firetacoma
Member
My new loco (first) says it will accept a maximum of 12v dc. The DCC system I am getting (hopefully delivered soon!) says it puts out 14-16 volts. Um. Am I missing something?
Track voltage?
- Thread starter firetacoma
- Start date
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Not to pick at nits, but that "ain't" what a bridge rectifier does. Rectifiers convert AC to DC. Bridge Rectifiers convert AC to full-wave-rectified DC. DC is not "a more normal sine wave."
Please explain just how a bridge rectifier can convert anything into any kind of sine wave, in case I have misunderstood.
Cheers!
If you read Jim's post you'd see that it was explained to him by an electrical engineer (aka he didn't understand it before the explanation), so I'm willing to cut him slack on that minor error. You are correct, that is what a bridge recitifer does - it's just a mix-up of terms that's all.
Just to expand a bit on what Jim was talking about originally, using a DCC unit to operate a DC loco is a process called "zero stretching". It is an NMRA standard that was allowed in to the DCC design so there was some some legacy compatibility to DC units. Most literature I've read however really discourages use of zero stretching as it isn't really too good for the DC motors.
Mark
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Selector
Well-Known Member
You shouldn't have any problems. If you are in DC, which I guess is the case in view of the maximum voltage stated, you simply control the voltage at the power box or control station with a rotary knob. You are highly unlikely to exceed the maximum voltage since at that voltage your engine will be whizzing around the layout at breakneck speed.
If you are in DCC and in a particular scale, the 12 volt max suggests you are in N scale, although even then it's a bit low to be honest. If you are in HO, that scale usually caps out at 16 volts, maybe a bit more. No matter the case, your base station will have a toggle or setting appropriate for each scale and will limit the voltage supplied to the rails to the maximum allowed under NMRA DCC guidelines...or it should be very close.
If you are in DCC and in a particular scale, the 12 volt max suggests you are in N scale, although even then it's a bit low to be honest. If you are in HO, that scale usually caps out at 16 volts, maybe a bit more. No matter the case, your base station will have a toggle or setting appropriate for each scale and will limit the voltage supplied to the rails to the maximum allowed under NMRA DCC guidelines...or it should be very close.
firetacoma
Member
Ah, that's what I wanted to hear! I am N scale using DCC. I will be sure to set it up as such when I arrives! Thanks for the help!
Fire, my brother's an electical engineer and explained how you have can DC and AC on the same track. The control station senses an engine without a decoder and then sends out AC with a heavily modified sine wave, so the back leg of the sine wave is really long. This is enough to fool the DC motor into thinking it's seeing DC since it wouldn't run on AC. The DCC locomotives have a bridge rectifier that converts the AC back to a more normal sine wave so they will run correctly. You can usually have no more than one DC locomotive running at a time on a DCC layout. As long as the locomotive is running, there won't be any problems. Don't let a DC locomotive sit for too long on a powered DCC layout though because you'll end up with a burned out motor. In N scale, this can take anything from a few minutes to a few hours.
firetacoma
Member
The DCC system I have on the way doesn't support DC engines at all. This engine is decoder equipped... I guess I'll know more when it comes time to set up the DCC system!
Not to pick at nits, but that "ain't" what a bridge rectifier does. Rectifiers convert AC to DC. Bridge Rectifiers convert AC to full-wave-rectified DC. DC is not "a more normal sine wave."
Please explain just how a bridge rectifier can convert anything into any kind of sine wave, in case I have misunderstood.
Cheers!
Last edited by a moderator:
CanuckMark
Member
If you read Jim's post you'd see that it was explained to him by an electrical engineer (aka he didn't understand it before the explanation), so I'm willing to cut him slack on that minor error. You are correct, that is what a bridge recitifer does - it's just a mix-up of terms that's all.
Just to expand a bit on what Jim was talking about originally, using a DCC unit to operate a DC loco is a process called "zero stretching". It is an NMRA standard that was allowed in to the DCC design so there was some some legacy compatibility to DC units. Most literature I've read however really discourages use of zero stretching as it isn't really too good for the DC motors.
Mark
Thank you, Mark. You're right, of course, my understanding of electricity doesn't go much beyond screwing in lightbulbs and I'm sure I did get my brother's explanation somewhat wrong. I knew I should have written it down. 
However, that "zero stretching" part was the the thing that fascinated me since I couldn't understand how a DC motor could run on a track that was being fed with AC. He said that the long leg of the sine wave would indeed eventually damage a DC motor although it wouldn't do so very fast as long as the engine was running. Apparently HO scale motors are a lot more tolerant of this zero stretching trick because they're bigger but N scale motors won't take a lot of sitting on DCC tracks doing nothing before the engine turns into steamer even if it's a diesel. 
However, that "zero stretching" part was the the thing that fascinated me since I couldn't understand how a DC motor could run on a track that was being fed with AC. He said that the long leg of the sine wave would indeed eventually damage a DC motor although it wouldn't do so very fast as long as the engine was running. Apparently HO scale motors are a lot more tolerant of this zero stretching trick because they're bigger but N scale motors won't take a lot of sitting on DCC tracks doing nothing before the engine turns into steamer even if it's a diesel. CanuckMark
Member
No prob Jim, if I can lean on you for RR tips feel free to lean on me for electronics help.
We're going a bit OT (that's my fault), but part of the reason the motor won't like zero stretch is what the motor "sees". A DCC signal is a +/- 12V (or close to that) square wave, where the 1s and 0s the decoder reads are transmitted as narrow pulses for a 1 or wide pulses for a 0. The decoder is fairly specific on the timing width of the 1, but the zero is much more lenient. The minimum spec is that a 0 is double the width of a 1, but it can be much wider then that. A zero stretch therefore is when the signal is held high on one edge (like a really long zero), essentially sending 12 volts DC to the rail. The problem is it is just that, either +12 or -12 VDC depending on which edge is held. To run the motor at slower speeds the controller sends narrow pulses in the opposite polarity - the motor receives the pulses of reverse voltage reducing the "average", and it is not a clean recified reduced voltage. So it tries to essentially go in reverse then forward very quickly, but with more/longer forward pulses. It'll overheat like a bugger.
Yeek...I hope that makes sense...I do go on y'know...
Mark
We're going a bit OT (that's my fault), but part of the reason the motor won't like zero stretch is what the motor "sees". A DCC signal is a +/- 12V (or close to that) square wave, where the 1s and 0s the decoder reads are transmitted as narrow pulses for a 1 or wide pulses for a 0. The decoder is fairly specific on the timing width of the 1, but the zero is much more lenient. The minimum spec is that a 0 is double the width of a 1, but it can be much wider then that. A zero stretch therefore is when the signal is held high on one edge (like a really long zero), essentially sending 12 volts DC to the rail. The problem is it is just that, either +12 or -12 VDC depending on which edge is held. To run the motor at slower speeds the controller sends narrow pulses in the opposite polarity - the motor receives the pulses of reverse voltage reducing the "average", and it is not a clean recified reduced voltage. So it tries to essentially go in reverse then forward very quickly, but with more/longer forward pulses. It'll overheat like a bugger.
Yeek...I hope that makes sense...I do go on y'know...
Mark
Thanks, Mark, that makes perfect sense. My brother was here this summer helping me with installing the DCC and he said you can't run a DC motor on AC current - period!. He about fell over when I put a DC engine on the track and ran it while a DCC engine was also running using the outboard throttle of a Bachmann EZ Command controller. He tracked down the zero stretching concept on the web and then it made sense. We did test an older BB Athearn engine by running it for a half hour and then measuring the temperature of the windings. It was about 130 - pretty warm but he thought it would probably last forever as long as it was kept running. Sitting on the track was another matter. It made an ominous buzzing noise the whole time and the windings went to 185 in less than ten minutes. His guess was, assuming the temperature curve was linear, that the motor would probably burn out the windings in about 40 minutes. I suspect the temperature curve isn't linear since I think it was Jeffrey that left an old Athearn sit on his tracks and it took something like 8 hours to burn up but I think that's pretty variable even with the same motor design. I guess the bottom line is not run DC engines a lot on DCC and don't let them sit idle for any longer than absolutely necessary.
I guess I kind of go on too.
He about fell over when I put a DC engine on the track and ran it while a DCC engine was also running using the outboard throttle of a Bachmann EZ Command controller. He tracked down the zero stretching concept on the web and then it made sense. We did test an older BB Athearn engine by running it for a half hour and then measuring the temperature of the windings. It was about 130 - pretty warm but he thought it would probably last forever as long as it was kept running. Sitting on the track was another matter. It made an ominous buzzing noise the whole time and the windings went to 185 in less than ten minutes. His guess was, assuming the temperature curve was linear, that the motor would probably burn out the windings in about 40 minutes. I suspect the temperature curve isn't linear since I think it was Jeffrey that left an old Athearn sit on his tracks and it took something like 8 hours to burn up but I think that's pretty variable even with the same motor design. I guess the bottom line is not run DC engines a lot on DCC and don't let them sit idle for any longer than absolutely necessary. I guess I kind of go on too.
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