Is DCC AC or DC current?

[Anonymous]

I thought I read that DCC track power was AC current, but if that's the case, how is it you can run DC motors (say older Athearn engines) with DCC just by adding a decoder? Do the decoders have some sort of converted that changes AC to DC? Or is DCC track power actually DC current and I misread something?

I don't know much about DCC electronics, and only know basic DC info, so alot of this is new to me.

Thanks!

[NZRMac]

DCC is high frequency AC that carries the digital control signal to the decoders in loco's etc. the decoders recieve the signal and convert the AC to DC which powers the motor etc. the digital signal controls the amount of DC to the motor, controls the whistle, horn etc too

Basically ( I'm sure Randy etc will tell you all the technical stuff!! )[:D][:D]

Ken.

[Anonymous]

Oh okay, well that makes sense then. I couldn't figure out how an older Athearn engine that was designed for DC running, could be swapped over to AC current DCC just by adding a decoder. If it's got a converter built into the decoder that would explain why.

[dinwitty]

model train motors are still DC, you just need the decoder installed.

DCC has a DC motor feature for ONE loco DC, howver it is recieving the full AC power
of the track, it just varies the bias of the AC to achieve DC control, and its not reccommended to do this as the motor always has current on it, it cant move because the AC is reversing the polarity too quick for the motor to move.
Cute trick to make DC motors to run on AC.
But it can heat the motor up, it wont turn to fan the motor, and can burn the motor out.

[rrinker]

The way a decoder works is actually similar to how address 00 can be used to run a DC loco. The big difference is the opposite polairty portion of the square wave is blocked by the decoder, to the motor only sees the pulse for the direction you command it to move. And with more modern decoders that have 'silent running' or one of the other names for the same feature do so at a frequency much higher than the DCC signal frequency. But the idea is the same - the longer the 'on' time, the more average voltage the motor sees and the faster it spins.

--Randy

[oleirish]

[%-)]After reading this post,it brings up a question?WHY was DCC made to run with AC insted of DC????
JIM

[Adelie]

I may be wrong, but I'm not sure it is possible to imbed the DCC signal in DC and provide motors reasonably steady current to run smoothly. It is pretty easy to use AC to transmit 1's and 0's, which is what DCC is based on.

[howmus]

QUOTE: Originally posted by oleirish

[%-)]After reading this post,it brings up a question?WHY was DCC made to run with AC insted of DC????
JIM

The simple answer... DCC needs to use AC in order to transmit the codes to the decoders. The power is not AC like you get from your house current but rather a very complex AC containing all the digital information that is needed to control the equipment.

The long answer is to go into a quite long disertation on the Hexadecimal System and data coding which you don't need to know to operate trains and is much to long a subject to be covered here. Besides that, I don't really understand it all myself. [:D] [:D] [:D]

[oleirish]

Thanks[2c][B)]
JIM

[cacole]

If you really want to know all of the nitty-gritty technical details about DCC, go to the NMRA's web site and read their DCC Standards and Recommended Practices that are used by the DCC system and decoder manufacturers.

But be forewarned, this will just get you more confused unless you have an understanding of electronics and digital communications.

http://www.nmra.org

[BentnoseWillie]

DCC is square-wave AC whose frequency is modulated to embed a digital signal. This supplies the +12V and - 12V (or greater) to run the DC motors. The wave has to be +/- to allow reversing. Each decoder rectifies this square wave and applies DC to turn the motor, on command. As far as the motor is concerned, the decoder is a powerpack like any other.

When address 00 is used to run a DC loco, the command station biases the AC toward one polarity - this doesn't affect the decoders because they're still getting their signal and voltage of both polarites. The non-decoder controlled motor "sees" the whole wave, generating a characteristic high-frequency "singing".

There is some concern about DC locos suffering motor damage if left idle on a DCC layout for an extended period. I haven't heard of a case where this has atually happened, however - probably because one is annoyed enough by the noise to remove the loco before it has a problem. [:)]

[rrinker]

QUOTE: Originally posted by oleirish

[%-)]After reading this post,it brings up a question?WHY was DCC made to run with AC insted of DC????
JIM

It's the whole reason WHY DCC is more successful than previous command control systems which did just as you describe - superimoise a low amplitude (think of it as the voltage) on top of a fixed DC voltage. Model railroads are a poor tramsnission environment - there's all the electrical noise from the motors and LOTS of parallel conductors (read - capacitor, which is a high resistance to certain frequencies). Witness the noise int he signal from most rtrain cams, even though the new ones broadcast an RF signal rather that through the track like the old Lionel system did. All this conspires to limit how well the signal can get through to the receivers in the locos. Bernd Lenz's stroke of genius was to combine the signal and the carrier into one. Each 1 and 0 in the data stream is represented by an equal time amount of positive-going current and negative-going current. Net 0 volts DC on the rails - 1ms + followed by 1ms - equals net 0 DC. (1ms is an example, not actual DCC timign information). Shorter times are 1 bits, longer than a certain limit is a 0 bit (see the NMRA specs for exact values). The spec also says that anythign over a certain limit is a 0 - there is no limit to the longest wave that can be used as a 0, outside of being able to maintain enough total cycles to meet the minimum bitrate (still awake? sorry, got a bit technical there). So on those systems that support it, you can run a DC loco by a technique called 0 stretching. It's exactly what it sounds like. Either the + half or the - half of those long 0 square waves in stretched out proportional to the throttle position. More + and less - equals a net DC positive voltage. And vice-versa. This allows a plain DC motor to turn, even through it is still getting pulses in the reverse direction - it gets MORE in the forward direction and thus moves.

--Randy

[NZRMac]

Told ya Randy would get technical!! Thanks guys I learned somefin too!! Where's that old dc loco......

Ken.

[Anonymous]

the converter isn't much, all it takes to change that way is a diode.

[rrinker]

A diode would work great - if you wanted full speed all the time. Decoders actually use a circuit called an H-bridge (since in a schemetic it looks like an H) and pulse-width modulation (PWM) to drive the DC motor from the DCC square wave. Most command control systems before DCC also used some sort of pulse system - because switching DC is HIGHLY inefficient and requires far larger components. At low speed, the motor might be getting 2 volts. Out of, for DC, 12 volts. That 10 volts is dissipated in the components as heat. It takes a good size heat sink that gets quite warm to handle even 1.5 amps - not tomention the 8a nd 10 amp large scale decoders! PWM allows the control devices to run either fully closed, or wide open. Minimal heat buildup because they are either passing NO current, or passing the load current with little drop (there's some - all semiconductor junctions have a voltage drop, but compared to droping 10+ volts, it's nothing). As the name implies, the width of the pulse is varied - the shorter it is, the less 'on' time there is, and the less average DC volts the motor sees. The longer the pulse, the longer the 'on' time and the higher the voltage the motor sees, and the faster it goes.
There IS a rectifier on a decoder, and a voltage regulator - to power the processor chip and produce fixed DC for the function outputs.

--Randy

[howmus]

An excellent article on how DCC works can be found here: http://www.loystoys.com/info/how-dcc-works.html

[Anonymous]

If you ever get into industrial electronics, then you can see somewhat how DCC works. The DCC system supplies the voltage to the motor controller (DCC Decoder), which in turn operates the motor in the locomotive. Since the motor is DC, the decoder can vary the voltage to the motor to control its speed. Because the motor gets the DC power only from the decoder, it is imperative that the motor be electrically isolated from the locomotive frame and the track voltage, otherwise the result is a fried decoder.