Super Caps - Fascinating Electronics

  Good post..i like the first video starting a car with a cap pack.

Continuing my education about the usefulness of capacitors in model railroading, and exploring their ability to store a charge, it appears that a series circuit of capacitors could store a charge at 9Vdc and behave somewhat like a battery.

What is the function of the resistors in the circuit? Is the formula below appropriate to size the resistors for the series circuit shown?

I believe it assumes that C1=C2=C3=C4 and that the resistors will have the same value.

For more than 2 capacitors

R=((NVm)-Vb) / (0.0015CVb)

where

R=megohms

Vm=maximum permissible voltage permissible in a single capacitor

Vb=maximum voltage across the group of capacitors

N=number of capacitors

See the idealized circuit in the post below.

A better way is to use Zener diodes across the caps, that insures that they get an even charge.  That's the reason for the resistors as well, but they tend to discharge the caps, the Zener diodes just limit the maximum voltage to any one cap.  FWIW, the BCR that uses superCAPS for this purpose uses Zener diodes.

  Do these require Zeners for ps2 2.4 boards?

They don't as the connection is internal.  They obviously insure the two caps are identical in internal resistance so that any charge/discharge will be equal.  The individual caps used in the 9V BCR can't be matched that close, so Zener diodes are used.

  Thank u GRJ will be ordering  from Digikey soon. Thanks to Bobbyogauge for this informative posting.

John,

How do I size the Zener diodes? My guess is via Ohm's Law. I am using 3.3F, 2.5V Series B Super Caps with R = 150 meg-Ohms. Power input is from the charging circuit in an MTH powered unit with 5V boards.

GRJ, why would you put caps in series?  Won't one suffice, if it has proper max voltage rating?

Since the Zener diodes are only there to equalize the charging current, they can be small 1/2W units or even lower.  The MTH boards charge at around 11ma.

Look for a 10V or higher SuperCAP with at least 1.5F of capacitance and tell me how many you find.  If you do find one, note the price.

Without checking the references, I accept what you say as factual.

As we all should; he knows his stuff!

John,

I revamped the circuit to replace the resistors with Zener diodes as you suggest. Did I get it right?

Nope, all the Zener diodes should have their cathode to the positive side and the anode to the negative side.  I'm not sure why you alternated the polarity of the diodes.

Thanks John. The sketch below reflects your tutoring.

Looks like how I wire them, none of them have blown up yet.

On your excellent graphic, I suggest you show the polarity stripe (cathode) on the zener diodes. 

Also, while you make reference to minding the polarity on the battery connector, a brief elaboration is in order that it is indeed red wire to negative, black wire to positive - because the battery connector is being used "backwards" to simulate a battery rather to connect to a battery.

This is the setup described by Susan Deats a year or so ago but with the diodes marked and the reverse polarity wiring clearly demonstrated also.   Joe

Stan,

Your comments are incorporated into the sketch above. Thanks for your input.

  So when install caps above does neg go to neg and pos to pos or? Glad i am following this post.

Willgee,

No the caps are in series, so it is negative to positive to negative to positive .......

You're trying to look like a battery with the 9V ones, so you have to reverse the connections.  For these, the + of the superCAP goes to the red + lead of the charging circuit.

Out of curiosity, how does the farad value figure in regards to our (this) application? Is a higher or lower value a benefit?

Milwrd

A higher capacitance equates to a larger "battery".

Careful, this topic will be deleted.  Seems one forum sponsor who builds these complained to Rich, I understan, and zap my and others battery replacement threads gone.

However Susan Deats used my information with permission, on her website out of OGR reach.

And Stan2004, yes the connector is wired backwards, black +, red - .

  i remember the "zap". This entire post has gone into my train electronics file.

That sucks. Didn't the vendor realize that do-it-yourselfers will always want to make their own and folks who cannot will still want to buy one?

Sounds like a real whiner.

Yes, and I don't think there are really all that many DIY'ers out there. It may seem like a lot at times from some of the threads here, but I think there are many more folks that won't go near a soldering iron than there are that will. I like some DIY projects myself, but I think some are better off left to the ready made products available.

I have purchased many capacitors from a forum sponsor and installed them in my MTH engines.  I will continue to do so.  There is no way I am going to risk my engine's electronics by making these components myself.  This is also the feeling of my fellow hobbyists.

Joe

Most folks don't want to tear into modern electronically controlled locomotives, which is probably why I'm so busy!

How do you work on trains and keep so busy on the internet?

24/7.

rrman,

Thanks for letting Susan Deats use your work. I converted about 50 - 60 engine/diesels using your/her instructions. More economical than the alternative.  Joe

You are welcome.  Glad to help.  So far all my home builts are still going strong.

I need a schematic, parts list with values and parts link.

I need to have 9 ready to install when my S2's, RS's, SD's, SW1's, GP-9 and Subways with PS2 go bad.

Thanks

SIRT,

Go look for Susan Deats postings.  Then check for her website or e-mail her for directions.

rrman,

I did a search of Susan Deats postings and found nothing about a capacitive circuit like the one in my diagram. How about providing a link to it.

Just did a quick search and found this. http://www.slsprr.net/

You should try it sometime.

Milwrd

Not needed...but it's because you say PS2 2.4.  The charging circuit on that board uses 5V to charge the supercap.  It was "discovered" that even though the OEM battery is charges up to ~2.4V (or so), you can use a supercap charged up to ~5V (or so) and operate the PS2 2.4 in shutdown with reasonably sized supercaps.  In an arguably logical attempt to better simulate the original battery, you could limit the voltage across the supercap to 2.4V using diode clamps.  But you'd need an impractically ginormous supercap which would take forever to charge and perhaps cost more than the engine!

A related (somewhat separate from balancing) function of the zener diodes is to limit/clamp the voltage impressed upon the individual supercaps.  So if you were using this 5V supercap in some other circuit that uses a charging circuit of more than 5V, then you'd want some type of voltage limiter such as a zener diode or whatever.

Try here Susan Deats Web Site - Battery Substitutes