Large Scale Central

LGB Powered tender question

Modeling Rolling stock
#1

I have a little LGB 0-4-0 steamer that I modified a bit and repainted. I had put aristo slopeback tender behind it and recently got a set of Hartland mini freight cars. It looks great and the kids who visit love that train–they can put things in the gondolas and ore cars and flatcars.

I wanted to have it pull a little better though, and so after reading on George Schreyer’s page I bought an LGB Powered tender on ebay. I managed to get it hooked up with the 75 mhz receiver that’s in the loco, so the receiver is being powered from and controlling both the loco and the tender. And I managed to get a small scale railways soundcard installed in the tender as well, by gluing the infrared chuff sensor to the wall of th motor block behind one of the wheels.

It does pull much better, and power pickup is much better, and I can live with the looks now that it’s painted to match the loco. But I’ve noticed that the powered tender is slower than the loco. Is there a way to sync them? I thought about this before I bought the tender, and just assumed they would run at basically the same speed. I suppose the only way to do it would be to change the gearing, which is beyond me.

Now that I think of it, would it be possible to drop the voltage going to the loco? using a resistor?

#2

Yes, you could try adding a voltage drop to the faster motor. Resistors limit current, not voltage. However a single diode in the power lead for the motor will drop the voltage .5V if I remember correctly. Add as many as you need to get the motors to run close, but remember the voltage dropped is wasted as heat and will effect your battery life a little bit.

You’ll actually need to use two diodes, one in each direction for each .5V drop in order to have the polarity of one diode always correct for the direction of power flow. Use diodes with a 2-3 amp rating.

#3

Thanks john–that makes sense. So one diode on each motor lead would drop the voltage by .5, two diodes by 1 volt?

#4

I’m not certain of the .5V part it might be .25V.

You only need to put them in 1 lead. Orient two diodes so the polarity marks are opposite, connect them together in parallel and splice into one of the power leads. The reason for the opposite polarities is to allow current to flow in either direction. Diodes are made to block the flow in one direction, and they have an insertion loss. We don’t want to block the flow at all, but simply use their insertion loss as a simple means to drop voltage.

If you really want to exactly match the speeds, a voltage regulator could be used. That would allow very fine adjustments. Regulators are more costly (perhaps $1.00 Vs. $.25) and will require more parts.

#5

Mike:

The forward voltage drop in rectifier type Silicon diodes (the most commonly available kind) is 0.7V. Here is information on diodes. Diodes have a very small leakage current in the reverse direction, but that can be ignored for this application.

You need to select a diode with a voltage rating well above the maximum it will see in the reverse direction and a forward current rating sufficient to handle the maximum motor power. I would suggest a 50V, 2 or 3A diode for a nominal 1A motor operating on <24V. Diodes are really cheap, but changing a failed one is a pain.

As Jon wrote, you put the diodes in one leg of the power to the motor. You can put multiple diodes in series, thus multiplying the 0.7V voltage drop by the number of diodes: 1 diode=0.7V; 2 diodes=1.4V; etc. You then put an equal number of diodes pointing the opposite way in parallel with the first series set. If the operating speed is different between forward and reverse, you can vary the number of diodes in either direction to even out that speed difference.

Another way of doing this is to use a single full bridge rectifier. I contains 4 diodes, with two in one direction and two in the reverse direction. Silicon diode full bridge rectifiers will give a 1.4V drop in each direction. There is a diagram on the website I linked above that shows the arrangement of the components in a full bridge rectifier. A rectifier’s rating should be in the 50V, 2 to 3A range for this application.

Hope this helps.

Happy RRing,

Jerry

#6

Thank you so much Jerry–that’s very clear. I’ve installed bridge rectifiers to run lights but never to drop voltage.

#7

Jerry,
Interesting and informational website.
In plain English, too.
Thanks.

I use this one to calculate resistors for LED’s, but works for regular lighting as well.
http://ledcalc.com/

#8

John:

I like that Electronics Club (Link to Homepage) website as it has lots of basic stuff for relative beginners, along with how to instructions for things like soldering and other electronic construction. One feature is the use of NE555 timers, which have lots of applications in all kinds of electronics, and especially for model RRs.

Thanks for the (now bookmarked) link to the LED calculator. The one I previously used disappeared a few months ago. I do quite a few power calculations of various kinds to support the machines I design, but an online source is great for both sanity checks and to pass along to others who need help.

Happy RRing,

Jerry

#9

Jerry,

Very neat website. I’ll be spending a lot of time there. Thanks for sharing!

#10

Just a followup–I installed four 4 amp diodes in anti-parralel on one motor lead of the loco. Worked like a charm–they are very closely matched in speed now. Thanks again!

#11

Isn’t “anti-parralel” - “in series”? Glad to hear it worked.