Capacitors. blah blah blah

[Chris333]

So back to me 

I ordered a few of the round button caps because I think they will fit in the cab of my Nn3 Plymouth.


Now is there a way I can wire the loco so it will only move forward and not risk blowing the cap. Like will a diode on one leg of the cap work?

[peteski]

Now is there a way I can wire the loco so it will only move forward and not risk blowing the cap. Like will a diode on one leg of the cap work?

Yup!
Take a diode (a simple rectifier diode rated for 1 Amp) like  any of the 1N4000 series diodes and insert it between the track pickup and the motor/cap combo.  If the right rail is positive when the loco runs forward (per the industry standard), then attach the anode (unmarked side of the diode) to the right pickup and the cathode (the marked side of the diode) to the motor/cap combo. The side where the diode is will be positive (so the positive lead of the cap will be attached to the cathode of the diode (don't worry if that sounds odd).


LEFT TRACK  >-----------------
                        |    |
                  CAP   =    O  MOTOR
                        | +  |
RIGHT TRACK >------->|--------
                 DIODE


I hope the above diagram shows up correctly.  The + shows where the positive side of the cap should be facing.

[Chris333]

Diagram came through, thanks.

One more question. Can I add the diode only to the cap and not the motor. Besides being easier to install I'm hoping the loco will still work in reverse, but without the cap.

[peteski]

Diagram came through, thanks.

One more question. Can I add the diode only to the cap and not the motor. Besides being easier to install I'm hoping the loco will still work in reverse, but without the cap.

No, can't do it.  If you added the diode just in the cap circuit then the cap wouldn't either charge or supply current to the motor (depending on how the diode was installed. If you can visualize this, diode is a one-way electric current valve. When the cap is charging then the current flows into the cap, when the track power is lost then the current flows out of the cap (in the opposite direction).   Even though the polarity of the voltage in the charged cap doesn't reverse (in reference to the entire circuit), the cap's current needs to flow in both directions.

[Chris333]

Oh yeah that does make sense. I'll stick to the diagram. 

[Chris333]

Well maybe I should have measured first 

The disc is smaller in some ways and bigger in others:



Is there a smaller cap I can use. Is say a 3v cap smaller physically?

[jdcolombo]

How much capacitance do you need?

Digikey has a 3.3v 330mf (.33 F or 330,000uf) that is 6.8mm in diameter and 3mm thick.  You might have to cut off the solder tab leads, though.

http://www.digikey.com/product-detail/en/DSK-3R3H334T-HL/604-1160-1-ND/3280653

Seiko makes an even smaller 80mf (.08 F or 80,000uf):

http://www.digikey.com/product-detail/en/XH414HG-IV01E/728-1050-1-ND/1889211

John C.

EDIT: I went back to your first post, and saw that the cap you are using now is 47000uf, or 47mf.  So either the 330mf or 80mf 3.3v caps should be more than adequate if the voltage is high enough. 

[peteski]

Chris, most of the higher capacitance or higher voltage supercaps are actually ganged sets of smaller caps (if you were to open one of those up).

Instead of trying to find a single cap which will fit the available space, you could just as easily make your own cap from several discrete smaller caps.  Digikey has a huge selection of suprecaps available.   Here is the link to the top level of the search http://www.digikey.com/product-search/en/capacitors/electric-double-layer-capacitors-supercaps/131084

Once you learn how to used the filters on that page (make sure to check the "in-stock" box too) you should be able to find some supercaps which will give you what you need.

For example, if you would like to have a 1F 5.2V supercap in your model, buy four 1F 2.7V caps. Those come in physical size packages. Connect 2 pairs of them in-series (so each set will be 0.5F 5.2V) then connect those 2 series-connected pairs in parallel. That gives you 1F 5.2V total capacitance.  But since each cap is fairly small (physically), you can try to arrange them to fit them all the available free space in the model.

When connecting them in-series, connect positive of one cap to negative lead of the next cap.   If all the caps are the same then the total capacitance will be the capacitance of a single cap divided by the number of series-connected caps. But the working voltage will be the sum of the voltages of all the series-connected caps.

When  connecting caps (or series connected sets of caps) then all the positive leads are joined together and all the negative leads are joined together.  The capacitance is the sum of all the parallel-connected caps (or the capacitances of the sets of series-connected caps), while the working voltage will be the same as the lowest working voltage of all the parallel connected caps (or sets of series-connected caps).  I gave the earlier example of using 4 1F 2.7V caps for ease in understanding how this all works.  Just some basic electronic theory.   

[Chris333]

Thanks for the help.

As far as what I want in my model I have no idea what voltage or farads I need. I listed the square cap at the beginning because I know it works. I have ordered a few more of those. Perhaps I can strip away the black plastic case and find I have a lot more room.

I'm not ever sure what exactly makes a cap "super".

[peteski]

I'm not ever sure what exactly makes a cap "super".

Supercap is a family of capacitors which have  very high capacitance in a small package as compared to standard electrolytic or tantalum caps. Gold capacitors or double-layer capacitors are within that family. Also, the document I linked to earlier has some pretty interesting info (even for non-experts).

[Chris333]

So then this won't work?
http://www.ebay.com/itm/161367606714

[peteski]

So then this won't work?
http://www.ebay.com/itm/161367606714

0.047 microfarads?  Sure, it will work, but it will probably have only enough energy stored to power the motor for few nanoseconds. 

[Chris333]

I was able to shave about .050" from the top and bottom of the square black box cap. I think it will fit, but I wanted to see how it runs so I rigged up some leads. I have no idea what Youtube is gonna do, but here goes.

/>
It is running at 1-1.1 volts in the video.

The thing is the coast effect isn't that great. In my boxcabs if I cut the power I get another 2-3 inches running. With this Plymouth I get a split second. What is different? The boxcabs have two 12V motors. The Plymouth has one 6V motor.

[jdcolombo]

It's not the voltage of the motor that matters, but rather the operating voltage (the voltage you are actually running them at) and the current draw.  If your 6v motor is inefficient and draws a lot of current, it will "use up" the energy stored in the cap faster than a very efficient 12v motor, each operating at the same voltage.  It's hard for us non-EE types to remember, but caps, batteries, etc. just store electrical energy.  The consumption of that energy depends on both the voltage and amperage of the consuming device.  That's why a flashlight that runs on 3 D-cell batteries won't work on 3 AAA's - the voltage is the same, but the AAA's don't supply enough current (amperage) to light the bulb (or won't do so for very long).  Or think of a glass of water.  How long it takes you to drink it depends on how fast you're gulping down and how big each gulp is (a little crude for an analogy, but . . .)

You need more capacitance if you want a longer run time.  If you are running this at 1v, then a 3.3v cap would work, and what you want is the highest amount of capacitance you can fit in the cab.  Capacitance is measured in Farads, and usually specified as either millifarads (mf, or 1/1000 of a Farad) or microfarads (uf, or 1 millionth of a farad).  1uf = .001mf = .000001F  or put another way, 330mf = 330,000uf.  The black cap you are using is 47000uf, or 47mf.  So to get more run time, you need more than 47000uf (and since you would be dropping the charge voltage from 5.5v to 3.3v, you need even more).

So, here's what I would do.  Measure the inside of the cab area as best you can.  I suspect you can fit two of the 330mf "button" caps that I gave a link to in my previous message in the cab (they are 3mm thick, so two of them gives you a "sandwich" that is 6mm thick).  If you can fit two of these, wire them in parallel.  It would be pretty easy to do with these button caps - you orient the caps so the + sides are facing each other, and solder them together with a wire lead.  That leaves the - terminals on the outsides of this "sandwich" and you can use a small piece of 30-gauge wire to go from one side to the other, and then twist the end of that wire with a longer lead and solder to the other side.  You now have a capacitor sandwich that is 6.8mm in diameter and 6mm thick (plus the wire, which should be minimal) with 660mf of capacitance, or roughly 14X the capacitance of that square black one.  I'll bet this does the trick - heck, I'd bet that ONE of these 330mf caps would do the trick, but two would be better (you could always start with one, and then if you need even more "coast" time, add the second if it fits).

John C.

[Chris333]

John,

I'll order those caps and try it. (just ordered 10) My space is 12mm wide, 10mm high, and 7mm long.