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So I’m setting up an HO scale train (trolley/cable car) along the ceiling of my game room. The track does not loop. There is 24 feet of track total. I’m using a 17v DC model train transformer to power the trolley car on the track and will use reed switches, a magnet on the car, an Arduino microcontroller and a relay board with four relays in order to reverse the current to the track when the car reaches one end. This is no problem to set up. My only concern is on the incredibly unlikely chance one of the relays (or code/CPU) malfunctions, it could theoretically cause a direct short of the transformer’s DC + and ground. To avoid any chance of this happening I want to install a fuse in-line but I’m not exactly sure what amp rating to use and whether to use fast or slow blow. It’s a Bachmann 6607 transformer that states 7VA total max output. The transformer will engage the track at the middle so the trolley will never be more than 12 feet from the transformer in terms of resistance in the track rails, plus it will have no resistance pulling additional cars, and it will always be one single-motor trolley car. Would, for example, a 250v, 4A fast blow fuse do the trick? Would a rating even lower work and if so what would be the lowest before risking an unintentional fuse blow from normal operation?

FYI, the trolley is a used Vintage Ahm HO scale Birney Trolley "Public Service # 70 if that matters.

One more off-topic question, there will be one curve in the middle of the track since it will be on two walls and will essentially be an L-shaped line. Given that its one trolley car with only four wheels on a single, centrally located wheel hub, will I be able to violate the 18" radius curve rule to make a tighter turn so the track hugs closer to the wall at the turn? If so how much can I fudge this (or should I just use trial and error)?

Thanks!
 

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I’m using a 17v DC model train transformer to power the trolley car on the track

My only concern is on the incredibly unlikely chance one of the relays (or code/CPU) malfunctions, it could theoretically cause a direct short of the transformer’s DC + and ground.

Would, for example, a 250v, 4A fast blow fuse do the trick? Would a rating even lower work and if so what would be the lowest before risking an unintentional fuse blow from normal operation?
can your 17 VDC supply provide 4A? if not, the supply is more likly to blow before the fuse.

considering that track exposes the wires of a power supply, unintentional shorts are likely due to derailments, mis-wiring during construction or laying metal tools across the track. An alternative is a resetible circuit breaker. But a less expensive option is an auto-lamp such as the 1056.

such a lamp is rated at ~2A and will limit current to that amount during a short, beside indicating that there is a short. So make is visible

of course it would be a poor choice if you expect the trolley to draw that much current. But a trolley is more likely to draw much less and at those currents the lamp will have less resistance (because the filament isn't hot).

Since the NCE PowerCab cannot supply 2A, i use a 3021 festoon bulb that rated at ~1A.
 

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I agree with Greg. The most simple short protection
is the auto bulb in series with one wire of your
track bus. It glows if there is a short across the
track rails and does not present a load to the
power pack and 'by passes' the short.

The typical motor in an HO loco or
streetcar would be around .4 of an amp at the speed
normally used, so of you really insist on a fuse you
could use a 12 v 1 or 2 amp auto fuse. The problem
with use of a fuse is the likelyhood of a derail causing
a short and blowing the fuse which you then would
have to replace. This could get to be annoying.
Using the 12 volt bulb eliminates that chore.

A 4 wheel trolley should have no problem negotiating
a less than 18" curve.

Don
 
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