Help determining which lamp to use as DCC circuit breaker, # 1156 or 1142?

Don,

  Using taillight bulbs is an easy way to limit current for 'power districts' on your layout.  If you are thinking of installing them to 'protect' your Powercab, the circuitry in the Powercab will shut down a lot faster that the tail light will do it.  Using the 1156/1142 lamps really is to 'isolate' sections of your layout.  This way if one engine derails on the right side of a layout with 2 power districts, then the engine on the left side will still run, as the Powercab may not shut down.

  Lets say you have the layout divided into 2 'power districts' and use are using the 1156(2.1 amp) lamps for each power district.  Your 1.7 amp Powercab will still shut down if there is a short, as the 1156 lamps will only limit the current to 2.1 amps(far below the Powercab output).  If you use the 1142(1.5 amp) lamp, it will limit the current flow in it's power district to 1.5 amps.  So, if an engine derails and shorts out the track in that power district, the 1142 lamp will limit the current to 1.5 amps. 

  Now, the 'practical' side of this.  The lamps do light up pretty fast, and start limiting the current as they heat up - but I suspect they will not do this as fast as the Powercab circuitry.  I used to have 8 power districts protected by 1156 lamps.  They worked fine(I had a 5 amp DCC booster though)  Even so, a 'dead short' would shut down my booster(it is faster than the lamps).  I now have a pair of PM42 units do the 'power districts'.

  If I was you, I would wire up 2-4 power districts and test with both 1142 & 1156 lamps.  If you have 2 people operating on your layout, not having your engine go through a booster reset and have the startup sounds start all over again because someone else derailed makes power districts worth it!

Jim

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Rich 

All the bulbs do is limit the total current to a given train block. If the bulb limit is 2.1 amps and your system can only supply 1.7 amps, then your train block will get 1.7 amps max. If you get a short, the bulb will light and 1.7 amps will flow through the short. Having the bulb be more than your system can supply in and of itself is not a problem at all.

The real key with bulb amperage is how many amps do you need per train block?

To answer this question, you need to know how many locos you have on a typical train. In my case, a typical train can have up to 6 units, split between the head end and mid-train helpers. Typically 2 of the units will be locos with sound. To compute amp needs, assume:

Non-sound loco typical: 0.25 amps

Sound loco typical: 0.50 amps

So for my typical train, it will need:

(4 x 0.25) + (2 x 0.50) = 2.0 amps

On the face of it, the 1156 bulb max is 2.1 amps, so I should be okay. However, that's not really the case.

Once the current flowing through a bulb approaches about 75% of its limit, the bulb will begin glowing a dull orange. When this happens, the bulb begins robbing current from the locos, causing them to slow. As the current increases and the bulb grows brighter, the bulb steals more and more power, leaving less for the locos.

So what I actually need is a bulb limit where 2 amps is 75% or less of the bulb's capacity -- 2 amps is 75% of 2.67 amps, so two 1142 bulbs with 2.8 amps will do nicely.

So why wouldn't I just wire two 1156 bulbs together, giving me a 4.2 amp capacity and be done with it? Clearly my train's 2 amps is way less than 75% of 4.2 amps!

First, I don't want 4.2 amps to flow during a short, because that could do more damage than 2.8 amps.

Secondly, there is booster capacity left to be used by the other trains in the same power district that need to keep running. If I get a short, there goes 2.8 amps. If my booster capacity is 5 amps, that means only 2.2 amps is left for other trains -- which works because my typical train needs 2 amps.

If I went with two 1156 bulbs in parallel, a short would consume 4.2 amps, leaving only 0.8 amps to be used by other trains.

So picking the proper bulb amp limit combination is a balancing act between how many amps each train needs and how many amps you want to be consumed of your total amp supply when a short happens. Make sense?