps (90 milliamps). While this probably wouldn’t run a large-scale train to any speed, it would still overpower an LED (even with the additional voltage drop across the LED) and you would still need dropping resistors if you use LEDs.


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## Dan Pierce (Jan 2, 2008)

The high heat (watts) created in todds explanation is why in large scale mosfets are used to 'pulse' the power. Much less heat created and no fan needed for large train operations. DCC decoders are over 5 amps with no heat sinks now.


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## mickey (Jan 28, 2009)

Todd, thanks. Now I know just how much I don't know about this. I thought it would be simple but now I'm glad I got the input.


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