IF the user remembers to install such a wire. But a backup is usually there, in the (usually) telephone cable used for the signal bus to the boosters. Which isn;t a heavy enough wire to handle the full current of a booster without significant loss, so it can cause other issues not immediately relatable to the lack of a heavy common.
NCE and Digitrax both mention hookign this up, Digitrax includes a terminal on the boosters for it. But neither makes a huge deal about it, and it seems to frequently get missed. I don't know what others like MRC and Lenz say. MRC tends to leave out the details that become important when you expand beyond the basic system, in the interest of makign their manuals smaller and simpler than everyone else's - except if you stick with it and have more than a small-ish layout, you kind of need to know that stuff to make the system work together properly.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
i only posted the diagram to show that the loco can draw power from one rail and one booster and the other rail and the other booster.
i believe the command station connection to all boosters (trackside) provides a common ground for all boosters thru which any "short" current across the gap flows back to a booster, otherwise, the decoder in the diagram above would not work.
i learned in junior high school, you cannot power a light bulb from the + side of one battery, the - side of a 2nd battery without a connection between the batteries.
rrinkerI wonder what the real problem is
excessive current creates heat which can damage electronic components if temperature limits are exceeded.
greg - Philadelphia & Reading / Reading
Not all boosters use the low side as common, sometimes it's in the moddle, with half the DCC signal positive relative to the common, and half negative realtive to the common. ANd there is another return path, or potential return path - the command bus to the boosters. In a common rail (actual common rail, in the DC sense) situation, it is usually recommended to use optoisolated boosters, so the only common IS the common rail.
I wonder what the real problem is - too great a voltage difference causing an SCR latchup situation with the protection diodes internal to the microcontrollers and/or driver MOSFETs, or just the losses in the diodes resulting in vastly different output voltages, or large curents flowing through components desiend to handle just a few mA. I'm leaning towards the latter.
Now picture that without the common wire between boosters. NOW where does the current flow, especially if the boosters are run with completely isolated (no earth ground) power supplies.
rrinkerReal question is, what do we do on the other side - with an all wheel pickup loco spanning the gaps, both gaps are going to be bridged.
not sure why this is the "real question"?
The decoder will see whatever voltage is present. While the wheels or frame may be creating the short(s), the decode is not in the path.
i'm curious about the voltage at the gap and current flow between the bootsters.
rrinkerWOuld be interesting to measure current flows even with just a realtively minor voltage difference, in the worst case - a loco parked spanning the gap (aka, a clip lead from one side of the gap to the other).
with some small resistance (> 0.25 Ohm) representing wire resistance
Yes, it certainly would be interesting to see what the limit is. ANyoen want to donat equipment that may or may not work once we're done with it? Actually I do have a spare Zephyr that has no power supply (I can power it from a bench supply). WOuld be interesting to measure current flows even with just a realtively minor voltage difference, in the worst case - a loco parked spanning the gap (aka, a clip lead from one side of the gap to the other).
Real question is, what do we do on the other side - with an all wheel pickup loco spanning the gaps, both gaps are going to be bridged. With a spliut pickup steam loco - you could have one gap bridged, but also the decoder/loco getting one side of its supply from Booster A, and the other side from Booster B. THe split pickup issue is there even if the two boosters are exactly matched in output voltage.
Even man of those with a simple toggle for 2 or 3 voltage options, like Digitrax, actually have an internal potentiometer to further fine tune the track voltage. This isn't in the instructions, only later was it brought up that if you needed a finer adjustment, you can open it up and make an additional adjustment. No idea on the actual circuit but I suspect the multiple position switch changes a fixed resistor that is in series with the fine tuning pot. Their newest command stations and boosters are electronically adjusted in, it appears, .1v increments.
since boosters may simply have two voltage settings (12/13.8), this issue may be moot (how many modelers alter any factory setting)?
It may be common and inevitable for there to be slight voltage differences (<< 0.5V) across the gaps of power districts. (10 ft of 18 g wire has a resistance of 0.064 Ohm and a 0.6V drop if passing 10A. 0.025 Ohm for 14G).
i am curious if there is a limit, how a booster handles any current spikes when the gap is bridged with metal wheels, or if this is not a concern and why?
Is it handled short term by capacitors in the boosters or does current in each district equalize thru the busses so that voltage drops makes the voltages equal?
I said 1.5V is no good. But it shouldn;t melt anythiung, you're talking the same side of the loco wiring, so a direct across some wiring, not across components of the decoder. If the currnet is high enough, it will trip the circuit breaker. This happens more often than not, and no one's melted their locos from this specific cause - happens when people have say a Digitrax Zephyr and then add a DB150 booster set on HO scale - that's a 1.5 to 2 volt difference since the original Zephyr was a fixed voltage, closer to the N scale setting on the bigger systems. It's just annoying when trains run different speeds when they are on different parts of the layout, so it is in everyone's best interest to get the voltage as equal as possible for smoothest operation.
.001V? Unless the adjustment pot in the booster is super touchy, that should be easily doable. Need a little better than a HF meter, but not crazy. My mid price handheld is only 4 digit, so it can;t get that accurate at over 9V, but my better one is 4 1/2 digits and can display millivolts up to 19.999 volts. I also have a 4 1/2 digit and 6 digit bench meters. The 6 digit is definitely better than 1mv accurate to a reference standard I have that was calibrated with one of those volt nut 8 digit meters.
I mean, if you wanted to. For some crazy reason. I do like precision measurement and test gear, but I've avoided going down that very expensive rabbit hole, I'm happy for most things I do to be accurate to .1 volt, it's close enough. The LAST thing I need is another hobby where even USED gear costs thousands. ANd may need repair - and then of course significant amounts spent on having it properly calibrated if you actually want to use it - traceable certified calibrations for high end test gear are not cheap. I'm happy just reading posts byt he people who get them and fix them up, I don't need one. I already have far too much test equiment.
gregc rrinker if one booster has a track votlage of say 14 volts and the other has a voltage of 15.5 volts, the loco will noticeably speed up or slow down crossing the gaps. you really think a 1.5V difference is OK when the wheels cross the gap and create a short between the rails thru the frame of the loco? doubt anyone could measure 0.001 V. But i bet you could measure a current increase with a 1.5V difference or measure a drop in booster voltage.
rrinker if one booster has a track votlage of say 14 volts and the other has a voltage of 15.5 volts, the loco will noticeably speed up or slow down crossing the gaps.
you really think a 1.5V difference is OK when the wheels cross the gap and create a short between the rails thru the frame of the loco?
doubt anyone could measure 0.001 V. But i bet you could measure a current increase with a 1.5V difference or measure a drop in booster voltage.
Well, he did say at 1.5 V "no one wants that", and that the loco would "noticeably speed up or slow down".
So I don't think he said it was ok. If you parked there, it might give a new meaning to the term "hot wheels"!
rrinkerif one booster has a track votlage of say 14 volts and the other has a voltage of 15.5 volts, the loco will noticeably speed up or slow down crossing the gaps.
Thanks for that info, Randy. Very informative.
Rich
Alton Junction
That difference between the outpus across the gaps is why a heavy (as thick as your bus wire) common is required between all boosters. Othersise, any current generated by that difference will try to find some other path between the two units, often throught he much thinner signal cable, and maybe through soem components not designed to handle it.
With boosters that are effectively infinitely adjustable, with a potentiometer, you can theoretically make the voltage the same. Or at least as accurately as your measuring tool allows. If you really want to worry, there technically are slight phase differences due to propogation delays on the signal bus from the command station to each booster, unless each cable is exactly the same length. But in truth, this isn't really an issue, which is why it isn't discussed, apart from having the voltages fairly close, if one booster has a track votlage of say 14 volts and the other has a voltage of 15.5 volts, the loco will noticeably speed up or slow down crossing the gaps. No one wants that. But to match them to .001 volt? Not necessary.
a "short" is a path between two points of unequal voltage drawing more than designed for current.
and normally the voltage difference is relatively large ~12V to ground or ~-12V to ~12V.
so we think of a short in terms of current where ~12V/ ~0 Ohm == ~Infinity.
so if the voltage difference in ~0.5 V, the current is still ~0.5V / ~0 Ohm == ~ Infinity.
so i wouldn't be surprised if the booster voltage on either side of the gap are not 0V (and of course in phase), then there's a spark and some current flowing. Presumably, the one booster gets loaded down and its voltage drops, eliminating the voltage difference and current across the gap.
then there's the case where there are multiple gaps, at different distances from the boosters and at slightly different (few tenths V) and no way to adjust the boosters.
it may not be enough to worry about for the reasons I mention above. But I've never read anything discussing it.
gregc richhotrain voltage is slightly different on each according to readings on my two RRampMeters have compared the RRampMeters to something more accurate? rrinker As long as the difference is less than half a volt, there should be no noticeable change in loco speed crossing between the two sections but that's essentially a short whenever the gap is bridged. can the booster voltage be adjusted so that the voltage is the same at the gap?
richhotrain voltage is slightly different on each according to readings on my two RRampMeters
have compared the RRampMeters to something more accurate?
rrinker As long as the difference is less than half a volt, there should be no noticeable change in loco speed crossing between the two sections
but that's essentially a short whenever the gap is bridged. can the booster voltage be adjusted so that the voltage is the same at the gap?
The voltages are adjustable, more or less. On my PH-Pro, the output voltage is factory set to 14 Volts, adjustable 9.5 to 18 Volts. Long ago, I adjusted the voltage to 13.5. On the second booster, an NCE DB5, the output voltage options are limited to either 12.0 or 13.8. I have never adjusted the voltage on the DB5. So, maybe that means that the RRampMeters are inaccurate since the voltage readouts are 13.4 and 13.6, respectively.
Locos crossing the gap from one booster district to another booster district should not short or stall. However, and correct my terminology if faulty, the power districts must be "in-phase" with the booster districts or the loco crossing the gaps will momentarily short. That happened in this instance where I moved the mainlines power district from one booster to the other booster. So, to correct that, I had to flip the two output wires on the mainlines PSX circuit breaker.
rrinker Well that would do it, a voltage difference with all else being equal will show a different current. As long as the difference is less than half a volt, there should be no noticeable change in loco speed crossing between the two sections, so it's not a huge deal. --Randy
Well that would do it, a voltage difference with all else being equal will show a different current.
As long as the difference is less than half a volt, there should be no noticeable change in loco speed crossing between the two sections, so it's not a huge deal.
richhotrainvoltage is slightly different on each according to readings on my two RRampMeters
rrinkerAs long as the difference is less than half a volt, there should be no noticeable change in loco speed crossing between the two sections
Michael, you are a man of few words.
But, you got me thinking. My two 5 amp boosters are not identical and the voltage is slightly different on each according to readings on my two RRampMeters. So, that likely accounts for the difference in total current draw after I moved the mainlines power district from one booster to the other booster. Hadn't thought about Ohms Law.
V=IRR=V/IR=12/0.14R=85.something ohms. Not a lot.
Disclaimer: This post may contain humor, sarcasm, and/or flatulence.
Michael Mornard
Bringing the North Woods to South Dakota!
rrinker Inaccuracy in the RRAmpmeter? Different wire lengths on the different busses causing less drop in the revised configuration? More (or maybe less) ringing with different track power bus configuration? Do you have snubbers on the track bus, and did changing the configuration maybe remove one? --Randy
Inaccuracy in the RRAmpmeter? Different wire lengths on the different busses causing less drop in the revised configuration? More (or maybe less) ringing with different track power bus configuration? Do you have snubbers on the track bus, and did changing the configuration maybe remove one?
When I moved the mainlines power district to a different booster, I did so by merely running two input wires to the PSX from the second booster instead of from the first booster, so the mainline bus wires remain the same. No big deal, but I am just curious how there would be a 0.14 amp difference in such an instance.
I have an NCE Power House Pro powering a DCC layout, divided into 7 power districts, and controlled by two 5 amp boosters.
I am in the process of adding signals to my layout, and these signals draw operating current from the DCC-powered rails.
This morning, I completed the mainlines and checked the current draw on the two boosters by readings on two in-line RRampMeters. The booster controlling the mainlines and engine servicing facility, where most of the locos are stored, showed a current draw of 1.80 amps. The second booster, which controls the four reversing sectons on the layout, showed a current draw of only 0.02 amps.
So, I moved the mainlines power district to the second booster. As a result, the first booster showed a current draw of 1.33 amps and the second booster showed a current draw of 0.35 amps. Somewhere in the balancing process, I wound up drawing 0.14 amps less than before the balancing process began.Where did those 0.14 amps go?