Help selecting a diode to control polarity

[Maletrain]

I am working on a battery power supply for my Power Cab, and want to make sure that it does not get the + and - feeds reversed at some point.  So, I want to  put a diode in the power feed to block accidentally reversed polarity.  My problem is that I am finding a bewildering amount of search results with some that seem to fit the bill not having the data I need to be sure they will work for me. 

I want to be able to run with as much as 2 amps, although I will probably run mostly at about an eighth of that.  The battery voltage will be 15 volts or less, so  I need to have a diode that does not break down until maybe 20 or 25 volts to be safe in blocking the voltage if it is reversed.

I did find these

https://www.mpja.com/Pack-of-10-1N5408-3A-1000PIV-Diode/productinfo/37119+DI/
https://www.amazon.com/ALLECIN-FR207-Recovery-Rectifier-Switching/dp/B0CKSQCPMF/ref=sr_1_2_sspa

which seem to be rated for 3 or 2 amps and can block 1000 volts without breaking down.  Do I have that right?

At this point, I am thinking of ordering from Amazon for speed of delivery.

Any thoughts or warnings from the more electrically educated members appreciated.

[peteski]

Any silicon rectifier diode rated for 50V or more will work. If you want to pass 2A through it, I recommend the diode you found rated at 3A.  Also keep in mind that there will be between 0.5-1.0V drop voltage across the diode (depending on the type of diode and the amount of current passing through it).

And depending on how much current is passing through it, the diode can get warm.

[Maletrain]

Nutz!  I ordered these last night to be sure I would get some before the weekend.

https://www.amazon.com/BOJACK-Schottky-DO-201AD-Electronic-Silicon/dp/B07Q4HYL1P/ref=sr_1_1_sspa

They are rated to block 40 volts, not 50. I am not going over 15, so is the 40 instead of 50 really a problem?

Anyway, thanks for being helpful, Peteski.

I do plan to install it so that it is hanging in the air by its leads inside an open back box, and will probably not have more than a quarter amp flowing through it.

[peteski]

No, 40V rating will not be a problem since this is a DC power supply and there are no voltage voltage spikes present.  I mentioned 50V because that seems to be the lowest reverse voltage I usually see generic silicone rectifier diodes rated for.  Maybe you have ordered Shottky rectifier diode. Those can have lower reverse voltage ratings.  Those have lower forward voltages, so that is actually beneficial (less voltage wasted by the diode means more voltage for your Power Cab and less heat generated by the diode).  Of course with only 250mA average load, the diode will barely get warm.

[Maletrain]

Thanks, Peteski.

When I get my contraption figured out  and tested, I will post the results here for others to use.  I am trying to keep it simple but still make it robust.

[Point353]

... generic silicone rectifier diodes ...

Would like to see a datasheet for those devices.

[peteski]

Would like to see a datasheet for those devices.

Was that directed at me?  Here's a datasheet for a generic 1A 50V silicone diode.
https://www.onsemi.com/pdf/datasheet/1n4001-d.pdf

[Point353]

Was that directed at me?  Here's a datasheet for a generic 1A 50V silicone diode.
https://www.onsemi.com/pdf/datasheet/1n4001-d.pdf

Is that for a silicone diode?

[peteski]

Is that for a silicone diode?

Ok, I finally got the joke.    Silly me.
Yes, those are the diodes which do not pass any current in either direction.   

[mmagliaro]

Ya done good, even if it was by accident.  Schottky diodes are great for things like this because you don't care about frequency response or reverse leakage current, things that never concern us in "train world" for a diode like this.    Lower voltage drop, less heat, all the good things Peteski said.   (It's the main reason I went with Schottkys in the rectifier section of my throttle circuit.  Dropping only .25v instead of 0.7 saves a ton of power and wasted heat)

That said, I do have a "wet rag" to throw here.  Parts off Amazon might be dodgy.  They could be substandard parts, Chinese fakes, or who knows what.   I would just suggest testing a couple of them by hooking them up across a power supply with reverse polarity, with a voltage as high as you can conveniently test with (maybe you have a 16v DC supply or wall wart?).   Just make sure it doesn't break down and short out.

[Point353]

I do plan to install it so that it is hanging in the air by its leads inside an open back box, and will probably not have more than a quarter amp flowing through it.

What is the worst case (highest) current that could flow through that diode if there was a short circuit in whatever devices it is being used to supply power?

[Maletrain]

I did run the PowerCab on a stack of 10 alkaline D cells at an outdoor booth today, and it worked fine.  I was a little cramped for time, and did leave out the 3 amp fuse that I intended to put in the battery holder, because the fuse holder I bought (with the fuses) would not close around  the fuses.

So, the potential current that could flow through that diode is whatever a shorted D cell can put out, if it was shorted just past the diode. 

But, the next device is the DC "motor shield" which I had set to trip at 1.6 amps with no delay.   The current has to get through that to get to the Power Cab.  And there is a PSXX DCC breaker on the PowerCab's track output that not only is set to trip at 1 amp, but is also current limiting to that 1 amp.

The quarter test on the rails briefly turns on the blue LED that indicates current limiting, which is quickly replaced by the red LED that indicates breaker open.

I did test that the diode blocked the 15.5 volts when hooked "backwards" across the battery leads. 

And I checked that the voltage on the open circuit from the motor shield was 15.04 volts, so Ok with the Power Cab rating of 15 volts DC.  It drops a bit when the PowerCab is powered.

I will write this up is a bit more detail in case anybody else is interested.

Briefly, the 10 out-of-date D cells managed to put 13.8 volts DCC on the track when first connected, and that degraded to 12.7 volts DCC after running 2 small N scale trains for 4 hours, with total draw on the batteries being about 0.24 amp and current to the trains indicated by the PowerCab reading at about 0.1 amp.  Testing the night before (with another set of batteries), I did manage to get the track voltage down to 11.9 volts DCC after more than 5 hours of running, without any problems with the PowerCab or locos.

So, this worked for me for the situation that was at hand.  Except for the diode and motor shield, it was made with equipment and scrap materials I already had.

One thing that was  a bit tiresome was keeping the 2 trains separated as the circled on the same small loop of track.  The loop was about 12' of Unitrack fed at only 2 locations.  The trains had widely different locos, a Bachmann 2-8-0 and an Atlas Alco S-4.  Scale speed was approximately 30 smph with the Bachmann set at 22 of 128 and the Atlas set at  80 of 128.   But, one or the other was frequently catching the other, requiring a lot of speed changes - glad I had them on 2 separate throttles.  I have not figured out what the electrical parameters were that caused the frequent changes in relative speed between the 2.  One possibility is different responses to voltage decreases.  So, if I do this again, I will probably try getting one of those constant voltage "buck boost" circuits.  But, I have read reviews of those failing and putting out as much as 80 volts when set for 12.  So, I am not going to trust one until I have experimented with it for a while.