A number of folks have asked me about this little life-saver over the years, so I thought I’d get it out there as a separate thread. This will limit the current flowing to your track and through your power pack during a short circuit, or other heavy overload to about 1.3-1.5 amps. It makes the old Troller Autopulse and Momentum 2.5 packs bulletproof. [:D]
The bulbs are the dual-filament Automobile brake-/tail-light bulbs (#1127, I think) that have been around longer than I have (a LONG time![:D]). Available at any auto parts store or Walmart Auto department.
The circuit makes use of an interesting property of incandescent bulbs: They pass current freely until the amount of current is enough to make the filament begin to glow, at which point, the more current that is flowing, the brighter the bulb, until it reaches full brightness for the voltage, then draws no more. So, they pass our track current readily, with no measurable effective resistance or voltage drop. Then when a short hits, they’ll pass current up to about 1.5 or 1.25 amps (I haven’t measured it exactly), and then light up to indicate the problem, You get both short circuit protection and visual indication.
A 1156 tail light lamp will limit you to about 2.2 - 2.5 amps. One thing to remember is that these are ‘slow acting’ current limiters(at least compared to fuses & electronic circuit breakers), so you do not have ‘instant’ protection. They are very handy though for ‘problem determination’ when looking for a short circuit - just follow the lighted lamp!
My layout had a string of 1156 lamps when I converted to DCC. I have since rewired and now have electronic power management(Digitrax PM42).
Jim, I was mistaken about the bulb number: the ones I use are #1157. I just ran a current check on one, and with both filaments lit, it draws about 1.2 amps. (It also gets a bit warm - mount with adequate clearance.) Thus, it will limit the current in a circuit to that amount. Remember, it is wired in series, not in parallel with the track. As to its “slow acting” property, it is certainly fast enough to protect most commercial circuits. I used one with a Troller Autopulse 1 power pack (notorious for burning out the power transistor if you look at it hard) for around 4 years, and with a lot of full-power short circuits, with absolutely no damage to the pack whatsoever. To be sure, it would probably be inadequate for DCC, but anything else… what can I say? It works.
If you need a lower-amperage limiter, get a smaller (lower-amperage) bulb…
