Hey Rails here's one that really floored my last night ! I visited a MRR club near LA I am thinking of joining. I brought along my Proto analog [DC in you insist] RDC to run if they liked... Said Hi and put him on the stall track of their roundhouse just to set it somewhere off the right of way as they were in a trance over their 2 long trains they were running/photographing..A member immediately pipes up " You better get it off there or the motor will burn up !! " I said " Why ? It's not warm and (as road is all Digital now [DCC if you insist]) it's not rolling full speed ahead (14 volts on rails)." He then says....." It's not that, the (get this now) AC will hurt the motor !" I said (appeasingly and to respect their governance over my not being a member yet)" What AC ? The rails are full power DC, no? " He/they said "No it's AC" I then went into a conversation on how trains which run/ran on AC were LIONEL , American Flyer, and such beause of two things. 1) track presented no reverse-loop polarity problems (read: dads could set up a quick, hassle-free train set for his boy with no intricate wiring work [ infact 2 wires from the transformer]) And 2) motor was not a perminant magnet and required alternating 60 hz current to spin the big sucker. The guys did not even seem to know whether I was even right or wrong. 2 of them insisted the rails were AC ... I said that maybe I am wrong and that the digital info to the encoders in the locos WAS AC .. If so , WOW was I ignorant of something which has been in our hobby some, what, 15-20 years WAS AC CURRENT coming from those LENZ and DIGITRAX controllers all this time !!! I thought digital communication was DC current which was converted to digital (0 volts off/0.5 volts on [basically])from the throttle which fiires through the full analog DC powered rails, rails !! Who's right ? Wont you Please, please help me help me help me OOooooooo !!!
I'm not an expert on DCC, but the rails have full AC power going to them at all times. The decoder converts it to DC to run the motor. I'm sure some of the more informed guys will chime in and give you the full explanation. Mike
MY WORD !! A C ? Where have I been ?? Why must the current be alternating just to get a message to the cab ? I still don't get it. Are you sure ? Why then can I use say an old MRC TECH II (analog DC) and be able to ring bell/whisle et.al. by guickly throwing direction switch while running a properly fitted loco (can't recall make now,[ Bachmann ?])? In this situ, rails ARE DC (analog) and yet one can 'send' a command. OR , have I hit the nail on the head ? I.E. qickly reversing the current makes current flip polarity as AC does (but at 60 times a second mind you), causing a WANTED disturbance to the loco
Hi Mark, Yes the rail are AC. As for it hurting your loco. The motor is trying to reverse direction 60 times in a second and at full power to boot. Thats the hum you will hear. Yes it will heat up after a while. It would be happiest running full steam in one direction. The decoder takes in the AC or DC and sends out DC. Try this test. Loco on DC layout. facing right. Right = forward Left = reverse Turn throttle and loco moves right or forward. Now lift loco and turn end for end. Loco now facing Left Right = reverse Left = forward Turn throttle up. Loco will move right or backward. AC current on DCC systems elimnates this. Forward is always forward, no matter which way the loco is facing. As for this AC current moving a DC motor. think of it this way. Loco is in a pipe with pressure from both directions. Loco will stay put. 50% pressure one way and 50% the other. DCC systems have the ability to change this ratio, ie,70% onr way 30% the other. this will push the DC loco away from the 70%. Hope this helps. I am not one of those experts you were told about. Just trying to explain it as it was to me. Have fun
I was under the impresion that it was still DC with a chopped signal. Square Wave DC/Pulsating DC. does the current in the rails actualy reverse or does it just drop to zero and then raise back up to 14V? Someone posted pics from an oscilliscope of the DCC wave but I don't remember if it had a negative value in the sine wave or not.
Nope. It is zero stretched, square wave AC current rectified by the decoder to DC for the motor and lighting. For a good primer, go to Tony's Train Exchange site on the www and read his DCC explained section. It is worth pondering for 20 minutes or so. Or, http://en.wikipedia.org/wiki/Digital_Command_Control As for operating DC motors on AC square wave DCC tracks, they'll squeel and squawl and groan...unlike their decodered cousins. Atlas has recently begun to ship engines with no decoders with a board that prevents them from operating on DCC tracks because they have gotten tired of repairing DC locomotives used on DCC layouts. Or, so I read the other day...don't know how true it is. In any event, leaving a DC motor parked with 15 volt HO level DCC current pulsing through the rails is apparently highly deleterious to those motors...they'll heat up by being excited magnetically, possibly melt the plastic around them, and certainly not do themselves any good.
being a Noob to DCC I did not know that the AC signal was rectified in the decoder. thought it was Pulsed DC. being an electrician by schooling, it makes total sense to me now.
If I have it correctly, the decoder knows which part of the AC signal to 'clip', thus leaving the directionally associated pulsed DC left as it goes to the motor. So, I believe it is truly pulsed DC, but only after the decoder deals with it based on the user's instruction of the direction of travel. If you want the engine to reverse, it just clips the other half of the current. I'm not sure if my terms are correct, maybe someone with more expertise will pipe up.
DCC is 'AC', but as stated above it is a square wave (OTOH the mains AC is a sine wave). DCC frequency (rate of change + to - and back) is about 8000Hz (8kHz), whereas the mains and what you get out of your 110V to 15V transformer is 60Hz (or in civilised countries its 230V to 15V at 50Hz ) DCC does NOT use the mains frequency, it generates it's own 'AC'. The high frequency means that most motors in non decoder fitted locos placed on DCC will 'block' most of the current (due to inductance*), whereas if you applied 60Hz 12V to them they wouldn't and would quickly cook up. Some DC motors (coreless is one example I believe) do not have enough inductance to effectively block the 8kHz power and will be damaged quite quickly. * Inductance is resistance that increases with frequency (it's the opposite of capacitance). If you want to know more it gets quite technical and ends up with imaginary numbers and phase shifting - and that's just for 'easy' sine waves
By the very definition of pulse or pulsed power indicates it operates with some sort of AC. However, you divide the sine waves, square them or round them it comes out the same. You don't have to take my word or anyone else's here. You can read up on DCC, buy a book and yes, you will learn it operates with a full load of AC on the track. I had hoped that DCC would deliver pure DC to the motors as they run much cooler and last longer. We are basically back... to a former pulse that used an AC chatter to control the motor for slow speed purposes. Not sure I'm happy with the flash back... but it seems to be the popular direction things are going. I was impressed with the performance of DCC on my layout. I must admit my DCC locomotives performed better then my Analong DC locos. Got to wonder what's up with that...right?
GOSH FELLAS, I didn't think I would start so much conversation about my opening thread title !! WOW, looks like many of us really don't know waz up with digital MRR control (DCC if you wish). I NEVER heard the term AC in our scale (except for the damned juice coming from the wall socket !!) until going to the club (above)..This is goofy !! AC in our HO rail ? ! O..K ...I thought too it was some kind of DC pulse which travels on the say, carrier current of 14 volts DC, no? A few of you sound very informed. But who of you will stand up and say, just like Brigham Young, " This is it !" ? Lay it on us as simp as pos..AC ..Are you 1000% sure ? WANTED :bottom line... Mulch tanks, Mark
If you read/have read through the two sources I suggested, you will know that I am willing to bet my next monthly pension check. ;-)
You don't need to. You probably don't know how the engine/braking/suspension management computers in a modern car do their stuff - you just drive it. Same with DCC - just enjoy the freedom The picture on the Wikipedia page Crandell linked to shows you it is AC - about half the volts are above 0 (the dotted line) and half below. It's just not like the AC a wall socket provides.
When DCC was developed, they did not have to use AC, but there were some compelling reasons why they did: If DCC used a pulsed DCC and you powered up a layout that had DC engines on it, they would all take off at a high speed. Using AC allowed the implentation of running DC locomotives on a DCC layout using "zero-stretching." Using AC simplified the requirements of the decoder design. With pulsed DC, the decoder could only pick up the encoded signal from one rail, so it would have to monitor both rails so that it could read the signal no matter which way it was place on the rails. With the signal being AC, the decoder can read the signal from either rail so it only has to monitor one of them. Using AC also allows for the 6 pin decoders design(where the lights pick up power from one rail instead of the blue lead). With pulsed DC, if you tried to power the lights from one of the rails, it would work when you placed the locomotive on the rails in one direction, but not in the other. I am not sure what you mean by "operates with some sort of AC." The DCC waveform from the command station to the decoder is very different from the waveform of the power from the decoder to the motor. The DCC waveform is AC, but the waveform from the decoder to the motor is not. The deocder output is pulsed DC. The pulses do cause it to have an AC component, and if this what you mean by "operates with some sort of AC" then that is correct; however, if you mean it actually reverses polarity, then that is not correct. To be clear: from the command station to the decoder is AC, from the decoder to the motor is not AC, but it does have an AC component(in simple terms, to have an AC component means that the output voltage varies around the average voltage). There is a simple reason that decoders use pulsed power instead of pure DC, and that reaosn is heat. While the motor would run somewhat cooler(but not a lot cooler, due to the inductance that Mike talked about, more on that in a minute) with pure DC, the decoder would have to either be a lot bigger or run a LOT hotter. With pulsed power, the decoder's output is either on or off, and the decoder has to dissipate very little power. To get pure DC(or close to it) output, the decoder could continue to pulse the power and use caps to smooth it out(the bigger solution), or it could regulate the output down to the DC voltage it wants to output. To regulate the voltage, the decoder would have to dissipate power to drop the voltage. When running on the old "pulse" power packs, motors did run a lot hotter, because of the 60 HZ frequency of the pulse. Most current decoders(any of the ones that say "SuperSonic Drive," "Quite Drive," "Silent Drive," or a variation of these) use a pulse frequency above 15,000 hz, which will result in very little additional heating of the motor. There are still some decoders that use lower frequencies(the Lenz designed decoders that some N-scale Atlas engines come with and the DCC Onboard decoder in the N-scale DD40AX are a couple that come to mind) and they will cause more heating thean the higher frequency drives, but still not as much as the old pulse power packs because they are still a much higher frequency than they were. I'll say it, it IS AC. I have seen people claim that it is not AC because it is a square wave or because it has a varying frequency. Neither of these qualities, however, disqualifies it from being AC.
CSX Robert, You read me correctly. I appreciate your response and explanation. I try to keep things simple but I always appreciate someone who can jump in and give us the details. Nice come back.
It's AC, This Is The Way...IT IS! Mark, I know where you are coming from...Grin! There have been so many wanna be leaders, leaders and finally governing officials who have stood up and said "This is the way". If you tried to examine what each one taught or believed at the end of the day...well...it would take more then a day. Denial is such a wonderful thing. You can hide there until you are ready to deal with the issue. Are you ready? Yes, it's AC on the track! That's the way it is. Now go back and read CSX Robert's explanation. He's nailed it and you needn't know more...unless you want to. It's all about a difficult learning curve and you hitched on somewhere down at the bottom. Time to study up and prove it, to and for yourself. Welcome to the world of DCC. wink:
