What are these Atlas DC board components?

[Dwight in Toronto]

Picked up an Atlas N scale GP40-2 at a local train show yesterday (straight DC, $100 Cdn).

Planning on installing a LS5 58923 Nano decoder, and hope to modify/utilize the existing DC lighting board.  I can see where and how to isolate the motor contact points, as well as the front & rear led’s.  The smd resistors and capacitors are evident, but I’m curious to learn what those two larger black rectangular devices are, labeled ‘M4’?  smd diodes perhaps?



Thanks in advance for the help.

[Point353]

Most likely diodes for directional lighting.
https://doc.hotking.com/wr/specification/specification-en/M1-M7-SMA.pdf

[Dwight in Toronto]

Thanks for the speedy reply.  I’m sure you’re right - diodes for setting fwd/rev DC lighting.  And the specs include a sheet for types M1 through M7.  That helps!

[peteski]

Yes, those look like "regular" diodes (then have a band denoting a cathode) for protecting the LEDs from reverse bias.  Not sure why they used such a large diode for this task.  With 1000 ohm resistors the LEDs will not consume more than (12-3)/1000=0.009A  (9mA) of current, so a diode less than quarter of physical size (or even smaller) could have been used.
Since the other components are identified on t he board, I wonder if the D1 and D2 designations are under the diodes.  Also strange that both capacitors are designated as C1, while resistors have individual designations. Seems that whoever designed the board was not paying much attention.

There are multiple ways of isolating the motor.  Easiest would be to cut the wide track power traces on both sides of the vias which connect to the motor lead pads on the bottom of the board.  Since the board contacts the chassis on both side, that will not affect power pickup (and besides, you will isolate the LEDs too, so again, this makes no difference.

If you are worried how reliable the contact is between the frames and the pads on the board, you can try to carefully cut the wide copper traces around the vias. That way you will isolate the motor connections while retaining connection on the board for the power pickup from both ends.

Or do what I proposed earlier, then use 2 pieces of wire to reconnect the front and rear power pickup traces.

Another way would be to drill out the vias, thus isolating the motor pads on the bottom from the power pickup traces on the top. Then solder the wires from the decoder directly to the motor pads (on the bottom of the board).  If you want to keep the wiring on top, then drill couple of larger holes near the motor pads where there are no copper traces in the center of the board,. then feed the motor wires through those holes from the top, then bend them on the bottom and solder them to the motor pads.  Try soldering the wires in the area of the pad away from the motor contacts.

[Point353]

Yes, those look like "regular" diodes (then have a band denoting a cathode) for protecting the LEDs from reverse bias.  Not sure why they used such a large diode for this task.  With 1000 ohm resistors the LEDs will not consume more than (12-3)/1000=0.009A  (9mA) of current, so a diode less than quarter of physical size (or even smaller) could have been used.

Since space is evidently not an issue on that PC board, a general-purpose 1A 400V rectifier in an SMA package - essentially a surface-mount 1N4004 - is probably the least expensive (and most readily available) part of that type you will find.

[tehachapifan]

I don't see it mentioned, but there's a LS5 drop-in decoder for this loco. Do you already have the nano and that's why you want to use it?

[peteski]

Since space is evidently not an issue on that PC board, a general-purpose 1A 400V rectifier in an SMA package - essentially a surface-mount 1N4004 - is probably the least expensive (and most readily available) part of that type you will find.

Nowadays there are plenty of much physically smaller SMD diodes available, and for comparable price. Even the old standby 1N914 or 1N4148 are available in SMD or leadless packages for few pennies each, and just as popular as the 1N4XXX diodes.  In that circuit you don't need more than 40V and 100mA rating (and I'm being generous). I wouldn't have used those behemoths on my boards, but whatever. 

[Dwight in Toronto]

Good stuff guys - thanks so much. 

On closer examination, I think Tehachapifan is right - I need to get the 58751 Atlas board.  As small as the Nano is, there’s just not enough room for that and the speaker.  I contemplated putting the speaker in the fuel tank, but that necessitates a lot of filing/“machining” (I don’t have a mill). And yes, I was pursuing the Nano only because I have one on-hand.  I now realize that the proper drop-in board is the way to go (I already did one Atlas GP with a 58751, and it was a breeze).

I was pretty much going to do the approach that Peteski outlined, but I’ll save the Nano for another day.

Thanks again for all the input.