Chopped and smooth SINE waves

Well lest see if I can do this without getting too involved...

First thing is that the voltage coming out of the transformer, any O scale transform will be a max of 18-22 VAC.  That's the main job of the transformer, to change the 120 VAC mains voltage to a lower voltage suitable and safe to run the trains. 

Next, at full power, a chopped wave and a pure wave transformer with the same max output will put (almost) the same amount of power on the track.  The chopped wave type works by "chopping" part of the wave off of a pure wave.  when the throttle is all the way up, nothing (almost) is chopped off.  (The almost comes in, in that in typical designs a very small fraction of the wave will always be cut, perhaps .1 percent.)  

Where the difference comes in is when the throttle is set to less than full.  Here on a pure sine wave transformer, the peak to peak voltage of the wave changes.  On a 'chopped wave' transformer, the peak to peak voltage remains at the 'full power' voltage at anything over 25% throttle.  engines with electronics inside that convert the AC track power to DC will 'see' full throttle voltage with as little as 25% throttle.  This is why modern smoke units function better on chopped wave type transformers, they get full power even when the throttle is set low.  

While not exactly your question This Thread discusses how chopped wave transformers function in great detail.  

To best answer your question, post war transformers generally are capable of supplying a bit more voltage than modern ones, usually about 20 volts, whereas modern ones are designed to supply only 18 VAC.  For post war locomotives this may make a difference, but over all it really doesn't.  The CW80 at max throttle (18VAC) will supply the same voltage as a postwar transformer set to 18VAC output.  

On a modern locomotive for conventional running the CW80, or other chopped wave type transformer, will provide better performance than a pure wave transformer when set to less than full throttle, and will provide identical performance at full throttle.  

JGL

Your example is probably not the best one, the MTH Z-Controller doesn't output anything close to a pure sine wave at full throttle, I was surprised at how much it's deformed.  Here is is at no load and with an 8 ohm load.  The brick has a pure sine wave output, I didn't bother to record those.

No Load Full Throttle

8 Ohm Load Full Throttle

I understand now. So as long as it is on full throttle it should put out plenty of voltage! Thank you all!

Im on a comand control layout for the most part so i wont need ro use a diffrent throttle speed most days.

I run pretty exclusively command, so I typically run at 18V on the track.  I do send my transformer through the MTH TIU variable channels so that I can elect to run conventional, that's useful for testing repair work.

Gunrunner:

I was told some stuff runs better on one kid of waveform versus the other. Is this accurate? Example -  I have a fairly new diesel whose horn functions just fine on the newer Lionel CW-80 watt transformer while on another layout powered by an old lionel transformer the horn does not work well at all.

Joe

Any digitally or computer controlled transformer will have a chopped sine wave. Even the "pure" sine wave of the Z4000 is chopped, its just chopped very finely.

Joe, 

The amount of ripple that is greater on the old supply.  Now saying that which old supply are you using? Also make sure they are phase together, with both transformers plugged in and power on, remove the C terminal wiring on both transformer, connect a AC voltmeter across the two open wires and they should read 0 volts if you have a voltage there it is out of phase. 

Make sure all you common leads are connected together and that you have not reversed the leads off the transformer.  You would then be putting the TMCC signal on the wrong track as it rides on the outside rails.  will cause some interesting problems. Same with DCS happens with the DCS signal sometimes it will operate and others it will not the latter is the norm. 

Call if you need some more help. 

The original post seems to indicate a possible confusion between watts and volts. 

Hi Dan!

It is two different layouts. One has the new 80 watt Lionel transformer so the horn on the diesel works while the other layout has a real old Lionel transformer - don't know model with which the horn does not function.

Joe

can you move a new transformer over there? it now sounds like the rectifier in the old power supply is bad.

I would like to apologize for my post. It might have confused you all, because i was confusing watts and volts I was not paying attention to what i was typing.

Garrett:

A couple of good videos on chopped versus smooth sine wave transformers are:

https://www.youtube.com/watch?v=OMR25JoCZiU

https://www.youtube.com/watch?v=1Ilj3p-Ov3c

From the videos I gather the basic difference is how fast the voltage climbs to max - let's say 18 volts. The newer Lionel transformers have chopped sine wave. If you look at John's oscilloscope pictures a modern Lionel transformer will climb to max voltage in a straight line then slope down from there. From John's pictures it looks like the Z4000 is not a pure smooth sine wave but partly chopped. The voltage waveform also goes up fast (the straight vertical line) but it only goes part way to max then curves up to the max voltage (this is the smooth portion).

It would be interesting if John could put a new Lionel transformer like a CW-80 on his oscilloscope and compare waveforms. I would be interested in knowing if the Lionel transformer is a pure chopped sine wave or looks like the Z4000 waveform.

From what I understand if you are running MTH DCS or pure conventional (ie no command control boards in the loco) the transformer waveform won't matter. If you are running TMCC or Legacy then according to the Lionel videos you should use a chopped sine wave transformer to get proper smoke and auto coupler performance. If you have a 'smooth' sine wave transformer and are running TMCC/Legacy you could run the transformer through a powermaster or a TPC both of which will transform the waveform into a chopped one.

Joe

Word is that the CW-80 is not compatible with DCS.