Easy - at full throttle, the BEMF decoder does not give 100% pulse width, maybe something like 95%. So if the loco slows down below the set speed, it can still increase the voltage to the motor to maintain speed. If it gave 100% at full throttle, the loco would just slow down because there would be no way to increase the motor voltage, it would already be at the maximum available.
I would tend to avoid lamp cord or zip cord for that very reason, much harder to attach feeders to it. ANd while it might nominally supposed to be #16 wire, there is plenty of chea stuff that has a very thick rubber insulation and not a whole lot of wire inside - probably closer to #20 than #16. Assuming the wire is even copper.
The leads on so many electronic components are not copper any more - clearly not, when I can use a magnetic parts tray meant for holding screws to catch the cut-offs after soldering a board. ANd they don't just dump out over the trash, they are magnetic.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
rrinkerAnd of course there have been numerous posts here about how a given loco is "too slow" - turning off BEMF 'fixes' that, because the decoder is doing exactly as you say, reserving a bit of the top end so that the BEMF has something to do to prevent a slowdown.
not sure how BEMF fixes a too slow loco or reserves "a bit of the top end". sounds like it can somehow increase the voltage to the motor above track voltage
BEMF can be used to measure the speed (RPM) of the motor. instead of the speed value to a decoder setting a voltage, it can specify a desired speed and let the decoder adjust the voltage to achieve that speed.
i'd hate to see the OP buy 12g wire, as suggested, and replace his 32 ft of lamp chord with it because it's a "better option". (i do wonder how easy it is to feeders to lamp chord)?
greg - Philadelphia & Reading / Reading
You need to apply a load to see a voltage drop, the drop is proportional to length of the run, resistance of the wire, and the current being drawn. A multimeter by design puts as little of a load on the circuit under test as possible, to avoid impacting the measurement. You could put 12V in to a 500 foot spool of temephone wire and a multimeter will measure 12V on the far end (a REALLY sensitive one could see a slight drop, but not your typical 3 1/2 digit handheld type.).
An auto tail lamp bulb is a good load simulator for what would be a multi unit lashup of HO diesels plus a little extra. Put that across the rails at the far end and then measure the voltage.
Why not get a multimeter and test the voltage at the controller output hook up the wire test at the other end and see what the difference is?
Joe Staten Island West
Which is exactly what code calls for here. Although I used #12 for all the new 15 amp circuits I ran for my basement remodel, and only use #12 in house wiring unless something larger is called for.
rrinkerThere was a recent thread on the NCE Group, with an O scale club using I think #8 wire for 10 amp boosters. Because the 'expert' is afraid of fires. The person posting used lesser wire for short runs to install block detectors, and as the story goes, everything ran perfectly fine for 3 operating sessions before he made it known that he used something other than #8 wire. Then he went back to the club the next time and the 'expert' was ripping it all out saying it was dangerous.
And yet the expert's house is likely wired with 20 amp circuits using #12 wire.
York1 John
In many cases, yes, since historically most scale models ran way too fast at full throttle, so there was plenty of headroom to just up the voltage a bit to compensate. More and more these days though, locos run close to or at realistic speeds.
And of course there have been numerous posts here about how a given loco is "too slow" - turning off BEMF 'fixes' that, because the decoder is doing exactly as you say, reserving a bit of the top end so that the BEMF has something to do to prevent a slowdown.
Yes, most people seem to tend to overkill - #12 should be more than adequate for anything, and #14 plenty for most room size layouts, and a 4x8 needs no more than #18. There is also the issue of how long you run the waire. That's why there are boosters. If the power wires are kept short, there's no need to use insanely thick wire.
There was a recent thread on the NCE Group, with an O scale club using I think #8 wire for 10 amp boosters. Because the 'expert' is afraid of fires. The person posting used lesser wire for short runs to install block detectors, and as the story goes, everything ran perfectly fine for 3 operating sessions before he made it known that he used something other than #8 wire. Then he went back to the club the next time and the 'expert' was ripping it all out saying it was dangerous.
rrinker2V is definitely too much drop, that's very noticeable as a speed change in the train.
of course it is. but that's at 5A? it would only be 0.5V at 1.25A and even that's a lot a current.
But we're talking about the DCC track voltage, not necessarily the motor voltage. if you're operating at less that max speed step, the PWM duty cycle can be increased to maintain the motor voltage. If the decoder uses BEMF to maintain speed, it adjusts the PWM duty cycle automatically.
2V is definitely too much drop, that's very noticeable as a speed change in the train. .5V is about a smuch as you really ever want. It was different with most DC systems, since you never ran wide open anyway, or at least anyone operating realistically did, if the train slowed at the far away point due to loss in the wire, you could just gradually creep up the speed control.
I used to smoke cigarettes and the pack was labeled LS/MFT!
sorry, but things need to make sense.
selectorline resistance takes place over distance at a given voltage.
don't understand what voltage has to do with it. The wire has a resistance regardless of the current flowing thru it or the voltage across is.
selector If the voltage drops enough
how much of a voltage drop is "enough"? is 2V at 5A too much
selector If the voltage drops enough, the short detection circuitry in the base unit of DCC systems won't detect the fault, it won't trip,
which part of the DCC system needs to detect the short? how can there not be enough voltage at the booster or a circuit breaker connected to the booster for it to not detect a short on its output
selectorAmong the first things to go are the components in a decoder.
if there's insufficient voltage reaching the decoder why should it be damaged?
selectorIf the DCC signal is so weak that the decoder can't read it over the noise,
how can a voltage drop introduce noise? the signal is the polarity reversals?
how much of a voltage drop is required for the decoder to be unable to see the polarity reversals carrying the signal?
how low can the track voltage go before the decoder stops operating (<~5V)?
selectorthe decoder will continue to meter the same voltage to the can motor as it did with the last instruction.
if it's not using BEMF, it's applying the same PWM signal to the motor and there is effectively less voltage across the motor.
If it is using BEMF to maintain speed, an equivalent amount of current (> PWM duty cycle) is flowing thru the motor (it's not running any faster or slower unless it was near max speed)
how can either condition be harmful unless possibly there is a significant (~10V) drop and insufficient voltage for the motor to turn?
selectorFinally, if you continue to draw the same current, but use less voltage to 'carry it', you'll get hot-running components, especially a motor doing work with the current, or the amperage.
why should the motor run hot drawing the same current at less voltage? isn't there less power (V*I) dissipated by the motor?
selectorIf an industrial motor is running hot, but you need it to perform the work it's meant to do, and with limited current, you'll have to change something, and that is done by raising the voltage.
don't understand why increasing the voltage and power (V*I) into a motor makes it cooler. is there another explanation (increasing its speed)?
And with a power factor of 1, ie a pure resistive load with AC, or with DC, P=VI if you solve P-I squared R by replacing R with V/I and simplifying. So if P remains constant, but V is going dowm, I has to increase to maintain the same power.
BigDaddyModel Railroader Georg Ohm, playing with his Marklin train set, didn't think current increased with a voltage drop, otherwise he would have invented a different law V=I*R
Then it must have been Hornby modeler Joule who invented the correct law; you maroons forgot something when you stopped using E for voltage and I for current for plain old DC and got your little letters mixed up.
V (or E) is simply IR. (That's little Georgie's law)
P = EI. (That's the thing that poor Betamax is being blasted for.)
Combining, P = I squared R (with an exponent, not an asterisk which connotes a form of multiplication).
One of you soi-disant electrical experts tell me what happens if you have a constant load in watts and the voltage starts sagging. Now how does that differ if you reduce voltage some other way?
(Seriously, now that I've had my little Manhattanite chop-busting fun: if all you're concerned about is the voltage drop from distributed resistance in the wire, then the available voltage goes lower following Ohm's law. It's when you actually try extracting power down near the end of the wiring run that the 'other' thing starts to become an issue ... see resistance heating and Kirchhoff's laws.)
Greg, line resistance takes place over distance at a given voltage. If the voltage drops enough, the short detection circuitry in the base unit of DCC systems won't detect the fault, it won't trip, and it'll let whatever has the least tolerance for current burn up. Among the first things to go are the components in a decoder.
Runaway decoders are decoders that continue to behave per their last received packet instruction. If the DCC signal is so weak that the decoder can't read it over the noise, the decoder will continue to meter the same voltage to the can motor as it did with the last instruction. It won't obey anything except a cessation of rail power, which the emergency button 'should' effect for the operator. If that doesn't work, there's always pulling the plug...literally.
Finally, if you continue to draw the same current, but use less voltage to 'carry it', you'll get hot-running components, especially a motor doing work with the current, or the amperage. If an industrial motor is running hot, but you need it to perform the work it's meant to do, and with limited current, you'll have to change something, and that is done by raising the voltage. I learned this from my father, a mining engineer, who instructed an electrician to pot up the voltage running to a crusher motor that was almost too hot to touch. When they raised the voltage by 40 volts, the motor began to cool. Same amps, higher voltage, cooler motor.
Pretty much a gimmick at audio frequencies, but as audio gimmicks go, it's a pretty mild thing. #10 is overkill unless you are running 1000 watt amplifiers.
Now if you said you were using directional speaker wires, or directional line cords, or those silly isolation pyramids under the speaker wire to keep vibration out of the speaker signal.....
Adequate wiring is important so circuit breakers can trip. Too much resistance int he wire means the current flow won't exceed the trip point of the breaker, so realtively high currents can flow through things not meant to handle them (pickup wipers, the thin wires used inside the loco, etc). This is why passing the quarter test is important. Set (do not push down) a quarter or equivalent size local coin across the rails and various spots all along the tracks, every foot or so. In each case, the circuit breaker should trip. If not, the wiring needs beefing up in that area.
SeeYou190there is no oxygen trapped in the insulation with the copper.
OFC refers to the copper alloy itself, not oxygen trapped by the insulation. That's why they call it Oxygen Free Copper instead of Oxygen Free Cable. The alleged advantages are very slightly improved conductivity and more resistance to corrosion. I understand it makes a big difference in particle accelerators.
I have the right to remain silent. By posting here I have given up that right and accept that anything I say can and will be used as evidence to critique me.
All: many thanks. Everyone is very helpful. You are the best folks. Andy
gmpullman Speaker wire is also a type of zip cord, too, but most places charge a premium price for speaker wire.
The speaker wire I use is more expensive because there is no oxygen trapped in the insulation with the copper. It is called "OFC" on the packaging. It is also 10 gauge made up of strands that are only 0.004" in diameter. Not sure if any of that really does any good or not.
betamaxIf the voltage drops the current will increase, meaning more heat produced by your decoders and booster.
I am not a DCC user, but this does not sound right.
-Kevin
Living the dream.
Some years ago our club switched from DC to DCC and we used #14 with about six to eight inch #24 feeders. Number 12 we thought was overkill and we had the NCE five amp Power Pro.
PFM turnouts. Non by DCC.
We ran as many as ten sound locos with no issues on code 100 rail.
At home I kept #22 when I switched to DCC. I only have the Power Cab. No big deal.
If your layout is wired, give it a try. That is what I did.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
I've seen some lamp-type zip cord in 12 gauge, but it is not very common. Speaker wire is also a type of zip cord, too, but most places charge a premium price for speaker wire. Some is extra flexible, too.
The drawback in my mind would be trying to differentiate "rail-A-rail-B" while under the layout when the color of each conductor is the same and all you have is a little rib to differentiate each leg.
This outfit (and others) offers a black/red zip cord down to 2 ga.
https://powerwerx.com/red-black-bonded-zip-cord
Still, there are better choices.
Regards, Ed
Andrew:
Our club uses 16 guage automotive wire. I don't know our size, but we have four power districts, with three subdivisions in each.
Our record is 18 people each running an engine. You are probably good.
betamax If the voltage drops the current will increase, meaning more heat produced by your decoders and booster. why? since there's less voltage why don't you say there will be less heat ???
why? since there's less voltage why don't you say there will be less heat ???
Model Railroader Georg Ohm, playing with his Marklin train set, didn't think current increased with a voltage drop, otherwise he would have invented a different law V=I*R
Henry
COB Potomac & Northern
Shenandoah Valley
betamax14 or even 12 gauge is a better choice.
why? 5 Amps is required to cause a 2V drop at 30 ft (18g wire has 6.4 mOhm / ft)
betamaxPoor wiring can cause problems such as runaway decoders and inadequate short circuit protection.
why? what's a "runaway decoder"?
Wiring is very important with DCC. DC wiring is usually inadequate for DCC. The lamp cord used for your is probably 18 gauge, which is adequate for feeders but not for the bus.
For the run you plan, 14 or even 12 gauge is a better choice. So upgrading your wiring should be considered. Poor wiring can cause problems such as runaway decoders and inadequate short circuit protection. DCC systems put a lot more current on the track than analog power packs, so this is important.
Since a DCC system can supply more power, voltage drop becomes an issue, because you can and will run more trains... If the voltage drops the current will increase, meaning more heat produced by your decoders and booster. More heat is never good with electronics.
Lamp cord is 18 gauge. Still, for your short-ish run, it would probably work ok. IF it isn't too much of a hassle, it might be worth your time and effort to bump it up to 14 gauge stranded.
Mark.
¡ uʍop ǝpısdn sı ǝɹnʇɐuƃıs ʎɯ 'dlǝɥ
andychandler. I use lamp cord as my DC bus.
So, is that the little skinny brown stuff that some "home"extension cords are made of?
Even if it is, I bet it's 14ga. stranded. I would say with the length your talking, it would be OK.
Mike.
My You Tube
Gang: ready to convert my DC layout to DCC. Total track length is 32 teet, HO scale. I use lamp cord as my DC bus. Can I use this for DCC or do I need to replace it with something else. Many thanks, Andy