ESU Mini Power Pack - Why does it work?

[RBrodzinsky]

The title might be somewhat facetious, but I recently did my first install using one.  After all the discussions about voltage ratings on capacitors, and using 20V minimum, or better yet 25V rated caps (for N/HO), the ESU mini power pack comes with a 2.7V 1F (yes - 1 Farad!), capacitor.

Now, I know how caps in series add their voltage ratings together, so 8 of these would exceed 20V, and still give 125mF of capacitance, but my question is how does a single 2.7V cap work in this environment (and it functions quite nicely)?  The power pack has circuitry on it, but still....

Curious minds would like to know

[peteski]

While I have not had a chance  to examine it I suspect that it is a pretty advanced power supply (DC/DC converter) which takes a 12V DC and charges the 2.7V cap (with less than 2.7V). When the power needs to deliver back to the decoder (interrupted track power pickup), the power pack circuit takes the low voltage stored in the cap and up-converts it to 12V DC.  The SuperCap never sees the full 12V.

I am familiar with a similar power supply used in a train camera circuit. In this case they use a small NiCad battery (1.2V) to store the energy and when needed it is converted to 5V which is the voltage needed for the camera.

[RBrodzinsky]

Thanks - I always wonder about "up convert";  I know it can happen, but it still confuses me (guess I will need to read up on it).

Give me analytical/physical chemistry any day! 

[peteski]

Thanks - I always wonder about "up convert";  I know it can happen, but it still confuses me (guess I will need to read up on it).

Give me analytical/physical chemistry any day! 

Here is some basic info (of course the actual circuit is much more complex).  There are IC chips available dedicated to that function.
http://www.learnabout-electronics.org/PSU/psu32.php
https://en.wikipedia.org/wiki/Boost_converter

Then of course the ESU circuit, besides the DC/DC upconverter, has to have a voltage regulator which will reduce the 12V from the decoder to the 2.7V to charge the cap, and a circuit which will automatically switch between charging the cap or boosting the cap's voltage supplying it back to the decoder.  Like I said, this is far from a simple keep-alive we were all used to.