I have 12volt ac transformers a friend put bridge rectifiers on them converting them to 12volts dc I have them operating my lights on my layout.I heard ac is better for lights what would be the advantage on ac?also do atlas switches work on 12volts ac or 12voltsdc?
jimbob
Many switch machine motors that I’ve seen require AC and can actually be badly damaged by DC. Also, on many power packs, the more accessories you connect to the AC lugs, it takes more power away form the DC side.
Unless your friend also added a smoothing capacitor, the only practical effect of powering light bulbs through a bridge rectifier is a reduction of approximately 1.2v in the available voltage from the transformer, aka reduced efficiency. The same can be said for using it to directly power twin coil switch machines (Atlas, Rix, KTM/Kemtron etc.)
Tortoise switch machines MUST be powered with DC. Those stall-motor machines operate just like the propulsion motors in analog DC locomotives. AC will not operate them, and may cause serious damage.
DC must also be used with the popular capacitor discharge “hot shot” circuits designed to give twin coil switch machines more Oomph (or a louder Clack.) The capacitor is polarity-sensitive.
IMHO, lights and switch machines should be powered separately from propulsion power, and analog DC with common rail requires each train to be powered by a separate transformer - rectifier. To power lights on my own layout under construction I have a number of train set power packs and wall warts. Turnouts are powered by a 3 amp AC supply built around a Radio Shack filament transformer.
Chuck [modeling Central Japan in September, 1964 - with the wrong AC frequency (model, 60hz, prototype 50 hz)]
As far as lights are concerned, there is little difference between AC and DC current. As you may or may not know, AC stands for alternating current and DC for direct current. This means in an AC circuit, the polarity can change direction of flow. Where as in a DC circuit, the polarity is fixed. In other words, current will flow from Negative to Positive. The advantage of using DC current is that the current flow is predictable (one lead alway pos and the other neg) to reverse the curent flow, A DPDT toggle or slide switch (wired in an X fashion) is often used.
Another example would be:
In a DC operated layout, the rails have fixed polarities (Pos and Neg) This is why a train (or any electrical dc motor) will operate in one direction, stop, and continue operating in the same direction when power is re-applied (even if you pick up the loco and place it back on the track facing the opposite direction). In an AC circuit, once the train stops, and power is re-applied, the polarity can invert and cause your locomotive or motor to run in the opposite direction.
BOTH Incandescent bulbs and Atlas Switch machineswork on AC or DC. Advantages vs. disadvantages: You did not mention Current (amps or VA’S.) 1. THE BRIDGE RECTIFIER drops the voltage to around 10.6Volts DC (A1.4volt drop) from 12 volts AC. I would run the lights on the 10.6 V. DC in order to make them last longer. 2. Atlas (and other twin coil machines) use a jolt of amperage to do their work, therefor you want amperage for an instant to do. Also they require momentary electrical contacts to avoid coil burnout. 2. I would use the AC for maimum current , or DC with Capacitive discharging units when needing to throw two or more twin coils simultaneously. Capacitors need DC to charge.
A couple of other points:
LEDs (Light-Emitting Diodes) use DC, but they will work on AC because they are actually only using half of the AC sine-wave. They are polarity sensitive, so you have to have the right side wired to the positive and negative posts if you’re using DC.
Some DC relays used for signalling or panel light control will NOT work on AC. I’ve found that these are also polarity-sensitive. Turnout machines will work on AC or DC, and they are NOT polarity sensitive.
I’m also in favor of using separate power supplies for trains and everything else. I recently installed an electromagnetic uncoupler from Kadee. Works fine, but it is a power hog when I turn it on. I solved that problem by using a Wall Wart (one of those plug-in “brick” external power supplies that comes with computer peripherals, cell phone chargers, etc.) from some discarded device.
Light bulbs last longer on DC (real DC - rectified and filtered) than on AC.
The advantage is measurable but not tremendous. AC by definition switches polarity 60 (50 in some parts of the world) times every second. The signal rises to a peak, falls to zero and drops to a negative peak. Then it repeats.
This causes two detrimental effects. 1) The filament in the light bulb is constantly heating up and cooling down. It never cools completely but temperature does vary.
2) The AC current can cause minute vibrations in the filament.
Bottom line, the filament is stressed more on AC and will break sooner under most conditions.
How much sooner varies, but the I have an Edison lightbulb that still works a century later. It ran on pure DC for many decades.
In practical terms though, the best thing that you can do for a lightbulb is to run it below it’s rated voltage. The choice of AC or DC is secondary.
Karl
I support your statement; I was thinking the same thing when I read the earlier posts.
Carl (the other electronics one[:D])