Well, it’s not my law, it’s Gustav Kirchoff’s, from way back in 1845. [:D]
It’s also the basis for putting a resistor in series with an LED - Ohm’s Law lets us calculate the value of the resistor, but Ohm’s law need 2 of the 3 terms to solve - voltage, current, or resistence. Know any 2, you can get the other. Voltage we know, but how do we get the current flow through the resistor? Kirchoff again. Now we have voltage and current and can easily calculate the required resistor value. Current is know because per Kirchoff, whatever current flows through the LED MUST also flow through the resistor. And really, we know the voltage also thanks to Kirchoff - the sum of the voltages through loads in series myst add up to the supply voltage. In simple termes, Vled + Vresistor = Vpower supply. The LED voltage is part of the specification of the LED so that is a known value, and hopefully you know what voltage power supply you are running it from.
It’s really not difficult to understand, and it makes it pretty obvious how to figure out what resistor is needed for a given LED with a given power supply. Unless the most basic of algebra eludes you - the sort of stuff they teach BEFORE they tell you it even is algebra.
Vled = Vresistor = Vpower supply. I know Vled is 3.5V, and Vpower supply is 12V. How do I get Vresistor? 3.5 + x = 12. or, basic algebra, x=12-3.5, x=8.5 It really is that simple. There are far more complext systems of equations that are much more difficult to solve but we don;t need that stuff for basic model railroading. These simple things cover most cases you will run into with model railroad electronics.
Randy, I am electricity challenged. I have a degree in civil engineering and the only courses that ruined my chance of graduating with honors was the first course in electricy and magnatism and Intro to Electrical Engineeringfor Non-EE majors. Kirchoff, Faraday, Ohm, name them all and they are all gobbledygook to me, which is why I rely on people like you to help if I have a problem.
When a short happens, current goes through the roof.
With an automotive lightbulb, as the current increases, the wire across the filament heats up. This produces both light and resistence. The resistence keeps your command unit from tripping. And the light lets you know you have a short. Kill two birds with one stone.
I believe simple would be an understatement…you have an impressively designed layout and I have been following you progress pretty closely.
To answer the OP question, I have used PSX and Digitrax PM-42’s (on club layout). I will never ever ever ever ever again use those (the PM-42’s). They were not my choice but I got voluntold to install them. They did not mix well with the Lenz system we were using at the time and were an even bigger headache with the inrush current of sound locomotives (mostly Tsunamis).
On my personal layout I have 10 PSX breakers (some were part of a PSX-4, a PSX-2) but I broke them all apart since they live in different areas of the layout. I have two main power points on the layout (the command station/booster and a booster). Each has 5 PSX breakers and a PSX-AR as I have two reverse loops on the layout.
I am a very big fan of the PSX breakers. The only drawback, which sometimes drives me up a wall is the hum of the onboard transformers. The high frequency noise gets to me sometimes.
Functionally they are wonderful. I have run up to 5-6 sound units in a single block, shorted it with a quarter and they all come back up just fine.
I am in the process of building an external display panel to show the status of the blocks and if they are shorted or not. I will say the PSX breakers are worth their price and they are super easy to connect (unlike the PM-42 which is a disaster without an acculites board). They can be daisy chained too for easy installation.
Randy’s point is that with the short and constant current through the 2 amp bulb means there is 2 maps flowing through the short, the object causing the short might not like that much current and vaporize. Springs in trucks for an example.
EDIT
I have an automotive 1157 bulb in series with my track for trouble shooting. The 1157 is a dual filament bulb, the brake side draws a bit over 2.1 amps at 12-14 volts the taillight side draws between .59-.61 amps or 600ma. It works pretty good for trouble shooting but I don’t use it as a fuse. I have a pair SPST switches to short across the filaments for normal running.
Mel
Modeling the early to mid 1950s SP in HO scale since 1951
Dick Bronson has a handy trick adding a few cents to the light bulb method but making it much dafer for your equipment - takes advantage of the dual filament bulbs. Initially, current flows through the 2.1 amp filament. If a short occurs, a PTC resistor switches the flow to the lower current filament. .6 amps at 15 volts is only 9 watts, like the large Christmas bulbs, before everything went LED. Those tend to not get too hot to touch. A lot better than 30+ watts.
Think of a stenosis, or an arterial narrowing due to plaque. That’s where all the pressure gets backed up. The filament in the bulb acts like a stenosis; it’s highly resistive to the current that the circuit can through-put per the maximimum permitted by the power supply. At low current through-put, the filament might only get warm and glow in far infra-red. A triple consisted diesel set hauling 40 cars up a 2.2% grade might need 1.5 amps. That’s substantially below the rating of the tail light bulb, but it might still glow dully in pitch black, or be warm to the touch. It’s still passing the demanded amperage that the three can motors need to do their ‘work’.
When the metal wheel jams at the frog and bridges two rails that should never touch, the full current rating for the system immediately wants to course through that point. However, back, earlier, nearer the bus, and in line with the wires serving that part of the track system, you soldered or marretted that tail light which will now glow brightly because all that current has to get through it to get past it to where the short is. Remember, we’re talking about amperage, not voltage. The voltage is the pressure in the pipe, but the amps are the water that turns the turbine blades…if that analogy works.
This is how I have come to look at the way the tail light bulb works. I only used it on my second layout, and don’t currently even need my boxed PSX-AR any more because I won’t be using a permanent reversing loop. Instead, I’ll rig a reversible crescent of track to meet up with what looks like the stub of an interchange ending at the edge of my yard.
Well plenty of people use light bulbs, and advocate for them, as opposed to an actual circuit breaker. So it counts.
There aren’t that many options - Old PowerShield, PSX series, EB-1, PM42, and On-Guard. Couple of more DIY ones floating around. And NCE also has the CP6 which is just the whole light bulb thing with lower current light bulbs for the PowerCab.
The lamp does not light without a short because there is no current flowing. During a short current flows from one rail to the other. This current flows through the lamp causing it to light.
I have five psx’s here and a psx-ar. Have installed about 10 more in two clubs. One had to be replaced in about 10 years.
Light bulbs are not a great idea, because they probably won’t work well with multiple units or lighted passenger trains. A bulb with low enough resistance so as not to bog down multple engines can pass quite a bit of current, and overheat damage can occur at the site of a short. I once put a new Broadway Ltd steamer on the track of a DCC layout without electronic protection, which had an intermittant short of a driver to the shell, and the axle overheated enough in several seconds to melt the driver hub. PSX’s are unlikely to let shorted sites overheat, and work fine with sound engines.
1156 Automotive bulbs have low resistence till you crank the current. Then their resistence shoots up limiting the current. They then stay “hot” till the current is lowered enough to allow them to cool then the resistence lower. I consider them an acceptable CHEAP solution and a lot of pros use them.
PSX’s are the cadellacs of breakers. They work well with sound engines. We had one on a turntable and 14 tracks all filled with sound before the PSX couldn’t handle the start up current.
I also use PSX’s block occupied hookups for signals on mine. The only disadvantage to this approach is I also have auto braking districts. I cross a red signal, my layout switches to DC for that block, forcing the train to SLOW to a stop. When I throw the DTDP relay, I loose the block detection, so I had to install a second set of detectors for the DC side.
HOWEVER all breakers I know of just break the single leg, not both. This is kind of dangerous if you have a backfeed situation and could damage your booster. I had several boosters at the club burn up for this very reason till I successfully isolated the sections that were causing these issues.
For accessory power (I run 5, 12, and 15V accessory power lines) I use glass tube circuit breakers attached to the main source distribution bus point.
I formerly used a PM42, and a pair of AR-1’s for the two reverse loops.
I’m currently (no pun intended) switching over to a Digitrax BXP88 (8-section solid-state circuit breaker with detection and Transponding for each section) and two Digitrax BXPA1’s (single-section solid-state circuit breaker with auto-reversing, detection and Transponding) for the reverse loops.
The changeover isn’t due to any issues with the PM42 or AR-1’s; they have worked perfectly for me even since converting to only sound-equipped locos on the layout.
It’s simply the easiest and most cost-effective way for me to implement detection for the eventual addition of signaling
I thought about doing this too. I still have to use a seperate feedback board for Loconet so JMRI knows which blocks are occupied. That just adds to complication. But my system is working as it is now.
PM-42’s break both legs of the protected track section.
Good old relays. Still useful.
PSX-AR should break both legs as well, it has to to be able to reverse (transistors wired as a DPDT switch. The other PSX series though, looks like only one leg, you can see the one big trace go right across the board from the input side to the output side. I also just noticed they have a capacitor across the rail A and rail B traces. Now that’s kind of interesting. Wonder if that’s to compensate for the inductive factor of the current sense transformer.
Where’s it say that in this thread? Gary’s were not failed, they were in programming mode (which fairly early on in discussing the issue I asked if the jumper was on…).