Re: MRC Railmaster - Overload keeps tripping

Stevert wrote:
David Starr wrote:

Puckdropper and Charles Davis have it right. With no load (load is locomotives or lamps) the resistance of the rails makes no difference at all to a voltage reading.

Sorry, but no they don't have it right. The resistance of the various conductors *does* make a difference, and Mr. Gartner has measured and documented those differences.

Let's take Mr. Gartners's calculated voltage drop of .028 volts per foot for code 100 nickel-silver rail at .5 amp (probably not far from the current draw of a typical Athearn Blue Box loco with that huge bulb in the cab).

Now multiply that by, let's say 9 linear feet of track - half-way around a small oval consisting of six 3-foot sections of flex track. And for the moment, let's not even consider the possible resistance/voltage drops introduced at any of the wire connections or track joints (also mentioned on Mr. Gartner's page) So now we have a voltage drop, caused by the resistance of the rails alone, of about .25 volt at the far side of that oval.

For purposes of this illustration, let's have that BB Athearn loco running at about half throttle, or nominally 6 volts measured where the feeders contact the rails. So with that .25 voltage drop, that Athearn gets only 5.75 volts at the far end of the small oval. That's about a 4% decrease, enough to notice if you look for it.

But even at half-throttle, that Athearn is still going unrealistically fast on that small oval, so you cut the throttle back to one-third, or 4 volts at the feeders. That causes the .25 volt drop to become a difference of over 6%. It's quite obvious now that the loco is slowing down and speeding up as it makes it's way around the oval.

But that oval of six sections of flex track is a little small, so you expand it some, and add some turnouts (additional rail length and more rail joints). And since now you have a "layout" instead of a simple oval, you've also added some cars, which increases that Athearn's current draw even more. You've gotten to the point where the voltage drop at the farthest point from that single pair of feeders causes a speed decrease that is more than obvious, it's a distraction.

C'mon, anyone who's ever set up a Lionel layout on the floor, or a big circle of HO track, with a single "terminal track" has seen this happen. Why do you suppose feeders are suggested for every section of flex track, or at a minimum every other section, even when the rail joints are soldered?

Yes, rail is a conductor, but not an ideal one, especially when it comes to nickel-silver rail. Certainly not better than properly-sized copper wire. I stand behind my original statements.

Stevert

We all agree that trains run better with good solid bus and feeder wires. No problem there. But that isn't the point under discussion here.
The original fault was the overload breaker on the power pack tripping, which suggests that something is pulling too much current, not that the voltage on the track is too low from voltage drop. The power pack and the locomotive were known to operate properly on another loop of track.
The suggestion was made to measure the voltage on the track with no load, which means no current should be flowing. The meter draws so little current that we can call it zero current. In this case, measured track voltage should be the same all over the loop of track. No current should be flowing, therefore no voltage will be dropped.
Should a voltage drop be measured with no load, that's a trouble sign. It means current is flowing when it isn't supposed to be, and that extra current might be enough to cause the overload breaker to trip.
In this case, (excessive current) it may be that some piece of track, or a turnout, or something, is allowing current to flow from one rail to the other, when it shouldn't. If this is happening, it may cause enough voltage drop to see with the voltmeter.
But the principle worth remembering is; with no current flow there can be no voltage drop. If you see a voltage drop, you know current must be flowing. The voltage drop is given by ohms law, V = I*R. When I (current) is zero, the voltage drop has to be zero too. For I = 0, it doesn't matter what R (resistance) is, zero times anything is still zero. That's why the resistance of the rail doesn't matter for voltage measurements at no load.
In this case we are looking for a current flow when there ain't supposed to be any. If everything is good, the meter will read the same voltage all around the track. If we have a current leak (which we have reason to suspect) then we may get a voltage drop large enough to see on the voltmeter.
Another way to trouble shoot this one is to put an ammeter in series with the track. This works better than the voltmeter, assuming one has a real ammeter that can read up to an ampere or two. A lot of us don't have ammeters, the common multimeters don't do amps, or they used to do amps but someone blew the shunt open so the amp range doesn't work anymore.

--
David J. Starr

Blog: www.newsnorthwoods.blogspot.com
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