I am using an MRC Prodigy Express system. I decided to try the auto light bulb system to limit current draw during a short.. I wired a 1156 taillight bulb in series with one leg of the power. When I short the tracks, the bulb lights up but it still trips the command station. I decided to try two bulbs in series - figuring that would be twice the resistance. I get the same result - the bulbs light up but the command station still trips. The demo I saw limted the current draw on the short enought to keep the command station from tripping and to allow a loco on another track to continue to run. Am I doing something wrong? Even if I can't get it to stop tripping the command station, is it a good idea to keep this system in place to limit any damage that might be caused by a short? Any help would be appreciated.
Thanks ahead of time to all of the helpful people on the forum!!!
Jim in Costa Rica
Modeling freelance Northern California late 1930s
The best thing is to get something like the PSX series of circuit protection from Tonys Train Exchange. Easy to wire in and the current levels can be adjusted for the best trip level desired. You will need one for each of the power districts that you have.
In my opinion (and not everyone agrees), you should get rid of the light bulbs and use a proper DCC circuit breaker.
The reason is that an 1156 bulb draws about 2.1 amps @ 14volts. While that might limit some command stations from shutting down, it still allows roughly 30 watts (depending on your track voltage) to flow through the short.
I use a 15 watt pencil for almost all of my railroad soldering. It gets plenty hot to melt a body shell or plastic truck frame. Why would I want to risk applying twice that heat to my models while I look for a short?
Jim:
The light bulb solution does not work well or at all with the lower amperage systems like the Prodigy Express and the Zephyr. As you have found the system short protector acts faster than the bulb can heat up. I'm told that the bulbs work OK with the 5 amp systems. There was a lot of discussion about this on Joe Fugate's forum.
Joe
If you really want to use light bulbs, you need lower amp ones than the 1156 for use with a lower power DCC system. NCE makes a unit that has 6 bulbs for 6 zones of protection, and it used like 1.6 amp bulbs.
Personally, I do not like the light bulb idea, as it cuts off nothing, power still flows continuously through the short. I'd rather a proper DCC breaker that actually cuts power. Or the alternal maethod shown by Dick Bronson on his RR-CirKits web site, using a dual filament bulb and a cheap PTC resistor. Under normal circumstances, trains run as usual. WHen a short happens, initially it is limited to the 2.1 amps of the stop light filament, but within a couple of seconds the PTC resistors heats up and the current flows through the .6 amp turn signal filament. Still wouldn;t work on the PA or original Zephyr, but a few cents each for the PTC resistors is cheap insurance and somewhat better then letting 2.1 amps flow through a shorted loco. At the typical 15 volts on the rails for HO, that's over 30 watts. Ever grab a 15 or 25 watt light bulb?
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
You really don't need to add circuit breakers. Prodigy Express has one built in as does every DCC booster on the market. PE is limited to 1.6 amps. A PSX circuit breaker has a lowest setting of 1.27 amps. Digitrax PM42 can only be set to a low of 1.5 amps.
Unless the layout is broken up into more then one power sub-district (think blocks) an additional circuit breaker is not needed. If so, then one per sub-district is required.
Martin Myers
Thanks everyone for the help. For now, I think i will stick with the built in breaker but might eventually add an after-market one to limit the wear and tear on my control unit. It will also probably eliminate having to unplug my control unit like I do now in order to reset it.
Thanks again everybody!
Jim,
Not sure how big or small your layout is, and how many folks may be running trains on it as the same time. By breaking up the layout into 'power districts', a short in a single section will not affect trains in other sections.
If the layout is small, and you are the only operator - Using the built-in breaker in the system is all you need. My layout fits in a 25' by 20' room, and I have 4 'power districts' - it is much easier to find the 'problem' when I already know what area to look in.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
"The reason is that an 1156 bulb draws about 2.1 amps @ 14volts. While that might limit some command stations from shutting down, it still allows roughly 30 watts (depending on your track voltage) to flow through the short."
Actually, that's not necessarily true. the bulb dissipates about 30 watts, not the short. The wattage at the short is 2.1 amps multiplied by whatever voltage is left. (Vsupply - 14). Since your supply is likely nowhere near 28 volts there will not be 30 watts at the site of the short.
The mind is like a parachute. It works better when it's open. www.stremy.net
You forgot the I-squared R law.
This is also a complex waveform, not a pure DC source, so things get interesting.
Why not just let the circuit protection built into the booster deal with these events, as it can do it better than a fuse or lamp can, and add the proper current protection devices if more than one power district is desired. Their operation is a lot more complex than that of a thermal device like a fuse or lamp.
Watts at the site of the short will depend on the resistence. But if the bulb is 2.1 amps then 2.1 amps will flow through the short as well as the bulb.
The whole light bulb concept goes way back as a way to protect power packs from shorts, the old selenium rectifiers could not stand much of an overload for any period of time and circuit breakers were notoriously slow and unreliable. And no one wanted to repalce fuses after every little blip. So, enter the serioes light bulb. It also helped save things if you overran your block, into one set for the opposite direction - especially with locos that picked up on one side of the loco or front truck and the other side of the tender or rear truck - you can theoretically get double voltage across the motor that way. It can happen in DCC with multiple boosters if they are out of phase and there is no common between them.
The Light will prevent problems onboard the locomotive since the current goes elsewhere.
LION likes the lights, but him does not use DCC. Him as a 15A regulated power supply. Since the layout is automated power is applied in each section between the stations. This is where I put the lights. It tells me WHERE the short is, after all I have 14 scale miles of track.
I have to use separate bulbs for each section, because with 12 locomotives drawing current, I could light the bulb in normal operations.
My problem had been in the speed control resistors. They are only 1/4W resistors and since there is current across them for only a few seconds every five minutes they have no chance to heat up. Our shop manager said they would work, and they do unless a train derails right on top of one. Then it will over heat, since it will draw too much current, but nowhere near enough to trip a breaker on a 15A source.
Oh well, you build after Rube Goldberg 's plan then you have to plan differently.
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Here there be cats. LIONS with CAMERAS
The current goes nowhere else, it goes through the loco, or the wiring inside, or a truck sideframe - whatever is causing the short. Kirchoff's Laws. Currents in series are shared, so if there is a circuit comprised of light bulb, shorted loco truck, and power supply, and the light bulb draws 2.1 amps, the same 2.1 amps will flow through all parts of the circuit.
The VOLTS may well be reduced - the other part of Kirchoff's laws is that voltages in series add, so depending on the lit resistence of the light bulb fillament, it will drop a certain amount of voltage. The difference between that and the power supply voltage will flow through the short.