Programming a stay alive equipped decoder

[Rallymatt]

This could go here or Hornby DCC but as this is the big topic here it is. As referred to elsewhere;

can you programme a DCC decoder via the normal methods, when that decoder is also equipped with a stay alive? Both in HM DCC and traditional DCC formats. There are comments that CV’s can’t be read, does that also cover writing values?

[Brew Man]

Can I suggest that members who posted relevant posts in the 'Prices' thread move them to here.

Mod note - if all you folk who posted over there go to your other post - select Edit from the 3 dots menu, then select all text and CUT, then go to the new thread reply box and paste your stuff in there that will work. Then delete your old empty post.

[96RAF]

Extract -

• External loads, such as constant lighting, smoke generators and other loads should be switched off. Decoders should shut down any loads connected to them when they enter service mode.
• Service mode operations should take place in a limited power environment. The NMRA specifies no more than 250mA for 100mS. The maximum current may be limited by the programmer using a resistor, but the manufacturer must state that it may not be 100% DCC compatible.
• Decoders equipped with capacitors, such as "keep-alive devices may pose problems. If possible, they should be disconnected prior to programming operations.
• A programming track booster may be required for some sound decoders.

[SteveM6]

I was confused by the assertion that once a stay alive is fitted you can’t read in programming mode as I had not noticed this with my 40+ locos fitted with stay alives.

Having looked into this it appears that some older DCC systems were designed in a time where sound wasn’t prevalent and have a slightly lower voltage(?) in programming mode. As the stay alives draw some of the available power, it can drop the amount of power available to the decoder. The result is that it doesn’t reach the threshold to access the higher functions of reading the decoder.

[SteveM6]

Colin, I'm replying to your last post in the Prices thread in which you asked about my stay alives.

I have fifteen TXS decoders fitted with the standard power bank plus another twenty + with homemade stayalives fitted to TTS decoders - these use five 3v 1f supercapacitors in each.

My system is run via an old Elite which has been updated with the latest firmware.

I've encountered no issues with programming or reading any of them.......so far.

[Brew Man]

Colin, I'm replying to your last post in the Prices thread in which you asked about my stay alives.

I have fifteen TXS decoders fitted with the standard power bank plus another twenty + with homemade stayalives fitted to TTS decoders - these use five 3v 1f supercapacitors in each.

My system is run via an old Elite which has been updated with the latest firmware.

I've encountered no issues with programming or reading any of them.......so far.

 

 

I take it these would be connected in series to give a total voltage of 15V and a total capacitance of 200,000 micro Farads. Is that correct?

 

 

[ColinB]

All I can say is amazing, it has never worked with any of mine with "stay alives" even the one I added to my TTS decoder. I think a couple of my Zimo sound decoders have "stay alive" and they seem to work ok but all the others I had to remove the connection to read a CV value. I am really surprised with that big a value of capacitance it works.

[Rallymatt]

Ok, just fitted a Zimo MX617N decoder with a 871007 KungFu stay alive hard wired to pads on the decoder. Fitted to a Hornby 08 TT:120 shunter.

My Dynamis Ultima won’t read CV values (not sure exactly if it should) but programmed CVs 3,4 & 5 on service track fine, Address and name altered fine. Runs sweet as a nut with the alterations fully activated.

On a side note, I had decent running (live frogs etc) but stay alive has really transformed its ability to behave like a shunter.

[SteveM6]

I have virtually no knowledge of electronics other than being able to solder and follow an existing diagram - proof that a little knowledge is a dangerous thing.

This is the diagram I am following and it seems to work well and is quite inexpensive to build - less than £6 per loco.

From what I can see, Hornby use the same basic diagram for the powerbank albeit with the addition of a transistor and the removal of two capacitors.

[Brew Man]

Thanks for the diagram, Steve. Yes, as I thought, 5 caps in series. There is quite a bulk there and almost impossible to fit into a small loco. I wonder if smaller value, smaller physical size super capacitors exist. That would reduce the overall size considerably.

[SteveM6]

You are absolutely correct that finding space can be a challenge but with the homebrew versions, the components can be spread around the available space. By doing this I have squeezed this lot into around 95% of the ones attempted. Where there is genuinely no space I fall back on a modified HM7K powerbank (I change the plug and solder it to the TTS decoder).

A couple of photos to demonstrate (taken before I added heatshrink insulation before anyone shouts at me). The first is a 61xx tank loco while the second is an A3 - a reasonable amount of space but doing it this way means I can retain the ballast weight.

[SteveM6]

Thanks for the diagram, Steve. Yes, as I thought, 5 caps in series. There is quite a bulk there and almost impossible to fit into a small loco. I wonder if smaller value, smaller physical size super capacitors exist. That would reduce the overall size considerably.

 

 

I’ve posted a couple of photos - awaiting mods approval.

As I indicated, Hornby use something very similar but manage to only use three caps - I presume the inclusion of a suitable transistor allows this. Unfortunately I don’t have enough knowledge of electronics to be able to incorporate that myself. I do know someone who does know but he’s not allowed to tell anyone.

[Brew Man]

I wonder who that could be thinking_face grinning

[Brew Man]

I can't see where the transistor would come into play as regards the capacitor count though. It is just a case of choosing capacitance values. The thing with capacitors is, if you put capacitors in series, the total capacitance doesn't increase, it decreases. So, assuming each capacitor has the same value, two in series gives a total of half the individual values. Three in series gives total of one third of the individual value, and so on. So if we could find a super capacitor of value 200,000 micro Farads it should be considerably smaller in physical size than five 1 Farad capacitors in series. No idea what the transistor brings to the party.

[SteveM6]

I am just theorising but I do know that capacitors don’t play well when fed with excess volts (?). I have assumed that as DCC puts out a nominal 15v, if that were applied to say 3 x 3v caps the result can be catastrophic.

Hornby use three caps in the powerbank of either 2.7v or 3v (can’t remember which offhand) and as far as I can see the only difference in the wiring is the transistor - does that limit or control the inward current?

I don’t know and freely admit that my knowledge of electronics is severely limited.

[Brew Man]

Yes, if it's being used used in a voltage regulator circuit. That's what the Zener diode does in your diagram - clamps the voltage across the capacitors at 15V.

[ateshci]

It's the most primitive approach with the least parts' count.

It supplies some 8V ore thereabouts to the decoder. Due to the decoder specs ( shutoff @~7V supply voltage according to NMRA standards ), approximately 12..15% of the available charge can be used. In most cases this is sufficient, however.

The surge current to charge the capacitors is hampering programming, however, so the best advice is to disconnect it then. Attached is the schematic, the components indicated are just an educated guess judging from performance requirements.

[Brew Man]

I must say I can't see any useful purpose to having the transistor and diode in circuit as the Zener diode and its series resistor should do the job fine.

[96RAF]

In HM7K decoder use it takes a switching signal from the decoder to start charging when the loco moves - since reverted to static charging after a connection delay, so the need for switching stays relevant.

[96RAF]

The PB zener is PN MMS74695-TP 8.7v and the 3x 1F caps are 2.7v.

[ColinB]

The reason there are 4 capacitors in series is because you can't get one with the right value and voltage rating. The zenor sets the voltage at the Zener value, looking at it, 15 volts. The resistor is the resistor that limits the charging current. The 1N4001 is the feed to the circuit when the voltage on the decoder drops below the value of the zener minus 0.7 the forward drop across the 1N4001 when it conducts. If you think the circuit is robbing current then increase the resistor value all it will do is take longer to charge up the time constant being R *C, the value of the resistor multiplied by the value of the capacitor. Yes, you don't need four capacitors you can use a lower value in a higher voltage rating. The second circuit is just a variation on the same theme where the transistor is an emitter follower charging the capacitors at constant current.

[Brew Man]

Right, if the unit is triggered on by a signal from the decoder then I get it.

[ColinB]

When the voltage on those 4 capacitors is 0.7 above the decoder voltage which is derived from the track, then the 1N4001 diode conducts and powers the decoder from the stored voltage in the capacitors.