Programming a decoder

[peteski]

Yes 1F. Mine are 6.5x10mm and all the one I find online are 8x12mm.

I had a feeling you would say that.
I had the same problem looking for these caps last year.  I found them at Mouser:  Part # 647-JUWT1105MCD.  When I ordered them, they were out of stock and I had to wait few months for restock.  I bought a 100.  If they are out of stock again, I suppose I coudl be talked into giving up four of them.   

[Chris333]

Dangit. 25 from there are the same price as 4 over sized one on e-bay.  In the 2-4-4-2 tender I should have plenty of room for the bigger ones. For the Hiesler I'll just buy a new KA3.

[peteski]

Dangit. 25 from there are the same price as 4 over sized one on e-bay.  In the 2-4-4-2 tender I should have plenty of room for the bigger ones. For the Hiesler I'll just buy a new KA3.

So you bought them on eBay already?

[Chris333]

Yep.

[Chris333]

Caps came in the mail today. Installed them and the KA3 still doesn't work. The black ground goes right to the row of caps and the blue wire goes through the circuit. I'm thinking the caps were fine and the problem is something else.

 Can I wire the blue wire right to the other end of the row of caps?


In the mean time my speaker box came in:


The chuffs are nice and deep, but I really haven't played with it much.

[peteski]

Caps came in the mail today. Installed them and the KA3 still doesn't work. The black ground goes right to the row of caps and the blue wire goes through the circuit. I'm thinking the caps were fine and the problem is something else.

 Can I wire the blue wire right to the other end of the row of caps?

No idea on your hookup question - I'm not familiar with those decoders. But the hookup info shown earlier is across what looks like a Zener diode right at the output of a bridge rectifier.  I know this is all mumbo-jumbo to you, but basically that is a good spot for hooking up a keep-alive unit.

Can you test the repaired unit?  Make sure to mind the polarity. Charge it with a 9V battery for like 30 seconds, or even a minute, then see if it will power up a small 12V motor for several seconds.  If it does, then the keep-alive is working properly and the problem is elsewhere.

[Chris333]

I mean on the KA3 itself. I just soldered a wire to the other side of the cap bank. If I try to charge the KA3 with a 9volt it doesn't work. But if I try to charge just the cap bank with no other circuit it does work. So I wired the original working Heisler up to the new wire I added and it worked!

So those caps were probably fine. Something else on the KA3 failed.

So instead of buying another KA3 I can just buy a bunch of caps.


Later on I will try hooking the KA3 up to the 2-4-4-2 tender and see what happens there.

[jdcolombo]

So instead of buying another KA3 I can just buy a bunch of caps.

A keep alive is nothing but a bunch of caps with a resistor/diode on the positive side to limit charging current [without the resistor/diode, very large keep alives will seem like a momentary short-circuit to the DCC command station when you flip the switch on and the caps charge; folks refer to this as "in-rush current".  The caps suck up all the available electricity in a way that makes the DCC system think there is a short.  The resistor limits this electrical suck-up).

Once you know how the electrical math works, building your own keep alive is easy.  Caps wired in parallel keep the same voltage but add capacitance.  Two 1F 2.7v. caps wired in parallel result in a 2.7v, 2F cap circuit.  Caps wired in series add voltage but divide the capacitance.  Two 1F 2.7 caps wired in series produce capacitance of one-half Farad (500 millifarads) and 5.4v.

Most commercial keep-alives use 4 or 5 2.7v or 3.3v caps with varying capacitance values.  Four 2.7v 1F caps wired in series produces 10.8v, 250 millifarads of output.  [Technically, the formula for series capacitance is 1 divided by 1/cap1 + 1/cap2 + 1/cap3, etc.  So four 1F caps in series produce 1/4 in total capacitance, or 1/4 of a Farad. 

You can make your own keep alives much cheaper than commercial units; but the commercial ones, like the KA3, are nicely packaged and have the resistor/diode.  I roll my own, because N-scale doesn't have enough room for the commercial versions.   And if you roll your own, you can stash the caps wherever space is available - you just have to wire them up correctly.  I've been known to put 4 caps in the 4 corners of a tender shell, because that's where space was available.   I've put two on one end of a tender and two on the other end.  Whatever works. 

John C. 

PS - the speaker box looks great!

[peteski]

As I mentioned earlier in this thread, there is more than just a resistor, diode and caps in the KA3. Exactly because 4 2.7V caps have 10.8V limit, there is a sort of voltage limiter in the circuit. It from what I see in Chris' photo I speculate that it is a transistor, resistor and a Zener diode (in addition to the other larger resistor and diode that John mentions).  Most DCC boosters but out 12V or more, whic within the decoder can exceed the 10.8V the Super Caps are rated for.   But if you use 5 series-connected caps rated 2.7V then you should be ok skipping the voltage limiter on the KA3 because that arrangement will be rated for 13.5V.  That is how my older TCS KA2 is designed. But even KA2 has 2 diodes (one for charging, one for discharging, a current limiting resistor for charging, and a Zener diode for protection).

[jdcolombo]

As I mentioned earlier in this thread, there is more than just a resistor, diode and caps in the KA3. Exactly because 4 2.7V caps have 10.8V limit, there is a sort of voltage limiter in the circuit. It from what I see in Chris' photo I speculate that it is a transistor, resistor and a Zener diode (in addition to the other larger resistor and diode that John mentions).  Most DCC boosters but out 12V or more, whic within the decoder can exceed the 10.8V the Super Caps are rated for.   But if you use 5 series-connected caps rated 2.7V then you should be ok skipping the voltage limiter on the KA3 because that arrangement will be rated for 13.5V.  That is how my older TCS KA2 is designed. But even KA2 has 2 diodes (one for charging, one for discharging, a current limiting resistor for charging, and a Zener diode for protection).

Yep, I forgot about the zener for voltage protection when using low-voltage supercaps in a keep alive.  Peteski, you had a diagram of a circuit for this somewhere on the forum, right?

John C.

[Chris333]

I thought DCC put AC to the rails and the decoder turned it into DC. Is that right?
And you hook up the caps right where the bridge retifier turns it into DC.

[jdcolombo]

I thought DCC put AC to the rails and the decoder turned it into DC. Is that right?
And you hook up the caps right where the bridge retifier turns it into DC.

Right.

But say the DCC system is putting 14v to the rails (a fairly common number).  The bridge rectifier will likely drop 1.4v in turning this to DC.  So the output of the rectifier will be 12.6v.  You don't want to feed 12.6v to a cap circuit rated at 10.8v (e.g., four 2.7v caps wired in series).  That would not be a good thing.  So you limit the charging voltage via a zener diode so that it doesn't exceed the 10.8 rated voltage for the caps.  As Peteski pointed out, if you are using FIVE 2.7v caps, then your voltage rating for the keep alive is 13.5v, which is higher than the output of the bridge rectifier, and all would be fine without the zener to limit charge voltage.

In N scale, we don't have room for the supercaps, so we tend to use 16v or 20v tantalum caps wired in parallel to increase the total capacitance.  But their capacitance is tiny (e.g., 100 or 200 microfarads).  We can only get a fraction of the total capacitance that a commercial keep alive (or a supercap one that you build yourself) this way (a microfarad is 1/1000 of a millifarad, so 5 200uf caps in parallel gives you 1 millifarad; the commercial keep alives with supercaps give you 250 times that amount or more), but it's enough to avoid momentary sound dropouts.  Not enough to operate the motor over dirty track, which you CAN do with a supercap keep-alive. The extra space you have in HO scale gives you keep-alive options that N scale simply doesn't.

John C.

[peteski]

Yep, I forgot about the zener for voltage protection when using low-voltage supercaps in a keep alive.  Peteski, you had a diagram of a circuit for this somewhere on the forum, right?

John C.

Yes, in https://www.therailwire.net/forum/index.php?reportsent;topic=37327.msg447874#msg447874 but the attachment is not showing up (I did ping the moderators to see if they can restore it).

Chris, you are correct about the bridge rectifier. The DCC signal is not a typical sine wave AC but more like square pulses which change polarity. But yes, the bridge rectifier takes those pulses and delivers a DC voltage with small spikes when the DCC signal flips polarity.  The decoder has capacitors in the output of the bridge rectifier to smooth hose spikes out, but not enough capacitance to be able to run the motor.  By installing the keep alive in the circuit you increasing the storage capacity to be enough to act like a miniature battery and power up the entire model for few seconds if the track voltage is lost.  But you can't just hookup the super caps directly to the bridge rectifier - you need to use the ancillary components to make it all work correctly.

[Chris333]

OK for simplicity for me    I will just buy another KA3 for the Heisler.

The butchered KA3 for the 2-4-4-2... what component on the board was the one that likely failed?


The white wire was added by me to bypass the circuit board.

[peteski]

Most likely are ether of the 3-lead components.  Without being able to do some more testing, it could be either one. So you hooked up the white wire to where the blue wire is hooked up?  The caps are getting charged and discharged directly by the decoder?