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"Keeping short circuits at bay with DCC" -- MR article

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"Keeping short circuits at bay with DCC" -- MR article
Posted by gregc on Sunday, May 07, 2017 6:30 AM

"Keeping short circuits at bay with DCC" in the June 2017 MR is on useful topic that can help many of us.   It's the first time I've seen an article decribing the use of bulbs for short protection in print.

But the problem I had using bulbs with my PowerCab was that the typical auto bulb (e.g. 1156) operates at a higher current than the PowerCab and the PowerCab would shutdown before bulb did any good.   This is anoying because it made it difficult to locate the short.

I think the article would be better if it discussed the need to select the bulb with appropriate current and identified different bulbs operating at various currents that would be appropriate for the system in use.

 

greg - Philadelphia & Reading / Reading

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Posted by maxman on Sunday, May 07, 2017 11:26 AM

gregc
I think the article would be better if it discussed the need to select the bulb with appropriate current and identified different bulbs operating at various currents that would be appropriate for the system in use.

I believe that there is a statement in the article saying that some experimention would be required to determine the bulb to use for different circumstances.

That didn't matter to me, because I tried the 1156(?) tail light bulb circuit and disliked it for the same reasons mentioned in the article.  I have the 5 amp NCE system and found that as I added more locos, within the command stations capacity, they would slow down and the bulb would start to glow.  Plus I didn't like the fact that a short would continue to draw current.  That didn't seem like a good idea to me.

I believe that the bulb method was derived prior to the invention of DCC circuit breakers.  They are a much better solution.

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Posted by richg1998 on Sunday, May 07, 2017 11:37 AM

I have the Power Cab for some years and tried the 1156 a few years ago. I did not like it.

Rich

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Posted by gregc on Sunday, May 07, 2017 2:07 PM

maxman
I have the 5 amp NCE system and found that as I added more locos, within the command stations capacity, they would slow down and the bulb would start to glow.

I had thought about mentioning this in my original post, although it was implied.  That the bulb needs to match the output power of the trottle.   My PowerCab needed a bulb that that drew ~1A.  A 5A booster would need a bulb (or multiple bulbs in parallel) to draw ~5A.

maxman
Plus I didn't like the fact that a short would continue to draw current.  That didn't seem like a good idea to me.

There's no difference to the throttle if a bulb or multiple locomotives are drawing 2A across the wiring and rails.   A properly sized bulb limits the current, unlike a fuse or circuit breaker which breaks the circuit.

A more complete article would have made this clearer.

maxman
I believe that the bulb method was derived prior to the invention of DCC circuit breakers.  They are a much better solution.

much more expensive solution.

greg - Philadelphia & Reading / Reading

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Posted by maxman on Sunday, May 07, 2017 2:26 PM

gregc
There's no difference to the throttle if a bulb or multiple locomotives are drawing 2A across the wiring and rails. A properly sized bulb limits the current, unlike a fuse or circuit breaker which breaks the circuit.

It isn't the throttle that is of concern.  It is the item that is causing the short.  I have a friend that burned down an Atlas HO tankcar because it was drawing a lot of current that didn't trip anything.

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Posted by richg1998 on Sunday, May 07, 2017 2:29 PM

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Posted by Stevert on Sunday, May 07, 2017 2:39 PM

gregc
maxman
Plus I didn't like the fact that a short would continue to draw current.  That didn't seem like a good idea to me.

 

There's no difference to the throttle if a bulb or multiple locomotives are drawing 2A across the wiring and rails.   A properly sized bulb limits the current, unlike a fuse or circuit breaker which breaks the circuit.

A more complete article would have made this clearer.

There's no difference to the throttle, but there IS a difference to your layout and/or the equipment on it!

As noted, the often-cited 1156 bulb draws about 2.1 amps.  At the 14 volts or so normally supplied for HO scale, that works out to about 29 watts of power going through that short.

Keep that 29 watts in mind when I mention that the soldering pencil I use for decoder work is a 7 watt pencil which easily exceeds the 370 degrees Fahrenheit needed to melt 60/40 solder.

So do you really want four times as much energy going through that short as it takes to heat a soldering pencil way past the point where it's capable of melting plastic, or even starting a fire on the layout?

All a bulb does is provide a load for the booster so it won't shut down in the event of a short.  It doesn't protect the layout or the equipment on it.

IMHO, money spent on a proper circuit breaker is money well spent.

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Posted by trainnut1250 on Sunday, May 07, 2017 4:01 PM

I have never been a fan of this approach since Joe Fugate was suggesting it here and in other places over a decade ago. The reasons:

 

1.      I don’t see the point: the stated purpose is to allow shorts within a district without shutting down the district for others running trains in the same power district. This is only really necessary on large Operations based layouts (or clubs) where there may be lots of operators and a short causes everyone to shut down while the offender remedies the situation.  With lots of operators, theoretically on a bad day, you might spend lots time waiting for the booster to reset. 

2.      You have to cut the layout up into blocks for the system to work. One of the reasons I went to DCC was to avoid wiring blocks…wiring these blocks to bulbs takes lots of time. This is especially true if the layout has been completed past the initial wiring stage. Isolating track sections from underneath is time consuming, there is always a wire you can’t see or a continuous rail conducting current that you weren’t counting on….

 

3.      The bulb is still letting current flow through the shorted section. I spend lots of time installing decoders in small locos. I want these installs protected ALL the time.  Especially when there is a short circuit.

 

I operate on several layouts regularly and none employ this system. Several of them are all one large block (including my own).  We don't spend lots of time waiting for shorts to clear.  From my perspective, I don't really see much need for the Bulb modification and, there are some serious drawbacks.

 

 

My two cents,

 

Guy

see stuff at: the Willoughby Line Site

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Posted by Jacktal on Sunday, May 07, 2017 4:18 PM

About "expensive solution"...locomotives and decoders are expensive too just as turnouts are.And that's not considering that I have a foam base under the trackwork and that I don't want my hobby to set fire to the house.I don't want any so called protection that will keep the heat on in the advent of a short.

I have a PM42 that works nicely.It has a cost indeed but not that much considering that my sound euipped Big Boy is worthed over four PM42s.

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Posted by gregc on Sunday, May 07, 2017 5:41 PM

As I said, i find a bulb a useful mechanism for dealing with shorts or high loads on my layout.  Other have to.   There are other perspectives that are equally relevant and valid depending on the layout and operator.

Electronics is challenging for many modellers and are expensive.   I see  far more forum threads with basic questions concerning electronics than on scenery or layout construction.

 

I think there is a need for more MR electronic articles. But recent articles have had errors and/or are incomplete.

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Posted by selector on Sunday, May 07, 2017 6:07 PM

I'm a little out of my depth here, but it seems to me that when Joe F. suggested that it was a viable alternative to more expensive fuse/breaker type appliances and DCC short detector circuits, decoders were the one item that he felt most of us wanted to protect.  We can deal with melted ties or dripping solder from fried feeders soldered to the rails nearby, but what to you do if close to 5 amps tries to run through the nearest decoder that is rated for 2 amps?  The point of the tail light was to reduce the probability that any one decoder in that power district (or block if you will, which was also part of the rationale behind Joe's proposal) would let out its magic smoke.  Far better to have trains slow and you may have to replace a tail light bulb @ $12 a pop, or whatever it is these days, than to replace what most of us would have had to spend to replace a newly marketed Tsunami at 2006 prices.

At least, that's what I took away from his argument in favour of them.

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Posted by rrinker on Sunday, May 07, 2017 6:40 PM

 That's why NCE came out with the EB6, it's 6 lower current light bulbs so it can work with the PowerCab. Although I would argue that with a small system that is designed to run only 1 or 2 trains, it's bette to just pay attention so you don;t run a switch and short in the first place. Running solo there is little need for any power district setups of any sort, unless you are running that one train with an full size 5, 8, or 10 amp booster. You don't want that level of power directly to the rails unless you are running large scale trains that draw 3-4 amps each.

 Dick Bronson at RR-CirKits has an interesting take on the light bulb concept, adding a few cents for a Polyfuse to the typical dual filament auto taillight bulb. He wires it such that under normal circumstances the current flows through the one filament. A short will be limited to the typical 2.1 amps, for a short time untilt he Polyfuse trips and throws in the other filament (actually, I have that backwards, the Polyfuse cuts OUT one filament) leaving just the lower current filament in the circuit - still a current limiter and not an interrupter but at a much lower level than the 2.1 amps of the 1157 bulb, yet not cutting power and partially lighting the lamp because you are running 1.5 amps worth of locos in that section. If you simply MUST use light bulbs instead of a actual circuit breaker, it's worth a look.

 Light bulbs were popular in the DC days, especally the early days when car batteries were the power source. Car batteris can delier a LOT of current instantaneously - a dropped wrench can literally weld itself to the terminals. So say nothing what a short would do to your nice brass loco (hopefully you always did want to try scratchbuilding littlerally from a metal ingot...). Circuit breakers were almost non-existent in those days - houses had fuses, and a one time use fuse isn;t too practical on a model railroad where incidental shorts happen all the time. So enter the light bulb. Limiting the battery current to a level that hopefully wouldn't turn your brass model into a puddle of molten brass. 

                              --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by gregc on Sunday, May 07, 2017 6:46 PM

I wish the article had addressed these points ...

selector
but what to you do if close to 5 amps tries to run through the nearest decoder that is rated for 2 amps?

there is no voltage across a decoder if there is a short between the rails that drops the track voltage to zero.

neither a 2A bulb or circuit breaker between the track and a 2A booster isn't going to protect a 1A decoder.

Stevert
So do you really want four times as much energy going through that short as it takes to heat a soldering pencil way past the point where it's capable of melting plastic, or even starting a fire on the layout?

if you have a 5A booster running multiple trains at a DCC voltage of 14V, there's 70W of power flowing through the wire, track, decoders and motors.   Most of it is dissipated as kinetic energy but some in heat.   There is practically zero watts of heat being dissipated in the wiring and track

when a bulb is lit, all that power is dissipated in the bulb.   The bulb gets hot, not the wiring or track.

 

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Posted by maxman on Sunday, May 07, 2017 6:53 PM

gregc
The bulb gets hot, not the wiring or track.

That's interesting.  According to the article "the weak point of ballast lamps is they don't stop current to the short...........that bulb can get hot, and so can the point where the short is occurring." I guess the article is incorrect.

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Posted by MisterBeasley on Sunday, May 07, 2017 7:09 PM

A good solid state circuit breaker like the PSX seems like a better investment.  But, nothing beats rock-solid track work and investigating every derailment so it doesn't happen again.

It takes an iron man to play with a toy iron horse. 

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Posted by gregc on Sunday, May 07, 2017 7:29 PM

maxman
According to the article "the weak point of ballast lamps is they don't stop current to the short...........that bulb can get hot, and so can the point where the short is occurring." I guess the article is incorrect.

yes, i don't understand why he said that.  I also don't understand why the article describes the bulbs he discusses as "ballast lamps" (google it).  The ones he referred to are incandecent lamps.

From a practical point of view, the short is not zero resistant, but practically no different that the resistance of a wire.  The short carrying 5A is not going to get any hotter than a similar length of track or wiring carrying the same current.  It's no different than the two pieces of metal making contact in an electrical switch or any other connection between two wires on the layout.

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Posted by rrinker on Sunday, May 07, 2017 7:44 PM

Kichoff's Law, if there is 2.1 amps flowing through the light bulb then there is 2.1 amps flowing through whatever is shorting the track. Odds are goof it's not THROUGH the decoder, but it certainly could be in some #30 decoder wire, or a circuit board trace connecting the frotn and rear pickups on the same side, or within a truck frame. That's still 28-30 watts, which through the metal parts of a truck sideframe can definitely get hot enough to melt the plastic sideframe attached to it.

                --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by gregc on Sunday, May 07, 2017 8:22 PM

I stripped a piece of 30g wire and held the center of it as I layed it across the rails.   My blub lit and the wire did not get hot (or even noticably warm).

heat is generated by resistance and there is so little resistance in metal (even 30g wire) that little heat is produced.

2A  thru 1" of 30g wire with 0.339 ohm/m is 0.02W.  (the ~30W would be a dissipated by the bulb).

Can't imagine much power being dissipated by any larger pieces of metal.

 

of course, if a decoder LED or motor lead hits the frame, I would guess the output transistor is going to blow before a circuit breaker opens.

 

greg - Philadelphia & Reading / Reading

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