1156 characteristics
Given a 'legal' HO scale DCC track voltage, putting a 1156 bulb in series with a block will limit current in that block to about 1.5 amps regardless of how much booster capacity is present.
When the filament in an incandescent bulb is cold, it is nearly a short circuit. As more current goes through the bulb the filament heats up and increases dramatically in resistance until at full rated current the bulb is glowing brightly and current is somewhere in the 1.3 to 1.5 amp range.
BUT
When you get up to around 1/2 to 3/4 amp through the bulb (very aproximate values!) the filament will start to glow a dim red color. At this point the resistance is already starting to soar.
The effect of this is to reduce the voltage on the tracks - the bulb and the locos on the track form a voltage divider. If there's 1/2 amp through a 5 ohm light bulb filament that bulb is taking 2.5 volts. (values are guesses) Yikes! 20% of booster output voltage is being dropped across the freakin' tail light bulb! Bulbs are a voltage controlled resistor...
So if we put 2 amps of locos (5 locos at full throttle on a steep grade with a long train), that tail light bulb is in series with them and with it's filament heating up nicely and brightly most of the booster output voltage is dropped by the bulb. In fact the bulb resistance becomes high enough that current won't stay at 2 amps for more than a 50 milliseconds before dropping back to the bulbs rated current at 12V.
So don't expect to use single 1156 bulbs in series with track where you really want full 5 amp booster output current to be available in a single block. Won't work. You can put multiple bulbs in parallel or use a higher current bulb. But probably you'll be needing to fork out the $$ for an electronic circuit breaker. The electronic jobs have nearly zero resistance at any current up to the trip point, which means they also are stealing (droppng) next to no voltage from the locomotives. And when an overcurrent situation arrises they will switch off more quickly than a 1156 bulb filament would heat up.
The following is surmise on my part. No actual studies. At one of the layouts I operate on I've seen the filaments of 1156 glowing dimly while consists are pulling long trains out of staging. The result of this is that giving the locos more throttle doesn't appear to do anything. But when that train loses rolling resistance - the grade lessens, or less of the train is on a curve or threading through turnouts, it's desire for current also drops, the bulbs stop trying to light up, and their resistance drops back to nearly nothing and suddenly the locos have an extra 2 to 5 volts of track voltage and they shoot ahead.
I believe that 1156 bulbs, when used in blocks where the current is flirting with that needed to barely start them lighting up, can cause pogoing to occur in loco operation. Increase the throttle, more current while the train accelerates causing low level bulb light up. Assuming the train reaches its operating speed and the throttle is backed off, the bulbs cool down, voltage increases and the train surges forward. Causing the opeator to drastically cut the throttle. Then the train slows down too much, throttle is increased and the cycle repeats.
I believe this can also happen going downhill with long trains. Some locos (proto 2000 especially) seem prone to this behaviour already. The long train bunches up behind the loco(s) pressing on them causing the gears to bind a bit. Which slows down the loco making the train press harder on the locos which makes the binding in the gears worse, etc. until the train either stops or gets slow enough the binding is eased. Then the loco(s) which still have the same throttle application, accerlerate forward releasing the binding completely which causes them to really shoot forward. Then the train catches back up to them and the binding cycle begins again. Now add in a 1156 bulb. If there's enough current draw in that block to bring the bulb up close to starting to glow, when the binding occurs current draw increases causing the bulb to light up a bit causing the loco to slow down more exaceerbating the binding problem. When the binding finally releases, the current drops enough that the bulb quits glowing and boy does that loco want to take off!
In summary, 1156 bulbs are a lot less expensive than electronic circuit breakers. But they have some behavioral peculiarities. If you're planning to run more than a few, can motored locos in a block at a time, you might want experiment a bit to see if the 1156 bulbs will be a benefit or a problem.
Now for some 'good' news... I wired my blocks with a spdt center off switch. In the up position in put an 1156 bulb in series with the track. In the down position the 1156 bulb is out of the circuit and the track is powered directly from a booster. In the center position that block is turned off. In nearly all situations including double headed locos going up my 2.8% grade with a helper shoving on the back, I've not seen any throttle lag or pogoing on my layout as evidenced by flipping the toggle switch between the 'bulb protected' and 'direct power' positions.
But I *have* seen bulbs glowing on other layouts.
One other thing about 1156 protection before I get off my soap box. They don't limit current instantly. Some DCC boosters (read Digitrax) are rather expeditious when detecting a short circuit and removing power. I needed to set my Digitrax boosters to their 'long' delay using an ops sw (sorry I don't remember which one off hand buy JMRI makes it simple to deal with these). Now when I can toss a quarter on the track, the bulb lights up limiting current and the booster doesn't crowbar so the other blocks keep functioning.
Hope all this is at least somewhat helpful.
Charlie
