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I have a DC transformer, from an HO train set, mounted way underneath my train table.   I use it to power my DC accessories.

Because it is so far underneath the table, I have to "pre-set" the voltage lever to about the middle range, and leave it there, for all of my accessories to work.  The problem is, with this arrangement, the accessories only operate at one speed.

Currently, I have the positive wire running to a simple on-off toggle switch on the side of my table, before going out to the accessories, so that I can turn the accessories on and off.  But, I would like to replace this switch with a DC potentiometer, so that I can control the speeds.   The negative wire from the transformer currently runs to a distribution block, which has attached to it all of the negative wires running out to the accessories.

My question is, why do these potentiometers have 3 places on their sides, to attach three wires.   I know that two of them are for the "in" and "out" of the power wire, but what is supposed to be attached to the third one?      Is it a ground?   If so, why would this rig need a ground?

  Is it for the "negative" wire from the transformer to attach to? Before the negative wire goes out to the distribution block?   

Thanks very much for any info.


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Terminal 1 - Negative

Terminal 2 - Positive Out

Terminal 3 - Positive In

Terminals 1 and 3 can be interchanged as they are opposite ends of the same resistor.  The middle terminal (2) is connected to the wiper that slides along the length of the resistor and varies the resistance from middle to each end.  This is a type of voltage divider circuit.

The potentiometer needs to have a high enough Wattage rating to handle the current drawn by the motor at its operating voltage.  Also the potentiometer's resistance needs to be chosen based on motor characteristics and input voltage.

Here's a link for more reading:

There are upsides and downsides to using a voltage divider to control a motor's speed.

Upside: Inexpensive

Downside: wastes power, creates heat

A variable DC-DC Converter is more efficient, but costs a bit more.  It's also easier to design the circuit.

Last edited by SteveH

In order to supply any useful current to the load, the resistance value of that pot has to be pretty low.  Wiring a low resistance pot across the power supply will generate tons of heat!  The pot has to have a reasonably low value provide any significant current to the load

I suspect he has the pot just wired in series with the load from the description.

You should not use a pot (potentiometer) to control DC voltage in this application. It just not the right tool for the job. A rheostat would do it, but it’s expensive and cumbersome. You could also use a variable transformer on the AC input to the HO power pack, but again that’s expensive and cumbersome.

Why not simply move the power pack to a more convenient location? All that is involved to do that is a few feet of wire.

@Mannyrock posted:

(I didn't know that a potentiometer itself actually drew current itself to operate.)

Well, only if you apply the full DC voltage across the potentiometer.

The simpler way would be to just use the center tap and one end connection in series with the load to split the voltage with the accessory, basically acting as a variable resistor in the circuit. Yes, some power will be lost as heat in the potentiometer while  the accessory is drawing power, but only to the extent such power is being diverted from the accessory, rather than constantly draining power from the source if it is wired across the positive and negative terminals.

All in all, though, I think Rich's suggestion is best: just rewire to bring the existing voltage control within reach. Wire is usually a lot cheaper than electronic components!

@Steve Tyler posted:

All in all, though, I think Rich's suggestion is best: just rewire to bring the existing voltage control within reach. Wire is usually a lot cheaper than electronic components!

Final thought: the only exception to the above would be if you need/desire multiple voltages, each matched to one or more of your accessories. In that event, I'd still favor moving the main power supply and "master control" within reach first, and set the throttle to produce the highest needed/desired voltage. Then, I'd send the output to an adjacent terminal block, and *then* run feeds either directly to the accessories that need the highest voltage, or through one or more potentiometer (as above) to accessories that work best on a lower voltage. [If you desire momentary or remotely controlled activation, the feed can be routed through an appropriate switch or control circuit.]

Keep in mind that if you wire more than one accessory through a single potentiometer, each accessory will receive even less voltage when any additional accessory on the same circuit is activated -- IOW, the voltage dropped and power absorbed by the pot will increase if you increase the current through the circuit. You've probably already seen a similar phenomenon when accessories dim or slow when the main throttle is opened to speed up the consist. The only remedy for this (other than separate power supplies or separate pots) might be to install a buck converter for each accessory. They're cheap (as little as less than a buck each -- no pun intended! -- from 'slow boat' sources), and will ensure each accessory is getting the optimum voltage. The only downside I can think of is that such buck converters are not as easy to adjust on the fly as a potentiometer (it requires a small screwdriver to adjust a tiny screw on a circuit board-mounted component), but even that can be facilitated by mounting the buck converters in an accessible location if such adjustment is needed.

In any event, good luck with rewiring/repowering your accessories!

Last edited by Steve Tyler

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