As I get back into trains I’m trying to upgrade my general knowledge of electricity into a better general knowledge of electronics. One of the little things I’d like to tackle is dropping voltage on a couple of AC things with a rheostat. If I’m doing my math correctly and allowing for some oversizing, assuming that I don’t push more than 14V and 2A through it I should be able to use a 50W 10 ohm rheostat in variety of ways. Is this a good “general purpose” rheostat to be purchasing? I do know there are other ways to tackle this type of problem (e.g., providing a different voltage from the transformer) – this is as much for my own learning as anything else.
As a side question, as I think about things it seems that (excluding dropping voltage on LEDs or maybe a bulb or two) there wouldn’t be many uses for cheap ceramic resistors in a Lionel layout. Drawing almost any “Lionel-sized” current at 12-14V quickly creates more heat than even a 1W resistor will handle. Am I thinking this through correctly?
IT consultant by day, 3rd generation Lionel guy (raising a 3YO 4th generation Lionel Lil' Man) by night in the suburbs of the greatest city in the world - Chicago. Home of the ever-changing Illinois Concretus Ry.
A rheostat is a current limiting device, not voltage limiting, so it will depend entirely on the load you intend to control at any given time. What are your plans?
Rob
As far as I am concerned using resistors or rheostats to drop voltage is almost always a bad idea. The voltage dropped by the will vary with the load. For example, the rheostat you specify might work OK to vary the speed of a PW loco drawing a couple amps, but even in its highest position it will only slightly dim a # 53 or 363 bulb! If you do use a resistor on a variable load, like a loco, the voltage drop will depend on the load the loco is pulling, and it will work in the "wrong" direction, it will make the motor slow down even more than usual on a heavy load. Is that really what you want?
Old transformers are inexpensive. Get a few, be sure the cords ared good and the circuit beakers are working, and set one for 10V, another for 12V etc. Much less headache than fiddling around with resistors
The problem that I expect you will have is that you will run out of resistance trying to reduce the voltage and current to any load that draws much less than 2 amperes. For example, suppose that you have a resistive load that draws .5 amperes at 14 volts with the rheostat all the way up, that is, a 28-ohm load. The lowest voltage that you can get with a 10-ohm rheostat in that case is about 10 volts. The rheostat will have an effect that is even smaller with lighter loads.
You could handle the 28-ohm load better with a 20-ohm rheostat; but the 20-ohm rheostat would need power rating of 100 watts to deal with the same maximum current that the 10-ohm rheostat could handle (which is 2.24 amperes). Or you could keep the 50-watt power rating and accept a lower maximum current of 1.58 amperes.
Have you considered using diodes wired in anti-parallel (cathode to anode) for voltage dropping? The voltage regulation is much better; and you can tap a single string of anti-parallel pairs for various voltages, with about 1/2-volt resolution.
Bob Nelson
My 322AC plus 5 passenger cars draws about 3A at 12V running around the layout at a typical toy train speed. So the equivalent resistance this shows to the transformer is 4 Ohms. Let's say we want to operate this train with a Lionel 18V constant supply. 18V at 3A requires 6 Ohms total in the circuit so the Rheostat needs to be set at 2 Ohms, using just 1/5 of the Rheostat windings but consuming 18W. The 50W rheostat is underrated for this application.
My personal opinion is the old style rheostats are not a good choice given all the modern alternatives.
lionelsoni The problem that I expect you will have is that you will run out of resistance trying to reduce the voltage and current to any load that draws much less than 2 amperes. For example, suppose that you have a resistive load that draws .5 amperes at 14 volts with the rheostat all the way up, that is, a 28-ohm load. The lowest voltage that you can get with a 10-ohm rheostat in that case is about 10 volts. The rheostat will have an effect that is even smaller with lighter loads.
You're correct - I guess I was thinking mostly about the maximum load I might want it to carry (which probably is more than I'd need in practice) and not thinking enough about how the resistance range would work on lower draw needs.
lionelsoni Have you considered using diodes wired in anti-parallel (cathode to anode) for voltage dropping? The voltage regulation is much better; and you can tap a single string of anti-parallel pairs for various voltages, with about 1/2-volt resolution.
My purpose in all of this is mostly experimental and learning (which I'm getting a bit of already...) I'll move into using diodes once I have the old-skool ways clear in my mind.
Laurastom My personal opinion is the old style rheostats are not a good choice given all the modern alternatives.
Speaking of old-style rheostats...does anyone happen to know the specs on the rheostats Lionel use to sell like the #95? They must be pretty beefy to acomodate their original intended use, which was to control track voltage on pre-war engines and such.
Lionel 027 Old transformers are inexpensive. Get a few, be sure the cords ared good and the circuit beakers are working, and set one for 10V, another for 12V etc. Much less headache than fiddling around with resistors
Agreed - there are definitely simpler ways to skin this cat. I'm looking forward to the headaches as a learning opportunity.
OK, I think I'm going nuts. I'm using my DMM to work out some Ohm's Law calculations on a simple 14v bulb. Voltage is set at about 8V (seems right - I have the transformer turned down), the bulb is drawing about .15A (seems about right for those bulbs), but when I measure the resistance I'm only getting about 8-9 Ohms instead of about 50. This is the first time I'm using the ohmmeter function in the DMM - am I missing something or is my meter a dud? Maybe it's time for bed...
JTrains Speaking of old-style rheostats...does anyone happen to know the specs on the rheostats Lionel use to sell like the #95?
Speaking of old-style rheostats...does anyone happen to know the specs on the rheostats Lionel use to sell like the #95?
Figure ~4.5 ohms, 20 watts.
You can't measure the resistance of a light bulb. Resistance goes up considerably due to heat
Banks, Proud member of the OTTS TCA 12-67310
Banks You can't measure the resistance of a light bulb. Resistance goes up considerably due to heat
Ah - thanks for the tip. That makes sense. I thought either my math minor had completely abandoned me or the meter was bad. It's interesting that a number of on-line "how to use a DMM" tutorials use light bulbs as one of their initial experiments.
Follow-up question: are there any other common train components for which trying to directly measure resistance might provide false readings? A smoke unit could perhaps be another example, for the same reason?
The resistance of metals generally goes up with temperature; but few temperature extremes are as great as what the tungsten filament of an incandescent lamp experiences. A smoke generator's resistance will increase, but much less and much more slowly than a lamp's. Even a lamp's resistance changes slowly enough that there is little variation during the 17-millisecond period of an AC voltage waveform.
For longer time scales, the current (DC or RMS) that a lamp draws varies approximately as the .55 power of the voltage. This can be used to advantage in a few places on a layout. For example, the relay-less control-rail signal circuit and the capacitive-discharge circuits that I have often described here. (The former is a variation on one by Al Kalmbach that used resistors. Substituting lamps greatly reduces the power consumed by the circuit.)
Many early incandescent lamps used carbon filaments, which behaved differently from metal ones, in that resistance decreased with temperature. This of course eliminated the current spike that happens whenever a tungsten lamp is abruptly switched on. This characteristic prolonged their use in telephone switchboards well into the 20th century, long after they became obsolete for general lighting, since they eliminated the clicks that otherwise would otherwise be induced into the telephone lines.
One item that behaves like a very strange resistor on a train layout is the universal motor usually found in older locomotives. The back-voltage that the motor generates is proportional to the field current (which is the same as the total motor current) and to the motor speed. So the motor voltage divided by the motor current--in other words, the motor resistance--is proportional to its speed. There is added to that a series component of resistance due just to the resistance of the motor windings. The motor torque is proportional to the square of the motor current.
lionelsoni The resistance of metals generally goes up with temperature; but few temperature extremes are as great as what the tungsten filament of an incandescent lamp experiences. A smoke generator's resistance will increase, but much less and much more slowly than a lamp's. Even a lamp's resistance changes slowly enough that there is little variation during the 17-millisecond period of an AC voltage waveform. For longer time scales, the current (DC or RMS) that a lamp draws varies approximately as the .55 power of the voltage. This can be used to advantage in a few places on a layout. For example, the relay-less control-rail signal circuit and the capacitive-discharge circuits that I have often described here. (The former is a variation on one by Al Kalmbach that used resistors. Substituting lamps greatly reduces the power consumed by the circuit.) Many early incandescent lamps used carbon filaments, which behaved differently from metal ones, in that resistance increased with temperature. This of course eliminated the current spike that happens whenever a tungsten lamp is abruptly switched on. This characteristic prolonged their use in telephone switchboards well into the 20th century, long after they became obsolete for general lighting, since they eliminated the clicks that otherwise would otherwise be induced into the telephone lines. One item that behaves like a very strange resistor on a train layout is the universal motor usually found in older locomotives. The back-voltage that the motor generates is proportional to the field current (which is the same as the total motor current) and to the motor speed. So the motor voltage divided by the motor current--in other words, the motor resistance--is proportional to its speed. There is added to that a series component of resistance due just to the resistance of the motor windings. The motor torque is proportional to the square of the motor current.
Many early incandescent lamps used carbon filaments, which behaved differently from metal ones, in that resistance increased with temperature. This of course eliminated the current spike that happens whenever a tungsten lamp is abruptly switched on. This characteristic prolonged their use in telephone switchboards well into the 20th century, long after they became obsolete for general lighting, since they eliminated the clicks that otherwise would otherwise be induced into the telephone lines.
Good stuff - many thanks for taking the time to share your insights and knowledge. I'll have to think through some of these points a bit more. One question, though: did you mean to say that carbon elements' resistance decreased as temperature increased? Else, what is its advantage over a tungsten filament?
Good catch! You're right: I meant to write "decreased". I'll edit the post.
lionelsoni Good catch! You're right: I meant to write "decreased". I'll edit the post.
I'm slowly learning...
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