Next electrical question: Arduino powered DC/DCC circuit breaking?

[mmagliaro]

Thanks Max!

I am not yet to the point of needing this.  Hopefully, by the time my club gets to where we are hosting meets, I will have found somebody who will build a couple of them.

I have copied the post into a Word document and printed it (as well as saved it electronically) so that I have minimized the probability that this will be lost by the time I need to act on it.

Thanks again.

You are quite welcome.  And I've got the PC board layout if the time comes when you want to have a PCB place make the circuit boards so you can just solder the parts on to make these.

But on the other hand... SCRATCH ALL THIS.

I have hit upon a much better design that will work in either track polarity, and so could be just tacked on to the outside of a DC
throttle as a simple add-on fast-trip device.   For any nerds reading this... since the ACS712 outputs a voltage above or below 2.5v proportional to the detected current, I just have to detect a WINDOW of voltages (say, from 2.2 to 2.8 for 1A trip, or 1.9 to 3.1 for 2A trip).  Window?   As in WINDOW COMPARATOR.   Can't believe I didn't think of this before.

[mmagliaro]

I am back on this.  After experimentation, I have a handy little overload circuit that handles DC, AC, and pulsey noisy DC, regardless of polarity,  has tight trip accuracy, and trips in about 1/4 second.  When sensing AC or DCC, it will be a bit slower owing to how it has to average the current with an oscillating waveform.  The more over the current limit you are, the faster it trips.  So if the trip point is 3A and you load it with 3.1A, it might take 1/2 second to trip.  But if you are well over the line (like 3.5A), or have a dead short, it will trip almost instantly.  It uses the ACS712 Hall Effect chip for current sensing so there is no voltage drop and no heat.
Ain't no Arduino, no programming.  Just a comparator chip and conventional analog electronics.

A 3D rendering of it is below (from the PC board layout program).  It's a little 2" x 3" board.  It can run on 12V (nominal, like 9 to 14v) DC or AC, it has a built-in 8 amp relay for switching off the power, and it can be set to trip anywhere from about 0.5 to 5 amp.  It has its own trip indicator LED and reset button, but you can also connect another one so it can indicate and be reset from a panel. 

You just put it inline with either of the two main wires going to the track (output from power pack, throttle, DCC booster, old Lionel transformer, whatever), and then set the overload point by turning an adjustable trimmer on the board.

I have a philosophy question for the group, but I already think I know what the answer will be.

The way I originally designed it, on an overload, it energized the relay to cut off the track power.  But now I have changed it so that when everything is OK, it energizes the relay and turns the track power ON.  If there is an overload, it de-energizes the relay to cut power off.  My thinking was that if this board is ever disconnected or it blows up, the "fail safe" mode should be "track power off".  The only drawback is that you have a relay activation every time it is turned on.  But seeing as how modern relays are good for tens of thousands of activations, I think if it ever fails,  it is better to fail in the "off" state.

[Ed Kapuscinski]

Whoa!

And also, I agree: fail safe.