Yet another Stay Alive question!

[Potrail2378]

Hi guys - I have searched 'Stay alive' and found various capacitor sizes/diodes/resistors, which is really useful. I also understand the uses of the capacitor. 

My question is - If a stay alive is fitted, when you stop the loco, will the loco 'run on' until the cap is discharged?

Thanks in advance. 

[37lover]

An expert will be along with more details but I believe Stay Alives only provide a relatively short period of back up power. If you were to action an Emergency Stop then yes the loco would run on a little further than without the Stay Alive but under normal operating, when you are reducing power for a gradual stop, you will not notice the difference.

[96RAF]

The stay alive keeps the decoder alive not the motor directly. Motor control is still through the decoder. The run on effect you see, if say you lift a loco off the track, is simply the stay alive fooling the decoder into thinking it is still on track, hence it keeps the last command sent in force.

[Chrissaf]

Something else you might possibly observe with a 'Stay Alive' fitted.

 

Let's say that under DCC control you bring your locos to a standstill pending powering down the DCC layout. You turn off the power to the DCC controller. What you might observe is that your 'stay alive' fitted locos lurch forward until the capacitors are discharged. This can happen when the decoder sees the proper DCC signal disappear [because it is turned off] but it still sees a DC voltage from the capacitor. The decoder can then switch into DC Operation mode.

 

The situation above and the other situations presented by others above can all be eliminated if CV29 is amended to disable 'DC Operation'. Some commercially purchased 'Stay Alive' even recommend turning off 'DC Operation' in their instruction leaflets.

[ColinB]

"Stay alive" is a capacitor that gets put across the rectified DCC signal that the decoder receives. Basically the capacitor retains the rectified voltage, if the DCC signal disappears. The size of the capacitor determines how long the voltage will stay, but in reality it is a relatively short time probably less than a second. If you had a carriage full of capacitors then you might be able to retain the power for tens of seconds, but the problem is it would take tens of seconds to charge. As the "Stay Alive" is across the rectified signal it feeds the whole decoder plus the motor, unless the "Stay Alive" has been designed not to feed the motor. If it didn't power the motor as well, then it would last much longer. The reason the motor doesn't run on, is that the capacitor can only power the decoder plus motor for a limited timed, so you never notice it doing anything. That is the reason they cause so many issues when trying to read CV values, the programming voltage is only on for a short period so the capacitor cannot charge up enough, so the decoder doesn't work properly. Sorry RAF96 it does power the motor, as the Chrissaf circuit shows the "stay alive" going directly across the output from the bridge rectifier, this powers the motor as well as the module. If it was across the voltage regulator (very small 3 legged chip, that I assumes provides 3.5 v to power the micro) then it would not.

[Chrissaf]

Sorry RAF96 it does power the motor,

 

Colin,

Rob never said it didn't power the motor. He said and I quote:

 

"The stay alive keeps the decoder alive not the motor directly. Motor control is still through the decoder."

 

He just said that the 'Stay Alive' did not DIRECTLY power the motor. Which is totally true. The capacitor powers the motor via the decoder control circuit with the decoder in essence acting as a speed control switch, which is exactly what Rob's post states.

 

Up until now I have kept my lips zipped with regard commenting on your posts, but I am afraid to say at the risk of upsetting you, that your constant butting in with statements made upon your mis-reading of other's posts is now getting very tiresome.

[ColinB]

Ok then, the short answer is, it does but you just don't notice it as the time scale is too short.

[96RAF]

@colin

Now do you believe me.

The stay alive keeps the decoder MCU and thus the motor and function switching control alive. Neither the motor nor the lights will work direct from the stored potential directly.

 

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[ColinB]

Yes, my apologies, I misread your post first time round..

[Chrissaf]

And just to break the myth that 'Stay Alives' are only good for less than a second and a few inches of travel.

 

The You-Tube video below shows a 'Stay Alive' running a loco for over 2 metres. Granted your average 'Stay Alive' will only do so many milliseconds if based on standard Electrolytic capacitors, but replace those with 'Super Capacitors' which are finding their way into many commercial 'Stay Alive' products [like the one in the video], then the kind of 'Stay Alive' that the video below shows is easily achievable.

 

 

One of my earlier posts gave constructional details on how to build a 'Super Capacitor' stay alive. See 7th & 8th posts down on the page:

 

https://www.hornby.com/uk-en/forum/post/view/topic_id/13404/?p=2

 

My constructional post was published more than four years ago, so 'Super Capacitor' based 'Stay Alive' technology has been around for some considerable time.

[walkingthedog]

Does that mean on turning off the power the loco will continue for two metres, rather negating the reason  for turning the power off?

[Chrissaf]

If you disable 'DC Operation' in CV29, then that doesn't happen.

 

The CV29 amendment is usually recommended in commercially purchased 'Super Capacitor' based 'Stay Alive' instruction leaflets.

[walkingthedog]

I see, makes sense. Thank you.

[96RAF]

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[Fishmanoz]

Might I just clarify whether a Stay Alive fitted loco will run on when you turn your DCC controller to stop. No it will not!  

It will stop according to the instruction given to the decoder whether Stay Alive fitted or not.  If it ran on when fitted it would run on while on the track without, the Stay Alive doing nothing more than maintaining the DC volts at the output of the bridge rectifier when there is an interruption to track DCC input to the rectifier.

 

But note my answer ignores the phenomenon known as DC Runaway whereby a decoder interprets a DCC signal as DC and takes off flat out as a result. That possibility is what responders above are referring to that "just might" allow the loco to run on until the Stay Alive capacitors are discharged.  And this can be avoided by turning off DC Running in CV29 as they say. 

[ColinB]

Yes Fishmanoz I agree with you entirely. I sat and thought about just after I posted the post and came to same conclusion. To me the only time you might get an issue is switching the dcc on/off or a really bad piece of track.