Fuse, Automatic Short Protection for DCC

[John]

https://voltscooter.com/products/circuit-protection-devices/fuse-automatic-short-protection-for-dcc/

Fuse, Automatic Short Protection for DCC

[mmagliaro]

This looks like it uses solid state auto-resetting fuses, like a "Polyswitches".   Has anyone tried one of these?

[Scottl]

I just use 0.5A blade fuses.  I have blown precisely 1 in 15 years.

[peteski]

This looks like it uses solid state auto-resetting fuses, like a "Polyswitches".   Has anyone tried one of these?

Some model locos (I've seen it in Bachmann and some other brands) have those type of circuit protection devices installed in the motor circuit (in both DC and DCC locos) for protecting the motor. They work similar to those old fashion glass bi-metal breakers that were used in old DC throttles, but these are smaller (heat up faster) for a faster response.  Still nothing like the electronic breakers built into DCC boosters.   These devices also have some non-trivial resistance in the "on" state, since they have to heat up to break the circuit.

Bachmann 44 tonner is one such loco.  The yellow disk is such a device (in a larger non-SMD version).  It is wired in series with the motor. I reused the original Bachmann part on my custom made PC-board.



Another one is Arnold SW1.  Here it is an SMD component marked a030 in the tapered part of the PC board.

[mmagliaro]

Pete,
That's what I thought.  The one in the Bachmann loco looks to be a traditional "Raychem"  "Polyswitch".
I also noted that the instructions for the voltscooter device say that it "should" automatically reset when the short
is cleared, but "if it doesn't", there is a reset switch.   My experience working with these devices is that
after they "trip", they can take a significant period of time to cool off enough in order to actually hold and behave
like they should.  They will trip very fast at a much lower setting when they are already hot, which makes sense
when you consider that they they are a thermoelectric device.

I don't see how this device makes a suitable protection circuit for a DCC application. 
These thermal devices typically don't trip at all until you are over 50% of their
rated current, and even then, they can take 30 seconds to disconnect the load.  They are fine for a "dead short",
but not for much else.  So this gets us right back to situations where there is a partial short that won't trip the DCC booster, but puts a huge current into the track for a long time before it shuts off, which does not look acceptable to me.

If you're going to do that, then why not just use some in-line fuses as Scottl suggests?

[peteski]

If you're going to do that, then why not just use some in-line fuses as Scottl suggests?

I agree.
Most boosters already have electronic breaker in them, but if their trip current is too high for the layout, then get one of the adjustable solid-state electronic breakers out there.  They are not that expensive, and when investing in the DCC system already, why not spend extra few dollars for the breaker.

And the reset switch is nothing more than a way to cut the current going through the breaker for several seconds (so it has a chance to cool down in case there is still a residual load, like a locomotive, keeping the current flowing)