The trusty Ohm meter

[ben10ben]

Charlie,
Thanks for the tip on the meters! I have a very nice Extech true RMS meter that I use most of the time, but there are definitely times when a cheap, easy to use meter would have its advantages. Looks like I'm going to be ordering a couple(dozen) of the Harbor Freight meters.

Dave,
For an ammeter, I've been using a Radio Shack analog model. Radio Shack sells a clamp-on analog AC ammeter that will measure up to 30 amps for $20, or $14 on sale(what I paid for mine). It will also function as, I think, a standard 7 function meter(AC/DC volts, DC amps, and resistance), and comes in a hard shell carrying case. I don't know about its accuracy, but for the times when I've needed to know a ballpark current draw, it's been just the thing. The clamp-on part also means that you don't have to disassemble your wiring just to take a reading.

[Anonymous]

John thanks for the info on making a voltmeter act as an ampmeter.

FYI The Habor Freight catalog for this week (yes I get about 1 each week!) now has a 7 function digital Liquid crystal display meter for $2.99. Wow you can't beat that. Get a 1/2 dozen.

Charlie

[lionelsoni]

Approximate resistances of copper wire are actually

10AWG .001 ohm/foot
12AWG .0016 ohm/foot
14AWG .0025 ohm/foot
16AWG .004 ohm/foot

How much current a wire can carry ("ampacity") depends on both its resistance and surface area, both of which depend on the wire gauge, and on the type of insulation. Approximate ampacities for copper and aluminum wire are

10AWG 30 and 20 amperes
12AWG 20 and 15 amperes
14AWG 15 and 10 amperes
16AWG 10 and 7 amperes

Wire should be sized according to the current at which the circuit is protected. For example, if your transformer's circuit breaker trips at 15 amperes, you should be using at least 14AWG wire.

Aluminum was popular for house wiring a few decades ago. Larger wires were required; but they were cheaper than copper wires with the same ampacity. Then it was realized that aluminum creeps away from terminals under pressure and oxidizes in the resulting gaps, creating high-resistance connections that heat up and start fires. Modern devices allowed to be used with aluminum have resilient terminals specially designed to keep the contact pressure high. But aluminum is little used today within buildings, because of its bad safety reputation.

It is still the usual metal for transmission and distribution lines and for the "drops" to houses. Where it is connected to your panel, there should be fittings rated for aluminum and a special grease covering the ends of the wires, to prevent oxidation.

[jkerklo]

"Interestingly, the current that enters my house comes through in an aluminum wire."

Yes, the large gauge required is expensive in copper. The electrical fittings in the meter and breaker box are designed for aluminum wire. The aluminum wire is usually one gauge larger than needed for copper.

If your power is via overhead wires, the light weight of aluminum is an additional benefit.

John Kerklo
TCA 94-38455
www.Three-Rail.com

[FJ and G]

Thanks, John,

This info helps. I'll likely juggle between backdrop painting and starting to lay my 3rd rail on Thursday (took day off) and over the weekend. The copper wire is very stable and you'd have to walk on it to bend it.

Interestingly, the current that enters my house comes through in an aluminum wire.

[jkerklo]

My sense is that 14 guage is about right for replacing the center rail. It is certainly a better conductor than tubular steel track.

More important, is its strength and the span between copper nails. Smaller, say 16 guage, would carry the current fine, but may need too close a nail spacing.

The important current carrying characteristic of wire is resistance / foot. Resistance means heat generation, which is the major limiting factor. Resistance is less as wire diameter is increased. Resistance increases as wire is longer. There is actually a simple relationship among wire guage: Resistance is halved every other even guage.
Which means, #14 carries twice the current as #18. (not perfect, but close enough).

Nichrome wire has a high resistance / foot. For example, #14 nichrome is about .15 ohms / foot, about 50 times copper wire. Not good for carrying current, but good for resistors or heating elements. Nichrome wire does not solder well.

Different metals have different resistances, about in order: silver (best, lowest), copper, aluminum (worst). Copper is the best cost/strength tradeoff. Aluminum is a good conductor, but the way it corrodes and changes size with heat are a serious detriment.
Also copper solders easily, aluminum with great difficulty.

John Kerklo
TCA 94-38455
www.Three-Rail.com

[FJ and G]

John,

Thanks. An off-topic question.

I'm using 14-ga copper wire for my center rail, soldered to copper roofing nails.

Is there a thinner wire that would carry enough amps to run my 3-rail trains? I know that I probably could get away w/16 ga copper wire.

But what about nichrome wire? Can it be soldered and would it work better than copper as a conductor and carrier of electricity.

As to carrying capacity of electricity, are all wires equal? e.g., can a copper, silver, aluminum etc wire of the same gauge carry the same current (amp-wise)?

[jkerklo]

You can use a voltmeter to measure current. Some "ampere" meters are actually mili-volt meters that read the voltage across a small value resistor, called a "meter shunt."

The meter shunt technique is useful because voltmeters are cheaper than amp-meters and a double-deck rotary switch can be used to select ampere readings from many circuits, difficult with inline amp meters.

Shunts are available, but you can make your own using copper or nichrome wire.
The resistance of copper wire is #16 - .005 Ohms/foot, #14 - .003 O/F, #12 - .002 O/F.

For example, two feet of #16 wire gives a .01 ohm resistor. 10 milivolts across this resistor indicates 1 ampere through the resistor. (V = IR, I = V/R, I = V/.01)

.01 ohms is not enough to have any negative affect on train operation.

Nichrome wire can also be used, with a lot less wire.

John Kerklo
TCA 94-38455
www.Three-Rail.com

[FJ and G]

Charlie,

Now all I need is an amp meter, but they are quite expensive. I'm using Z750 bricks that don't have any indicators.

[Anonymous]

Harbor Freight has digital VOM for $4. That makes it easy to have a couple for when you can not remember where you left it!! I always keep one in the kitchen to check battery voltage plus one at the layout and one in the shop.

One could also build in one into the control panel for each transformer to show voltage. To bad they do not read amperage.

Charlie.

[Anonymous]

Don't forget to check your rail-to-tie points on your insulated
sections as well. Sometimes the insulating material will go
bad and cause a connection (short?) between the insulated
rail and the metal tie, thus closing the sircuit you want open.
The solution is to merely replace the insulation material. I
use the cardboard from parts cards such as from Radio
Shack parts. The cardboard is the same thickness as the
original material and is usually finished at least on one side
making it less prone to breaking down.

Don't forget your non-derailing rails on your switches, too. An
insulating pin needs to go between the special rail and the
section connecting to it or else the switch will chatter or not
function.

Yup, the old VOM is a good troubleshooting tool! You can also
check for resistance over long stretches of track. This will tell
you if you are suffering any line loss due to corrosion, loose
pins, etc.

Have fun. Good luck.

[FJ and G]

I dusted off my voltmeter this weekend to check my track-laying progress. First, I checked all of the soldered rails (I solder all railsections together). I was pleased to see that current flowed through all of the rails and the solder even looks prototypical, like welded sections of track.

On my insulated sections, there were 2 problem areas where current was getting thru (must have butted up the sections too tightly. A quick dremel cut will fix those.

I checked the frogs, poiints, control rails, etc.

For those of you with normal track (unlike my abnormal track), an Ohm meter can also help you pinpoint problem areas.