Cheap and easy short protection for your DCC layout

[wazzou]

[mmagliaro]

Can anyone tell me what the trip time and trip current are for these solid-state breaker circuits used in your DCC systems?  That is, if the current limit is 2 amps, how much current has to flow before it trips?  2.1 ?  Or does it have to go about 50% over the limit (3A in this example).  And how quickly does it trip?

Are these things actually current sensing circuits like a classic "crowbar" circuit, or are they just using things like the Raychem Polyswitch solid-state auto-resetting devices?

I ask because I long ago gave up on the bulb, thermal breakers, fuses, and Polyswitches because none of them trip quickly enough for me.  I want something that trips instantly when the current goes, say, 10% over the line.
What I actually did for now (and it's definitely an odd solution, I admit), was use an analog current meter in the line, inside of which I built a little physical "whisker" such that when the meter goes over my current limit, it touches one whisker against another (as a switch) and turns off the current via a latching relay.   I still left the main thermal breaker in there just because.  It may be hokey, but it darn well trips when the current is just a hair over my limit and it trips instantly with no excuses.

[Greg Elmassian]

The NCE site has a little information, but not much.

DCC specialities gives a little more explanation of how the PSX-AR works, and outlines at least the methodology.

My Zimo MX-10 has settable parameters for the sensing.

All of these have manuals online, plus you can contact them by email to try to get more details if you really want to know. As far as I know they are current sensing, not PolySwitch thermal state changing devices.

Regards, Greg

[Point353]

Can anyone tell me what the trip time and trip current are for these solid-state breaker circuits used in your DCC systems?  That is, if the current limit is 2 amps, how much current has to flow before it trips?  2.1 ?  Or does it have to go about 50% over the limit (3A in this example).  And how quickly does it trip?

Are these things actually current sensing circuits like a classic "crowbar" circuit, or are they just using things like the Raychem Polyswitch solid-state auto-resetting devices?

I ask because I long ago gave up on the bulb, thermal breakers, fuses, and Polyswitches because none of them trip quickly enough for me.  I want something that trips instantly when the current goes, say, 10% over the line.
What I actually did for now (and it's definitely an odd solution, I admit), was use an analog current meter in the line, inside of which I built a little physical "whisker" such that when the meter goes over my current limit, it touches one whisker against another (as a switch) and turns off the current via a latching relay.   I still left the main thermal breaker in there just because.  It may be hokey, but it darn well trips when the current is just a hair over my limit and it trips instantly with no excuses.

What you seem to need for your application is precision over-current protection rather than simply short-circuit protection.

Out of curiosity, what would get damaged by a current draw of ~2.2A that would otherwise function properly at 2A?
Also, how fast do you need the protector to trip?

Specifically which polymeric PTC device(s) did you try?
There is quite a variety of such products available.
http://www.littelfuse.com/products/resettable-ptcs.aspx
http://www.littelfuse.com/products/resettable-ptcs/radial-leaded.aspx

[mmagliaro]

What you seem to need for your application is precision over-current protection rather than simply short-circuit protection.

Out of curiosity, what would get damaged by a current draw of ~2.2A that would otherwise function properly at 2A?
Also, how fast do you need the protector to trip?

Specifically which polymeric PTC device(s) did you try?
There is quite a variety of such products available.
http://www.littelfuse.com/products/resettable-ptcs.aspx
http://www.littelfuse.com/products/resettable-ptcs/radial-leaded.aspx

I don't "need" super precise overcurrent protection, but I have never liked the slow behavior of typical throttle short-circuit protection.  If there is a dead short at higher throttle settings, then the breaker or other device trips quickly.  But otherwise, it will sit there for 5 or 10 seconds or even more before it trips.  If there is a "near short" that is pulling 3 or 4 amps, 5 seconds is deadly.

What you'll see at those Polyswitch specs is that their hold/trip current ratings are typically such that the trip is 2x the hold.  So a 2A device doesn't trip until you hit 4 amps.  That may be okay for dead-short conditions, but for anything else, it's useless.
I have a few of those, and that's pretty close to how they actually behave.   A 2A device will flow 3A for a minute or more and "maybe" trip and maybe not. 

So, to reiterate, I don't need super precision (even though I asked for a trip at 10% over the rating).  But I want something that is closer than what a breaker or Polyswitch provides.

Sorry for hijacking the thread, but it is related.  The bulb approach works, but doesn't work for me because it doesn't really shut off the current.

[Scottl]

This is interesting.  I use a 1.0A blade fuse to protect my dcc system and locomotives.  It trips when something shorts on a turnout and is well below maximum current for the system.  I bought fifty fuses on eBay for a pittance and have blown perhaps five since installing this solution.

[mmagliaro]

This is interesting.  I use a 1.0A blade fuse to protect my dcc system and locomotives.  It trips when something shorts on a turnout and is well below maximum current for the system.  I bought fifty fuses on eBay for a pittance and have blown perhaps five since installing this solution.

Regular (i.e. "fast blowing") fuses are always a good option for something that will trip quicker than thermal breakers and closer to the max current rating you want to limit your system to.  But of course, any time they blow, you have to plug in a fuse, which can be a problem if you have a short that isn't easy to find.  You might go through 10 fuses or more pretty quick.  (But then, you bought 50, so you are probably covered!    )

[Ron McF]

Good-grief! A novice reading much of the foregoing could be forgiven for thinking that if he uses auto-lamps for short circuit protection then his locos will certainly be destroyed the first time he encounters a short-circuit.

Here are links to info related to the light bulb over-current protectors.  If you actually bother to read completely through this info, you'll find warnings there, very similar to what we are telling you here.

https://sites.google.com/site/markgurries/home/technical-discussions/dcc-circuit-breakers/light-bulbs
http://cs.trains.com/mrr/f/744/t/215408.aspx
http://model-railroad-hobbyist.com/magazine/mrh-2014-11-nov/di_short-protection
http://www.modelrailforum.com/forums/index.php?showtopic=10173
http://www.wiringfordcc.com/track.htm
http://www.rr-cirkits.com/Notebook/short.html

But hey, if it works for you - keep on using it. But I also felt that it was my duty to present my warning.  I'm done here -there is nothing more I can say about this subject.

I've read most, probably all of those articles, and have even linked to several of them on my own blog.  The point of the "warnings" is to highlight the limitations of this system of protection, and so long as you understand and respect those limitations then the dire predictions of your locos melting down when you run into a turnout that's closed against are extremely unlikely to eventuate. Indeed, many of those warning were written by people who advocate or even use auto-lamps for protection.

I use auto-lamps in conjunction with PTC fuses (as described by Dick Bronson here: http://www.rr-cirkits.com/Notebook/short.html) on my own layout. During our operating sessions we frequently get shorts caused by operators running into incorrectly aligned turnouts, and the devices shut down that area (and only that area) exactly as intended.

The huge advantage that auto-lamp/PTC fuse devices have over electronic solutions such as PSXs is their cost. They are so cheap that you can install them in droves - every few feet of track if you want to (as Joe Fugate apparently does). This can be especially advantageous for (say) a large walk-around style of layout, where a short caused by an operator who has run into a turnout thrown against him will not affect anybody else. He knows that he has caused a short, and being on-the-spot, he can fix it long before any damage occurs.

But using such devices for easily accessible track does not preclude the use of electronic devices for locations that pose a higher risk, such as hidden tracks. On my own layout I have a separate auto-lamp/PTC fuse device protecting each town, and a visible/accessible staging yard. The other staging yard is hidden, and relies on the DCS circuit breaker for protection.  A short there will shut down the entire layout, which I'm happy with, as it usually means that the ladder, throat or approach track is blocked; and in such cases I don't want any trains to enter that area.

So, if you're not a rich man, consider using auto-lamps in conjunction with PTC fuses in those parts of your layout where they can be safely installed.

[peteski]

Ron,  I believe that I repeately stated that the choice not to use the type of current limiter described in this thread is my own personal choice. I base that choice on many instances of damage, which I personally witnessed,  to N scale models caused by couple of Amperes of current flow over a period of few second.  My statements were meant as a warning to modelers who might chose to depend on this simple and inexpensive current limiter (which is not a true circuit breaker) for protecting their models.  I was not planing on posting any more replies here, but your post indicates to me that I again have to explain my stance on this subject.