1156 Bulbs with N Scale

[ft-fan]

I have read through several of the clinics on DCC in this forum, and also quite a bit on the WiringForDCC site. Most of these favor using the 1156 bulbs for short detection. I have also read one site that does not prefer this method and says to use a breaker instead. (The breaker-only method is obviously more expensive.) My question relates to this method in an N-Scale layout. The question is, will using these bulbs for short detection remove enough of the current to keep the smaller N-scale locos safe from harm? Since they don’t use much current, it seems to me that a smaller current load continuing to run will be there to burn out the electronics and/or motors in the smaller engines. Does anyone have any experience with this? Is this a correct conclusion?

Thanks for your help.

FT

[jrbernier]

  The 1156 bulb is a 'slow acting' current limiting device.  That said, it is excellent for an N scale layout as it will limit the total current in the protected block to about 2.5 amps.  So, it you derail or put a metal tool across the track, it will limit the current draw in the protected block to about 2.5 amps.  With N scale this is fine, as I doubt you will find very many engines combinations/number of consists that will even draw 2.5 amps.  With some of the larger scales you can get to that 2.5 amp limit pretty fast with a consist of sound decoder engines!

  The short protection really is not there for the engine/decoder - it protects the DCC command Station/Booster and prevents a booster shutdown that stops all of the engines on the layout.  The short is not in the engine, but rather external like a derailment or switch set wrong.  If there is a short in the decoder, it will be fried right now - most N scale decoders have about a .75 amp continuous and 1 amp maximum rating.  Maybe some of the more expensive circuit breakers can shut down the power fast enough, but if there is a problem in the decoder; it is going to fry and lose it's 'magic smoke' anyway!

Jim

[jfugate]

Jim is right, the 1156 bulb trick limits the current draw to 2.1 amps for an 1156 bulb. And it's even better than it sounds, since the bulb also becomes the load in the circuit when there's a short, meaning the bulb will steal most of the energy and leave very little left in continuous flow to do any damage at the point of the short.

The bulb takes a few hundred milliseconds to come up to temperature and begin taking the current, so for a fraction of a second, there could be more of the 2.1 amps available at the point of the short to do damage. Once the bulb comes to temperature, leaving the short for even a minute or so will do little harm because the bulb has become the load and robs the short location itself of energy.

Cold or hot the 1156 bulb will limit the maximum current to 2.1 amps. As more current is drawn by locos on the track, the bulb filament starts to heat up and it will limit the total voltage available to the locos, which can cause them to slow if their total current draw approaches the 2.1 amp limit.

If you regularly have 2 amps or more in a given booster power district being drawn by all the trains, this can be a problem. Busting things up into several train-length sub-blocks per booster district can help. Here's a video clip that explains how this works.

You can also tune the current limit by using different bulb combinations per short protected section. Here's a guide:

1. One 1152 bulb - 1.4 amp limit
2. One 1142 bulb - 1.5 amp limit
3. One 1156 bulb - 2.1 amp limit
4. One 1157 bulb - 2.5 amp limit (if both filaments wired in parallel)
5. Two 1152 bulbs - 2.8 amp limit
6. Two 1142 bulbs - 3.0 amp limit
7. Two 1156 bulbs - 4.2 amp limit
8. Two 1157 bulbs - 5.0 amp limit (if both filaments wired in parallel)

Armed with this information it is possible to make the short management on your DCC layout be more or less exactly the current limit you want. 

[ft-fan]

Jim and Joe, thanks for the replies. I appreciate what you have said. Joe, I have read your clinics a couple of times, and today watched that video clip for about the 6th time in a month. But I am still having a hard time getting my hands around this. So, now we get to publicly expose how "challenged" I am when it comes to electronics. I hope someone can help me make more sense of this.

We'll start with a couple of assumptions:

1. We have a DCC system which gives 5 amps to my smallish layout. There are no power districts, so the whole layout has 5 amps coursing through it. 

2. I have an N-scale FT engine (not sure why I picked that one ) with a Digitrax dn163m0 decoder in it. This decoder is rated at 1.5 amp/2.0 amp peak.

3. I have a small loop with a couple of sidings. Let's say I have 6 power management zones, one of them is a small yard.

Scenario 1: I have a bunch of cars in the yard and the engine is hooking up to some of them. One of the cars (with metal wheels and axles) derails and causes a short. The light bulb comes on, which limits the current draw to 2.1 amps (which means that the resistance of the bulb is 2.9 amps?). But this 2.1 amp limit is still bigger than the peak rating for this engine (2.0 amp). Or, am I reading this wrong and the limit is what the bulb draws, leaving 2.9 amps on the track. This would be even worse.

Bottom line: Is 2.1 amps still flowing through this engine which is now essentially stuck? And won't that tend to do some damage to either the decoder, the motor in the FT, or will the breaker in the control station blow?

Scenario 2 is similar, but say I have broken the layout into four zones using a PS-4 from Tony. So now, this zone is only going to get 1.5 amps, the bulb will take all of that to light, and won't even light very bright as this is only about 1/2 of what the bulb needs. So in this scenario, I am thinking that the PS-4 will not shut off the power, the bulb will take everything and the engine should be fine.

Can someone help me see if I am wrong in either of these cases? I am especially confused on why I would want to use an 1156 in Scenario 1.

Thanks again in advance for all your help.

FT 

 

[rrinker]

 In short (no pun intended) yes, the current is flowing through whatever is stuck. Not the motor of the loco, but the short circuit path via the metal wheelsets. With the light bulb in the circuit, you are simply making a complete circuit from the booster through the light bulb. Think of the short on the track as a switch. Derail and short across the track, you close the switch, and power gets to the light bulb.

 Technically, a working DC motor is many momentary near-shorts per second, depending on how fast it is turning. It's a 'near' short because the armature windings do have some resistance, it's not the same as a metal tool laid across the track.  But they happen too fast for the light bulb to turn on. Now, if the cause of the short is the motor freezing, or gears getting jammed, then you DO have 2.1 amps with the 1156 bulb going through the motor (and decoder with DCC). Either the decoder is goign to let go or the motor coil will be cooked with 2.1 amps going through it. Some N scale decoders can handle a 2 amp stall current, but most are less. 2.1 amps is enough to melt windings even in HO motors. This is the downside of the light bulb method. In the case of a direct short like a metal wheel bridgeing the rail, or running the wrong way into the frog end of a turnout, the light bulbs work just fine and won't harm your equipment.

 

                                          --Randy