I had thought the normal

I had thought the normal values for super capacitors was 2.6 and 5.2 volts, but the article references them at 3.6 volt ?

Now you’ve learned something

From the Wikipedia entry on Supercapacitors ...

okay, good to know ... it's

okay, good to know ... it's been about four years since I did anything with them ... advances are coming, lol

I have heard it said....

That what we were learning in data processing class was already way outa date

Duration?

About how long will this StayAlive keep a typical, modern, locomotive running?  I understand it varies widely, but are we talking 1-2 seconds (or less), or 10-15 seconds?

StayAlive typo

An page 6 of the article:

Should be Zener not Zerner I think.

Duration? - A BIG IT DEPENDS

How long a Stay-Alive (SA) will power a locomotive depends on the motor characteristics, the motor load (current draw), how much capacitance in the SA, the characteristics of the capacitors in the SA and more. A extreme example - the not so small SA below will power one of my loaded Large Scale SD-45s for almost two seconds, but would power most HO SD-45s for more than a minute (if you could only get it in the engine). Some of the typical numbers quoted for commercial SAs run from 0.5 to 2 seconds, but mileage (in/mm) will vary greatly. I doubt that any of the DIY SAs described in the article would give you more a couple of seconds in moderately loaded, modern HO engine.

8.75 F at up to 24 V (12 x 100 F caps in series)

If you know the load (motor current), equivalent capacitance you can calculate an approximation for the duration. Remember that you are dealing with a dynamic system, so the EE 101 maths can get complicated very fast.

For a small headache: Capacitor Discharging- Explained

For that Migraine:  Capacitor - Wikipedia

Series v Parallel

The author does not indicate why a series circuit is used instead of parallel.  It seems to me a parallel circuit would increase storage capacity, but obviously I am missing something.  Is series used due to the higher voltage of the DCC system exceeding the working voltage of the capacitors, or something else?  Great article, I've thought about making my own for quite a while not knowing exactly how the circuit was configured.  Bob J

Re: Series v Parallel

The supercaps are rated to much lower voltage than the rectified decoder one. If you would put them all in parallel you would blow them all up. In series you get the capacitance of one of them but at *N the nominal voltage. With

N = ceil( power supply voltage / capacitor rating voltage)

Something else to keep in mind, the capacitors have to be _identical_. For even more peace of mind you can connect identical, large resistors in parallel with each capacitor, so that the voltage is the same across each capacitor, regardless of its internal resistance.

Series

In serries the total capacitance is the reciprocal of the sum of the reciprocals.  Digikey has a handy tool to calculate that easily: https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-series-and-parallel-capacitor

So 4 220uF caps in series is the equivalent of 1 55uF capacitor with 4 times the voltage rating.

Series v Parallel

OK, I think I get it now.  Even though the total capacitance is only equal to one super cap, the voltage put out is the sum of the charged voltage of each cap. Like a battery's output voltage being the combined voltage of each cell, but its total amp hour capacity limited by the amp hour of one cell.  Also, like a battery, the cells must be matched  evenly, as the first cell to discharge limits the battery's output to that lowest capacity cell.  Bob J.

Series Resistance

You got it with a good analogy.

Another capacitor characteristic that will affect the performance of Stay-Alives (SAs), is the internal or series resistance (ESR). It is also one reason I would not use the 0.22 F (220,000 μF) used in the article. Although the description of the 0.22F 3.6V supercapacitor ( 555-DXS-3R6H224U) states that it is a "Low ESR" device, but I do not consider 25 Ω low. 

How might this affect the performance of the SA? Lets set some conditions with the decoder (motor) drawing 200 mA with an applied DCC voltage which will results in 12 VDC at the output of the decoder bridge rectifier. Although more complicated, a fair equivalent load for the decoder operating under these conditions would be resistance of around 60 Ω (12 VDC / 200 mA). A SA circuit built with four (4) of the 0.22 F capacitors in series would have a series resistance of 100 Ω, so the total circuit resistance would be ~160 Ω. After the SA is fully charged (~12 VDC), any time the power drops out (dirty wheels or track) the SA will attempt to deliver 200 mA to the decoder through 160 Ω. Again simplified, this means that about 5/8 or 62.5% of the power will be dissipated in the capacitor while only about 3/8 or 37.5% will be available for powering the engine. 37% - Not very efficient. There are supercapacitors with lower ESRs, but they are very expensive. One can also build SAs with parallel / series caps which can lower the effective series resistance, but of course it will require more space.

I am not saying that DIYers should not be building and using SAs based on this design, I am just pointing out that designing efficient SAs is more complicated than indicated in the article.

Note to MRH Staff: Although the tool bar uses the Omega (Ω) character for the "Insert Special Character" function, the "Ω" character is not included in the symbol table. ????  It would also be nice to have "μ". 

discharge through diode

The resistor is used to limit the charging current. Discharge should go through the diode instead. That makes the not-so-low ESR caps usable for this purpose, both for deliverying higher current and for not causing an apparent short (due to the inrush current) when you boot up the system.

Mind the diode direction, it's a critical piece of information.

Charge limiting resistor not relevant during discharge

You missed the point! I fully understand the function of the diode and the charge limiting resistor. Since I was discussing what happens during discharge the charge limiting resistor is not included in the analysis, and resistor and diode do not in any significant way effect my simple analysis of the affect of the capacitor series resistance (ESR) during discharge. Yes I did leave out the voltage (and thus power) drop across the diode, but with a zener this loss will be minimal. So again - using capacitors with 25 Ω ESR in these simple SAs will result in significant power being lost in the capacitors.

good point

Yes, you make a very good point and the summed up ESR indeed becomes a quantity to take into account here.

For my education, the data sheet mentions "Internal resistance: Less than five times of the value at 20°C" and 25Ω as the max ESR. Does that mean that in normal operating conditions we can expect however a much lower ESR than the worse case scenario?

ESR Spec

The "Internal resistance: Less than five times of the value at 20°C"  spec applies to the "Characteristics at high and low temperature", but the data sheet is not clear on what this actually means over the specified -25°C to 85°C range. A temp vs cap plot would be helpful (several other mfgrs provide this info), but the Elna America web site does not provide any additional information. Although I have been known to run trains under extreme conditions (< < 0°C) in the garden, most of our SAs will probably be operating in the 20°C to 30°C range, so the ESR will probably be closer to 25 Ω.

Elna America does provide some guidance on the use of these caps including this Calculation of Discharge Time PDF  which some may find useful.  

I did want to mention that the ESR for the 100F caps I used in my Large Scale SAs is in the range of 22 mΩ, or about 1/1000 of that of the button cells.

DIY vs TCS

So Ken, or any other EE out there, what's your opinion of the designs used by TCS for their KA products, and the caps they use? They cost around $30, vs the < $10 for the DIY version -

I'm wondering if the performance is going to be substantially better in the TCS version due to better caps and better design, or whether it's not substantially better and for the extra $20 you're getting about the same performance only without having to build it yourself.

And, without asking you to go out and design one yourself, do you think you could do a lot better than the DIY design in the article if you had a $30 parts budget and took the time to hunt around for a really good part?

I'm asking because I basically have no frame of reference to tell whether the differences you're pointing out are a really big deal or lie at the performance margins. Does the $10 BoM get you nearly all the performance that you'd get with a $30 BoM or is it only 1/3 as good?

DIY vs TCS Tue, 2020-06-16

Commercial SA designer have to create generic products which may of my not fit into your engine. TCS now offers ~12 KAs and ~8 Motherboards with additional caps (KAs) built in cover a wide range of applications but not all. The do a very good job, but the performance of the TCS Keep Alives (KAs) are not substantially better than what a DIYer can design, build and install. However, for some installations I really do like their (expensive) Motherboards.

Probably! I can often fit SAs built with SMD tantalum capacitors (very low ESR) in N scale engines were none of the commercial products would come close to fitting. For some installations, a $30 budget might net> 50% increase in available energy. In some old S scale engines there is barely room for a decoder, but I have manage to squeeze in enough SMD tantalum caps (~$20) to keep the engines running for several seconds under load. For other installations the net energy gain over the commercial devices might be negligible, but the ease of installation may be improved which would still be a plus. I have some LifeLike (E6s, DL109s) engines where I had to mill away some of the frame to mount any SAs. The performances of the (smaller) DIY SAs was about what I would have expected from the commercial units (if they could have been installed).

For around $10 DIY SAs can equal the performance of the $30 commercial devices. Again in situations where commercial SAs will not fit, DIY SAs present a win-win situation.

Hi KenCan you tell us more

Hi Ken

Can you tell us more about these SMD tantalum capacitors - and also how do you get enough of them so that the total voltage is about 12V - I know putting them in series will increase reduces their total capacitance.  So which tantalum caps do you use?

Thanks

Shelton

Shelton said: "- I know

Shelton said: "- I know putting them in series will increased their total capacitance".

No, putting caps in series reduces total capacitance. The total capacitance will be less than the cap with the smallest value. Sum effect is like one cap with the plates spaced farther apart. Capacitance and plate distance are inversely proportional.

Thanks - made the fix to my

Thanks - made the fix to my post.

  "In some old S scale

Hi Ken, Could you be more specific as to what S scale engines might need the smaller size keep alive caps? ....DaveB

Questions - will get back soon - I hope

Shelton, DaveB,

I am not ignoring your questions, but at the moment I have been distracted by a errant organ which is probably going to keep me occupied for some days.

No worries Ken - hope your

No worries Ken - hope your getting better.

Regards

Shelton