21st Feb 2020, 11:36 AM
Playing around with DCC powered things.
I've been meaning to electrify a number of things from DCC track power and finally got the first project off my workbench. My plan is to start with simple, lower component count and current things and work up to some more interesting stuff I have in mind.
With that, my first attempt is a simple FRED. But of course I want a good FRED, so that means using a "supercapacitor". Basically I don't want any flicker, in fact I'd like several seconds of hold-over ability. But I also don't want a battery, they go bad, have to be changed. The only real solution is a super-capacitor.
I've finished my prototype. It was made with parts from Amazon.com as that's a great place for assortments. For $10 I got 14 sizes of super-capacitors to play with, which is very cool. The finished circuit is below, and it's on my bench and blinking away. My spreadsheet calculations say the flash should be every 1.08 seconds for 0.18 seconds of duration. I think that's about what a real life FRED does, but honestly I'm not very sure.
Theory of operation is pretty simple:
- Rectifier D1 turns the track square wave into DC.
- Voltage regulator U1 gives me 5v regulated output.
- SuperCapacitor C3 stores the energy like a rechargeable battery. R3 is a 10ohm resistor to limit inrush current to prevent damage to C3.
- A good old 555 IC, U3, provides the "blink" function. R1, R2, and C2 control the blink rate -- and current usage!
- R3 is a current limiter for the FRED LED, which the final would be mounted off-board on the coupler via the FRED+ and FRED- terminals. For testing though, it's D, stuck right in the bread board.
I found https://www.allaboutcircuits.com/tex...d-led-flasher/ a very valuable resource in configuring the 555. Once I understood the theory, I made a spreadsheet to calculate R1, R2, and C2. The values are based on items in my parts bin though, I can probably tweak a bit more by using values I don't have on hand.
And, it all works. And runs nicely for 20+ seconds when unplugged.
But, it's way too big. The smallest super-capacitor in the assortment I bought was 0.1F or 100mF. It's the size of a coin-cell battery. I mean yes, it would fit fine in a lot of places, like a boxcar, but I really want to make a small board here I can hide on more interesting cars. So I want to end up with all surface mount components.
What does the next revision look like? Well, after browsing mouser.com and digikey.com I found the highest capacity literal surface mount (like a square thing that would lay flat) is a 11mF SuperCapacitor, 3.2mm x 2.4mm. Woot! Except that is 1/10th the power of my 0.1F, aka 100mF super capacitor. Boo. So I really need to reduce current by a factor of 10 to get similar performance. Can I do that?
Yes! I think so. Two simple changes. First, swap out my TLL 555 for a CMOS version of the part. The TTL takes 5ma per the data sheet, the CMOS 0.250ma. Second, my LED today is a nice big T1.5 that's great for prototyping, but takes 20ma. The real LED will have to be much smaller, and can be much more efficient. So I found a square shaped, surface mount RED LED that is 1mm x 0.6mm (wow, soldering that will be "fun") and only draws 1ma. This should take me from 25ma of draw today, down to around 1.25ma. That's actually a 20x reduction in draw!
The other change is necessitated by the smaller super-capacitor as well. I'm currently running a 5v regulator because my super-cap assortment was 5.5v rated. The SMD supercap is a 3.3v rated. So I'll also be swapping out the voltage regulator for a 3.3v part.
In short, circuit stays the same, just swapping out for higher efficiency parts all the way around. If it works on my bench, I think I'm going to order a set of custom made boards to make up 20 or so as physically small as I can make them with all surface mount components and FRED up a number of my rolling stock items.
Now, I said this was the first baby step to understanding how all of this worked. My next goal is to update the lighting in some of my passenger cars using the same trick. Bright white LEDs, a small control board, and a super-capacitor (or two, or three) that will keep them lit for 10-20 seconds with now power ensuring true flicker-free operation on even the worst track. From there, I have more plans!
Last edited by bicknell; 22nd Feb 2020 at 08:30 AM.
The Following 11 Users Say Thank You to bicknell For This Useful Post:
BruceNscale, Bryan, Gary Rowan, Heiko, JohnE, NtheBasement, Paul Schmidt, RailKing50, Scotian_Huntress, Silent1961, SP-Wolf
21st Feb 2020, 12:01 PM