What capacitors to use for homemade keepalive?

[Trainman440]

Hello, I'm looking to build some keep alives at home. Im just going to use a conventional resistor-diode charging method, and have the voltage output be unregulated. Im trying to find the best capacitor type to use for this keep alive. 

Im split between using either:

1. SIX 2.5V caps wired in series (outputs 15V) or

2. FIVE 2.7V caps wired in series (outputs 13.5V)

Between the two, option 2 would be cheaper yet also offer much more capacitance. That is what I would prefer to use.

I plan to be adding these into my TCS WOWsound decoders. Track voltage is 13.6V, but I do plan to bring my engines to other club layouts which might have different track voltages. 

My question is, is option 2 safe? Its only 13.5V which is below track voltage, BUT to my understanding, since its being wired to the blue common 12V wire and ground, the capacitors therefore shouldnt ever exceed 12V, making it safe.

I would prefer to not have my engine be blowing up on club layouts. 

Any thoughts?

Charles

PS also anyone have a suggestion on what 2.7V 1F capactior to use? Im not sure if the manufacturer use special caps or if any generic one will do. To my understanding, I want to find one that has low ESR.

[Mark R.]

You will need to use the zener diode in the circuit as well as it will clamp the voltage at 13 volts. Without it, you run the risk of a voltage spike taking out the capacitor. 

I built what you described once using a single 16v capacitor in a caboose. After ten minutes of running, the capacitor exploded !  After cleaning up the mess, I replaced it with a 25v capacitor and it is still working years later.

Had I known about including the zener diode at the time, I could have avoided that melt down.

 

As for what capacitors to use, these would be recommended as they are physically smaller than most others of the same rating ....

https://www.mouser.ca/ProductDetail/Nichicon/JUWT1105MCD?qs=zimaWZoi%252BnPSejLfj4P%252BuQ%3D%3D

 

Mark.

[jjdamnit]

Hello All,

current keeper recomendation

Hope this helps.

[Trainman440]

jjdamnit

Hello All,

current keeper recomendation

Hope this helps.

 

Respectfully, this wasent very helpful. One poster uses 5 2.7V 1F caps, another posts a video of someone who uses 5 3.0V 1F caps.

That's the exact dilemma Im having myself, trying to figure out who to listen to. Neither party explains why they chose what they did. 

I already saw the post above, the reason I asked the question in the first place was because two people replied in that post using different caps and Im trying to figure out who to follow. 

Currently Im leaning toward the 3.0V caps just for the little bit of additional overhead, as Mark mentions. Although I do plan to be putting in a 12V Zener diode, so I dont think using the 2.7V caps would be such a bad idea. 

Thanks, 

Charles

[Mark R.]

As I mentioned, it usually comes down to physical size. The 3.0v caps are bigger than the 2.7v caps. In some cases, it may be a non-issue, but in most engines, there isn't much room to spare.

I just did a quick look, and the 3v version is 8mmX12mm and the 2.7v is 6.3mmX9mm. In this case, the voltage difference is rather moot, but the physical size is kind of important.

Mark.

[Overmod]

See the parallel thread on 'current keeper' design here.  In particular note that Hornblower provides a list of part numbers and sources, and a schematic.  [As he has duplicated the post here, I have now removed the quote from it I posted.]

Keep in mind that, in my experience, supercaps/ultracaps are exquisitely overvoltage-sensitive.  That implies (1) be very conservative with the cumulative voltage rating, and (2) ALWAYS use a Zener or comparable quick-acting voltage clamp to prevent any spikes.

The voltage rating has nothing to do with the amount of charge the capacitor holds, or the current that charge can provide.  It is only the "Vne", as it were, and that 'never exceed' means even in short spikes.    These devices hold their tremendous charge in nano-sized structures, and it does not take much overvoltage to penetrate the internal insulation...

 

[hornblower]

Trainman440,

I believe you are overthinking this matter.  Larry Pucket has been doing this for a long time and knows what he's doing.  I myself have already built and installed 10 of these keep-alive units and they work wonderfully!  I use the Nichicon Supercapacitors as they are the smallest I've been able to find.  They are rated at 2.7 volts and 1 farad so five give you 13.5 volts total.  The Zener diode limits the voltage to 13 volts to protect the circuit.  The Mouser Electronics part numbers are as follows:

Nichicon Supercapacitor 2.7v 1 Farad

    Mouser P/N: 647-JUWT1105MCD

Zener Diodes 13v 5w

    Mouser P/N: 863-1N5350BRLG

The other parts I use are as follows:

Taiwan Semi Rectifiers (diode) 1A 1000v

    Mouser P/N: 821-1N4007G-KR0G

TE Connectivity Carbo Film Resistors 100 Ohm 1/2w

    Mouser P/N: 279-CFR50J100R

Below is a photo of the schematic I use.  

 

If you stagger the positions of diodes and resistor as shown in the schematic, you can get these three components to fit pretty tight to the tops of the capacitors.  I first assemble the capacitors in series, then place a piece of tape over the tops of the capacitors to eliminate shorts with the remaining components.  I then solder the Zener diode as close to the negative end of the capacitors as possible.  Next, I place a length of shrink tube (tape also works) to the long lead of the Zener diode before soldering this lead to the positive end of the capacitors.  I then solder the regular diode and resistor together in a staggered configuration no longer than the long lead of the Zener diode.  I then solder the diode/resistor assembly to to the positive end of the capacitors making sure it fits between the body of the Zener diode and the positive end of the capacitors.  I then solder a length of blue wire to the other end of the diode/resistor assembly and a length of green wire to the negative end of the capacitors.  Finally, I cover the entire assembly with shrink tubing.  If you tuck the diodes and resistor tightly against the top of the capacitors, the finished circuit is about 1 1/4" long, 1/2" wide and 1/4" thick.  It will fit in a lot of HO scale locomotives.  One benefit of building your own keep-alives is that you can alter the way you stack the capacitors to get them to fit in tight spaces.

I have installed these circuits in a number of Blue Box EMD F7 locos I remotored using Mabuchi or SMC can motors.  These locos run so well with the keep-alives that a pair will crawl around the layout pulling a 10 or 12 car train at about 10 SMPH for hours without hesitation.  What is rediculous is that I can leave these trains running while I power down the layout only to watch these trains continue to run for another 2 feet!  The LED headlights will stay on for at least another 15 minutes.  Kind of cool!

[wvgca]

in either case the supercaps will give a couple of seconds of run time, enough for -most- cases.... the final decision on what supercaps will be required depends more on physical space available that anything else ..

most designs are going towards multiple caps rather than a single 5v cap with step up on the decoder ..

[Trainman440]

Gotcha, thanks for the replies guys!

The Nichon caps are out of stock from all retailers it seems, ended up getting some 3V 1F caps which were 6.3mm in diameter. 

Charles

[hon30critter]

I just want to add one final point to this discussion. When you look at the schematic diagram, all the capacitors are grouped in a straight line. In reality, the capacitors can be put anywhere inside the locomotive where there is enough space for a single cap. For example, you could have one in the nose, two in the cab and the other two in the back of the shell or wherever. As long as they are wired together in series the circuit will work fine. Just make sure that everything is insulated.

Cheers!!

Dave