Capacitors for DCC stay alive

[robwill84]

Lately I’ve been studying up on using capacitors as energy storage backup for DCC. This is just going to be a catch-all for info I have found. It’s an interesting topic that I can’t seem to find a whole lot of information about.

First, ts useful to understand how capacitance is measured. 1 farad is equal to 1,000,000 micro farad, or uf. So if a capacitor is rated at 0.1 uf that’s 100,000 uf.

This is the DCCwiki article: http://www.dccwiki.com/Energy_Storage

Next another article showing some different types: http://www.members.optusnet.com.au/mainnorth/alive.htm

Types of capacitors include:

Electrolytic capacitor- this is the basic type of capacitor that is included with Soundtraxx Tsunami sound decoders (220 uf). Useful for keeping sound decoders from resetting. Even in the 4700 uf range, can only supply backup voltage for a fraction of a second.

Tantalum capacitors- these are compact capacitors, typically around 7x4x4mm. Can be wired together in parallel to form a compact package. Handy for installations where electrolytic caps are too large, but a small amount of capacitance is still desirable.

Double layer, gold, super capacitors, etc- these capacitors have lower voltages, but much higher capacitance.  These are what TCS uses for their keep alives. For example the KA1 uses 6 X 220,000 uf super capacitors at 2.5 volts each, wired in series, for a total keep alive capacitance of 36,667 uf at 15 volts.
The KA1 provides a whopping 200,000 uf at 13.5 volts.

So anyway, that’s what I’ve dug up, without doing any actual experimenting myself. I’d like to hear from anyone that has some experience with stay alives, what kind, what capacitance and the results. I’ll post my own results here from time to time.

[TrainCat2]

You do understand that placing a capacitor across the module inputs (in conjunction with the wire resistance) creates a Low-Pass filter that can eliminate your DCC command signal.

May I suggest some instructive reading:
http://www.electronics-tutorials.ws/filter/filter_2.html

Be sure to read the section at the bottom of how the capacitor will distort the command signal.

[robwill84]

Interesting info to consider, thanks. The second link addresses problems in programming a tsunami decoder with a TCS keep alive installed. His solution is to simply disconnect the keep alive if the problem persists. Not a very elegant solution, but effective. Would be a real pain to have to do every time you need to program.

[peteski]

Like Bob said, the capacitor will have to be placed behind the decoder's bridge rectifier.   The positive connection is easy - it is the blue wire. But the negative side of the bridge rectifier is not readily available on most DCC decoders that are not designed for add-on caps.  But if you are familiar with electronic design, you can find the negative side of the bridge rectifier fairly easily.

Small caps (few hundred micro Farads, like the ones used on sound decoders) can be installed directly across the bridge rectifier's output. But there is more to attaching a very large cap than just installing it directly in the decoder's circuit. If you were to buy one of those keep-alive circuits (for example from TCS) and disassemble it, you'll see that beside the super-caps, it also contains a resistor and a diode.  That additional circuit limits the charging current of the large capacitor  while the diode allows for a fast discharge (when the current is needed to power the motor).  Otherwise, the charging current of the large capacitor would exceed the maximum current of the bridge rectifier.

As far as the experience with the keep-alive circuits goes: they do work. 

EDIT: what I described above is pretty much covered in the wiki article you linked to.

[Big Windy]

I want to put a little something in my KATO passenger cars when I install lights.Can anybody suggest a circuit and components that would reduce flicker but not have my layout go down like a New York power grid in August.

[TiVoPrince]

I want to put a little something in my KATO passenger cars when I install lights.Can anybody suggest a circuit and components that would reduce flicker but not have my layout go down like a New York power grid in August.

Seconded
especially if there is a nicely etched PCB  to securely mount components and fit between track pickups and those wonderful 12v LED strips...

[trainforfun]

Seconded
especially if there is a nicely etched PCB  to securely mount components and fit between track pickups and those wonderful 12v LED strips...

TiVo there is an article in the last MR ( november ? ) about these LED in an HO car used with a DCC module to control them .

[peteski]

Adding a DCC decoder will not magically prevent flicker.  I added a capacitor and a resistor to the original Kato white LED light kits to prevent flicker.  I'll I'll dig out that write-up later an post it here.

[TiVoPrince]

Adding a DCC decoder will not magically prevent flicker.  I added a capacitor and a resistor to the original Kato white LED light kits to prevent flicker.  I'll I'll dig out that write-up later an post it here.

Thanks
that would be what I am looking for.  Something to keep lights alive across contact voids as they roll along.  If there are other nifty features that can be included, I'm in for that too...

[peteski]

TiVoPrince, PM me your email address and I'll send you the instructions for the deflicker circuit (it is a PDF file).

[peteski]

I want to put a little something in my KATO passenger cars when I install lights.Can anybody suggest a circuit and components that would reduce flicker but not have my layout go down like a New York power grid in August.

PM me your email address and I'll send you instructions for de-flickering your Kato lighting. Assuming that you are using the first generation white LED units (not the new, extra bright ones).

[TiVoPrince]

Peteski
sent along a clear and concise diagram using stand alone components.  Along with some great 'nub' polishing tips.  Thanks again Peteski, Superliners work a treat now. 

Sourced the NTE parts on the peg at my local Fry's.  Just too impatient for mail order is time.  Used the 50v cap and bridge to avoid overvoltage problems should I ever forget and leave command station on the HO setting.  Those 12v 9LED strips are very current thirsty (I think it may be the resistors on the strip) and while they don't go dark there is a bit of dimming even with everything immaculately clean and a 470mfd capacitor as they roll along.  TiVoPrincess approves of the change.  But, now she wonders where all the passengers are.

I removed some blue tint from the LEDs to compensate for the resistors in the circuit.  Built three so far and now I need to mail order more parts to complete the fleet...

[peteski]

Peteski
sent along a clear and concise diagram using stand alone components.  Along with some great 'nub' polishing tips.  Thanks again Peteski, Superliners work a treat now. 

Sourced the NTE parts on the peg at my local Fry's.  Just too impatient for mail order is time.  Used the 50v cap and bridge to avoid overvoltage problems should I ever forget and leave command station on the HO setting.  Those 12v 9LED strips are very current thirsty (I think it may be the resistors on the strip) and while they don't go dark there is a bit of dimming even with everything immaculately clean and a 470mfd capacitor as they roll along.  TiVoPrincess approves of the change.  But, now she wonders where all the passengers are.

I removed some blue tint from the LEDs to compensate for the resistors in the circuit.  Built three so far and now I need to mail order more parts to complete the fleet...

Thanks for the kind words!  I'm glad that the circuit is TiVoPrincess-approved! 
The reason I have not just posted the circuit diagrams and construction photos here is because I'm writing an article about these circuits, to be published in a magazine. It is in the pipeline, along with several other articles I'm writing. I just need to spend some more time to on them to get them finished.

BTW, a cap rated for 50V is a huge overkill!  One rated for 25V (or even 20V) would have plenty of safety margin, even if the DCC system was in H0 mode.  Caps rated for a lower voltage are physically smaller than the same capacitance cap with higher voltage rating.  Since space in N scale models is at a premium, it makes sense to use the 20 0r 25V rated caps. For example, a 470uF cap rated at 50V is most likely physically twice as large than a 470uF cap rated for 20V, or the same physical size as a 1000uF 20V cap.  So you either need less space for the same value capacitor, or you can fit a capacitor with twice as large of a capacitance into the same space.

As far as obtaining parts, I highly recommend mail order through either Digikey or Mouser Electronics. They have no problem with small orders, and the available parts selection is mind-boggling huge!

[TiVoPrince]

As
stated I used available bits, safe but seriously bulky.  Shopping online for correct voltage bits right now...