Osquin

Thanks both, that looks like it might be the source of the problem. I am indeed trying to use 5.5V caps (I thought I could get away with fewer in series to help with the space issue Nigel mentions), and looking up the spec sheet for them it says the internal resistance at 1kHz is <75 ohms, so I guess that is likely to be significantly above the 5 ohm max that Hamburger mentioned. Somewhat back to the drawing board I guess, but I am definitely learning useful info here, so I appreciate your inputs! Maybe I’ll try the tantalum option in the next iteration.

So thinking about the suggestions made so far - if the diode was the wrong way round would the circuit charge quickly (through the diode) and discharge slowly (through the resistor) so the voltage on the capacitors would stay higher for longer but not supply much current to the motor? I see the voltage drop v quickly. I guess I can get the soldering iron back out and turn it round to see. On the super capacitors, I guess it is more than likely that they are not really designed for this purpose, but if this is the case, is there anything that one should look out for on a spec sheet that would identify those that might be useable from those that would not? Thanks again for any advice!

Thanks for the suggestions I’ll try to follow up over the weekend. Sorry if it was a misplaced post, it was simply that I found and followed the various bits of advice on building stay alives from you guys on this thread to get to the point I did, so it seemed like it might be the best way to ask for your help. Also my first day on the forum, so maybe I was overly reluctant to try to start something new. I am not sure I know yet what the TCS units actually are, so please be assured I in no way intended to cast any doubt on them!

I realise this thread is a bit stale, but I came across it after a google search and have eventually come down to building the stay alive in one of Arts posts. I came this route after building the most simple circuit (diode, resister, 4700microF electrolytic capacitor) but finding it only gave about 1/4 wheel turn on a Hornby class 8 with a Hornby decoder (not TTS). So I acquired some of these super capacitors and built the version in this thread, I think with a total capacitance of about 0.2F. I installed it in place of the electrolytic circuit, but on test got less, indeed almost no discernible response. Having checked the wiring etc, I attached the decoder plus stay alive to an ESU decoder tester which has a small motor with flywheel on it. In this configuration, my super capacitor circuit kept the motor running for well over a minute, and I then found the electrolytic version kept the tester motor running for 10 seconds or so. So this seemed to be as expected, the super capacitors provide longer stay alive by a factor of order 6. So the question I am hoping that someone here might be able to answer is is there any reason that the supercapacitor circuit shown here would have worse performance than the electrolytic version when installed and tested in the model train? Is there something about a proper load, the style of motor, the back emf, that would result in the super capacitors discharging faster than the electrolytic? With a voltmeter across the capacitors I can see the electrolytic keeps above 5v (the threshold I assume is needed for the dcc to stay alive) for a second or so, but the super app version drops immediately the power is cut. Anyone seen this, and if so any clues as to what I have done wrong?

It might be of interest for me to report here (my first post) that over the last couple of weeks I have (unexpectedly) had success in repairing a couple of Hornby decoders that I blew up in my first experiment with building and fitting stay alives (one died after wiring the wrong way round, the other after an inadvertent short between the track and the motor connection). Turns out that what fried was a couple of the tiny (I think they are called surface mounted devices, SMDs) diodes that make a bridge rectifier on the decoder board. Since I was anyway soldering the stay alive connection to one contact of these bridge diodes I thought I’d try to replace them. Found some equivalents for the 1N4001 online, it was a very fiddly soldering job, but much to my amazement both the fried decoders now work again. Not sure how much I saved in the time vs cost of replacement equation, but it was kind of satisfying to have experimented and succeeded!