Capacitors

[lashedup]

So in my continuing quest for knowledge (and to prove my wife wrong that I *do* ask for directions when lost) I want to better understand capacitors. I have a CalZephyr OBS that I installed a Digitrax DZ126T in for the Gyra Light.  Can a capacitor be added to any decoder or only those with keep alive wires/pads? If so what would be the wiring scheme and do’s and don’ts? 

Thanks in advance!

- jamie

[jdcolombo]

A keep alive can be added to almost any decoder IF you can find the appropriate solder points.  The positive connection is usually simple - you just connect to the blue wire on the decoder, which is the positive common for all function wires and serves as a positive voltage source to charge the cap.

The difficulty is the negative.  For a keep alive to work, the negative has to be soldered to the negative supply for the decoder after the bridge rectifier (the bridge rectifier is what takes the DCC signal - a form of AC voltage - and "smooths" it into DC current that is useable by our DC motors, LEDs and such).  Many modern decoders now have pads for the negative connection from a cap, but if there is no pad, you can sometimes still add a cap by finding the correct solder point for a negative connection.  Unfortunately, I'm not familiar with the DZ126T, so I can't tell you where that solder point might be.

If you have some basic knowledge of electronics (particularly the ability to identify electronic parts, like caps and diodes), you can take a look at this page and see if it might help you identify where an appropriate solder point would be for the Digitrax:

http://web.archive.org/web/20120729061658/http:/www.members.optusnet.com.au/mainnorth/alive.htm

John C.

[lashedup]

Thanks John.  Here is what the DZ126 looks like without the shrink wrap. Doesn't look like fun. This thing is pretty small...

[jdcolombo]

The bottom photo on the LH side shows what I think are the four diodes that would make up the bridge rectifier.  The inner (RH side of two of these diodes represents the negative rail connection.   Without the decoder in my hands, though, I can't tell which.

Peteski might be able to tell us from the photos.

@peteski Where are you?

John C.

[RBrodzinsky]

I would agree that those are the rectifiers, looks as if track power are soldered directly to them.  Based on the blue wire coming from the center of board, and this is PURE SPECULATION, my guess is the outer pads are ground, and the inner pads common +.

[peteski]

EDIT: I like to thank Marc for catching my mistake.  Here is the corrected post.



Yes, the 4 large components by the wire harness are the rectifier diodes.  Before going any further please verify that the blue wire is connected to the ends of the diodes marked in blue.  If yes, that is the positive side, and the green-marked ends of the other diodes are the negative (common or ground) for the keep-alive caps.  Then I also suspect (the photo is not very clear for me to be certain) that the yellow marked copper trace, the end of the small cap marked J1, and possibly the solder pad marked yellow are all ground connections. But double check (measure connectivity of those to the green-marked rectifier diode ends).

[nstars]

Peteski, I think that Rick’s explanation is more logical and that the 2 outside pads are connected as well as the 2 inside pads. The 2 bottom diodes are for example already connected on the wire side, which makes it unlogical to have them also connected on the other side of the diodes.

Marc

[peteski]

Peteski, I think that Rick’s explanation is more logical and that the 2 outside pads are connected as well as the 2 inside pads. The 2 bottom diodes are for example already connected on the wire side, which makes it unlogical to have them also connected on the other side of the diodes.

Marc

You are correct Marc.  Thanks for pointing that out.  I corrected my post.

[nstars]

Given the location of the blue wire on the other side, this looks very plausible.

Marc

[peteski]

Given the location of the blue wire on the other side, this looks very plausible.

Marc

Yes, and also it appears that the outside diode (top left of the photo) seems to be connected to the wide trace which connect it to the negative pad of tantalum filter cap marked J1.  Of course we need confirmation of whether the blue wire is  in fact connected to the center 2 diodes. But to be 100% certain we need to find out if the blue wire is in fact connected to the 2 center diodes.  If that is verified, than we know that the other 2 diodes (marked with green rectangles) are the common/negative/ground of the decoder.