# DCC System Provides AC or DC to Rail?



## jimtyp (Jan 2, 2008)

I have an NCE 10amp DCC setup. When I put a volt meter on my track and set it to AC, it reads 21.8 (which I believe to be correct) When I switch the meter to DC, it registers some huge number, like 43. So DCC systems provide AC to the rails? I thought it would have been DC? 

The outputs from my decoder appear to read correctly when I use DC on the volt meter - 19.1


So AC to the rails but then DC from the decoder?


----------



## JPCaputo (Jul 26, 2009)

DCC puts a square-wave to the track. Essentially a continuous series of pulses extremely fast. 

For a regular meter, that does not read true-rms (averaging), AC will read better, however it can read significantly off as well. DC can be wild as you have seen. 

The best meter to read DCC is one that reads True-RMS. will measure the pulsed DC properly when set to DC. 

A better place to measure the voltage is off of the power supply feeding the system. 

The decoder smooths out the pulses on the track to a measurable frequency. Usually around 1 - 10 KHZ. Most meters can read this effectively.


----------



## jimtyp (Jan 2, 2008)

Thanks JP. Got another question. The reason I'm asking about AC or DC is I'm trying to get a smoke unit to chuff and puff in sync. I found a board that may work but says it requires 18v AC as input. I have an output that I measure as 19.1v DC from the decoder. If something wants 18v AC, would 19v DC work?


----------



## JPCaputo (Jul 26, 2009)

Do you have the manual of the smoke unit, or a schematic for it? 

For putting DC on an AC device, it really depends on the particular device. With the schematic, it is possible to determine if something will work or not, and that requires some electronic knowledge. 

Trying it will risk releasing the smoke built into every electronic device.


----------



## jimtyp (Jan 2, 2008)

I don't have the schematic, and since the company is trying to make money off the product I suspect they won't give me a schematic. The only thing I know is that it says it takes 18V AC as input and outputs 5V DC.

Is there a way to convert the track DC voltage to AC? So ~22V DC to 18V AC?


----------



## JPCaputo (Jul 26, 2009)

There is a chance it may work. You may or may not need to switch polarities, Depending if the input is a bridge rectifier ( uses both positive and negative parts of cycle ) or half wave ( either positive or negative half of cycle ) put a low current fuse in line with it, if it called to use 18v 1amp, put a .75 amp or 1 amp fast blow fuse when testing. 

Also use power direct from the power supply with nothing else connected. Just incase so you do not damage the DCC equipment. 

Do this if you are willing to risk the smoke unit. 

Be careful with the chuff and puff input and put it to what voltage it wants. 

By chance is it a Lionel unit, or one made for 3 rail AC trains? 

It may work just fine, BUT Do this if you are willing to risk destroying that smoke unit.


----------



## jimtyp (Jan 2, 2008)

Thanks JP. Yes it is made for a Lionel 3 rail AC 18V. There is a separate board in front of the smoke unit, that is what takes the 18V AC. It then outputs 5V DC to the smoke unit fan. 

Since my DCC puts out a constant voltage should I put in a resistor to get it down from ~22V to 18V?


----------



## JPCaputo (Jul 26, 2009)

It should be ok without the resistor. It will be like running the lionel at full speed. 

I will get into a bit of useful theory. When 18vac (rms) is rectified. It becomes roughly 25volts. Because the ratio for single phase rms to peak is 1.4x.


----------



## jimtyp (Jan 2, 2008)

Thanks JP. I'm definitely electronically challenged so I appreciate your expertise.


----------

