Old DC locos stay alive

[Tay00]

I’m looking for help please.

I want to fit stay alive circuits to my old DC locos, Princess class, Britannia etc. Thinking maybe fitting stay alive components in the tender with wires connecting to the loco.

I can only find info on here for DCC.

can anyone help with suitable components and circuit please

tia

[Chrissaf]

It is not technically possible to fit DCC stay alives to a DC locomotive. The reason was (is) explained in the Stay Alive FAQ found at the top of the DCC forum.

DCC stay alive capacitors are polarity sensitive, but the wiring between the track and the motor on a DC loco have reversing voltage polarities depending upon which direction the loco is travelling. Applying a voltage to a DCC Stay Alive capacitor the wrong way round will make the capacitor swell up and potentially explode.

Any circuit designed for a DC loco would ideally need to be based upon a microprocessor controller. The track voltage would need to be bridge rectified to provide a consistent polarity for the 'stay alive', then the processor would need to detect the originating track polarity and switch the motor voltage polarity using an H bridge motor controller. All of these elements are found in a DCC decoder. Theoretically, you could fit a DCC decoder with 'Stay Alive' to your DC models then rely on the 'DC operation' feature of the decoder to run it on your DC layout. But your old DC locos may not be easy to add a decoder and is an expensive and potentially complex workaround.

Then even if you did use a decoder on your DC locos just to get 'Stay Alive' functionality, you create two more issues. One is that you lose low speed fine control as the decoder needs to see a basic voltage level on the track before it can power up and function as a decoder. Secondly, the loco will continue to 'run on' due to the 'Stay Alive' when you shut the DC controller down to its stop position.

[atom3624]

Hi Chris, might this product, or a variation of it help?

I often insert cab lighting, and don't mind it being directional - faint on for the direction of travel.

I asked how could I install the cab lighting and have it lit at both ends - led lighting being pole sensitive (directional).

I was told to install a bridge rectifier

For lighting, this is the product: https://www.illuminatedmodels.co.uk/products/bridge-rectifyer-circuits-5pk?_pos=1&_sid=06106172d&_ss=r

For a higher load on a locomotive, it may need a much bigger capacity of the same thing?

Al.

[Chrissaf]

A motor is completly different to lighting. A bridge rectifier will make its output voltage constantly one polarity, this is great for LED lighting but 'chocolate teapot territory' for a DC loco motor. The motor has to see a reversing voltage polarity to control which way the loco travels. If you place a bridge rectifier between the wheel pickups and the motor this will only allow the loco to move in one direction of travel.

[threelink]

I'm not au fait with the electronic wizardry of stay alive circuits. However, as a dab hand with old DC stuff, I always found that a fly wheel could work wonders in keeping a loco moving over a bit of dodgy track where electrical continuity was a bit iffy. Not always possible to fit, but worth it where they can be fitted

[Rog RJ]

There is/was a thread on rmweb forum where someone has apparently been successful in making a stay alive for dc. It is very simple, very few components. I think his name is AndyMsa

[Chrissaf]

I searched through Andymsa's posting history on rmweb, 42 pages of it and can find no reference to DC Stay Alive, the bulk of his posts seem the be DCC focused.

Any such circuit would [by the law of physics] have to be a Bi-polar capacitor. A Bi-polar capacitor is one that can tolerate being charged up with either voltage polarity applied to it. The issue with Bi-polar capacitors is that to get a µF value large enough to be of use as a 'Stay Alive', the capacitor would need to be a large physical size and therefore not practical for most 00 models.

[Rog RJ]

Sorry, wrong Andy. Just found the thread. Not sure now as to whether it's fully working. Uses back to back super caps. https://www.rmweb.co.uk/community/index.php?/topic/159701-keep-alive-for-dc/page/2/&tab=comments#comment-4253910

[Chrissaf]

An interesting concept. Note that his circuit is creating a Bi-polar capacitor array to overcome the reversing polarity issue. He goes on to say that he hasn't tested the circuit for long term reliability.

Rog(RJ) link made clickable:

Keep-alive for DC? - Page 2 - Electrics (non-DCC) - RMweb

[RDS]

On the video it looks a lot better than any of my attempts with Commercially available Stay Alive on DCC!

[Rana Temporia]

There are some quite large non-polar caps available now but not quite sure how big?

Using Q=CV, Q=It and V=IR it should be possible to work out what size cap would be needed to say keep a loco moving over a plastic frog or other dead spot. There was something in an old railway modeller about an electronic flywheel but the user has to use a DPDT switch when they wanted to change direction.

As mentioned above, the residual charge would continue to move the loco when power was removed but if we are only using something that is needed for say 0.25 of a second the effect would be minimal. Mechanical flywheels/big motors introduce this effect as well due to the kinetic energy stored in the mass of the wheel/armature. Try driving a 1960s Jouef/Playcraft Pacific, D6100 or even their 040 tank loco to see how far the loco goes when power is removed. One person in an old magazine had a bulb wired into the circuit to absorb some of the energy. If it achieves nothing else a non polar cap across a DC motor will help stop lights flickering.

[ColinB]

You probably could do it with a combination of diodes, electrolytic capacitors and transistors if you really wanted to.

[Brew Man]

You probably could do it with a combination of diodes, electrolytic capacitors and transistors if you really wanted to.

 

 

joy You make it sound so attractive, Colin

[ColinB]

There is no answer to that. I must admit the big advantage of DC ( I run DCC ) is that it is simple so why add complexity. I think I was told once that you can make a non polarised eletrolytic by putting two in series with their same polarities connected together. Never tried it, so I don't know. The trouble is with DC there would be no way to stop loco immediately, at least with DCC you can hit the big red button on an Elite.

[Stephen in Kerry]

I looked on YouTube the other day, to find a definition of 'stay alive'. Plenty of expert train modellers telling me how to install it, but none of them told me what it is or what it does.

First rule of teaching: Never assume your students (audience) know as much as you.

But, having read here about 'diodes, electrolytic capacitors and transistors', I'm fairly sure, now, that I don't want to know! scream

[Going Spare]

@Stephen

If you search the internet for Model Railway Stay Alive, you will find a brief explanation. As you have chosen DCC, it probably is advisable to know at least the basics.

[Chrissaf]

".....but none of them told me what it is or what it does."

This FAQ gives some background info on what a 'Stay Alive' is and what it does when used with a DCC Decoder. Some DCC decoder brands [non-Hornby] provide factory fitted 'Stay Alive' options.

FAQ - How Do I add a 'Stay Alive' to Hornby decoders?