improvised "Keep Alive" for DCC?

[jbconn]

Hello group

I've been kicking around how to add a keep alive functionality to a Bachmann rail detector van my son desparately wanted for Christmas. It's so small and light, with limited pickup, that it stalled on most of my insulated frogs while running in DC.

I added a TCS T-1 decoder (with ground wire added by me), I didn't think I could fit one of the TCS keep alive modules, so I built my own. It actually improved things, but not enough. It will cross one #6 or two #4 frogs, but then stall on the next one. I think it's capacitance (~47 mF, 47000uF) is too low, or maybe recharge rate is too slow, but those were the largest caps available from the local electronics store that would fit the shell.

Has anyone looked inside to see what's inside the TCS KA1 and KA2 modules? I'm thinking of ordering some 1F 2.7v ultracapacitors if I can fit a bank of five in the van.

Also considering replacing the stock weight with lead to improve pickup.

Your thoughts on this would be appreciated.

[Mark R.]

You're on the right track, but you will need some additional circuitry in order to use the 2.7 volt rating of the cap. Blown capacitors put on quite a show !


Mark.

[Mark R.]

A quick bit of research shows that the commercial variety of Stay Alive circuits do indeed use the 2.7 volt super-caps. They use a DC-DC converter to step the low voltage on the capacitor up to the higher voltage required by the decoder. This makes it possible to use a smaller than would otherwise be required capacitor because all of the energy can be extracted by upping the voltage, and the capacitor can be charged quickly because there will be a large voltage differential between the capacitor voltage and the track voltage.

Now, how do you build a homebrewed one ? .... not sure on that yet. Unless you're good at working with SMD technology, the end result would probably wind up being bigger than the ones you can buy.


Mark.

[jbconn]

My first attempt, I connected the smaller 16V caps in parallel. I had thought to connect five 2.7V caps in series to spread the (approximately 13.5V) voltage drop across all five. This would reduce the total capacitance from 1F to about 0.2F, if I recall my EE correctly.

But, it's been a very long time since that class...

[nwcanuck]

I was recently thinking about using a capacitor on a non sound equipped decoder myself and wouldn't you know it, I was reading the March edition of Model Railroad Hobbyist and this is the exact topic that they are covering this month in the DCC impulses section. Might be worth the read.

model-railroad-hobbyist.com/node/12497

[pacbelt]

You should consider the TCS KAT-12 decoder! It's a DCC decoder AND Keep-Alive in one! It's roughly the same height and width as say, a Digitrax HO decoder, and about .080 thicker...
I have some. I can, and have, run a Atlas C30-7 with this decoder over a 14 inch long piece of masking tape!! Not even a headlight flicker!!!

I made a video for my Facebook model railroad page. If I can figure outhow to post it here, I will!!