# Can some 'splain to me how "stay alive" type capacitors work in a DCC environment?



## JeffHurl (Apr 22, 2021)

Layman language, please. I know what capacitors and diodes are and what they do, so I'm not completely illiterate when it comes to electrical components, but I'm certainly NOT an electrical engineer, LOL . 

Anyway, I've been intrigued by the concept of adding a capacitor to store up enough power to negate the effect of, say, a dirty spot in the tracks, or a non-powered frog.

In DCC, where does a capacitor go? Seems like if you put it somewhere between the track and the decoder, the digital signals would get "muffled" by the capacitor and wouldn't even get to the decoder. And if you put it between the decoder and the motor, how would you control the speed? 

Seems like the capacitor would need to be inserted somewhere downstream of where the digital signal is read, but upstream of where the current to the motor is controlled.

So if someone wanted to add a stay-alive-like capacitor to a DCC locomotive, how would you go about doing it?


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## mesenteria (Oct 29, 2015)

The command station sends 'packets' of information to each decoder periodically.....literally 'periodically'. In the absence of new packets/information, the decoder retains its last instruction and will continue to do what it was compelled to do based on the last instruction. When power is cut, the cap will continue to supply power to the decoder, but no more packets will present themselves since there's none of them coming up from the rails. Since a decoder meters rectified square wave AC, the cap provides the power reserve to the bridge rectifier. That's my understanding. See:


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## wvgca (Jan 21, 2013)

pretty much what he said ..


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## Roundhouse Foreman (Jan 6, 2015)

There should be a resistor or two to help delay and extend the discharge of the capacitor as they usually snap when discharged. The resistors slow the capacitive discharge and extend the time viable voltage is available to the circuit.
A pass time at sea was to toss a "loaded" capacitor across the room and see what happened when it was caught by an unsuspecting radioman.

RHF


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## JeffHurl (Apr 22, 2021)

I see... so temporary packet loss isn't really an issue since the whole purpose of these things is to keep a reserve power that may only last a very short time anyway.


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## Roundhouse Foreman (Jan 6, 2015)

BINGO!


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## wvgca (Jan 21, 2013)

Roundhouse Foreman said:


> There should be a resistor or two to help delay and extend the discharge of the capacitor as they usually snap when discharged. The resistors slow the capacitive discharge and extend the time viable voltage is available to the circuit.
> RHF


actually the resistor goes on the charging side to limit inrush current ..no other purpose at all ..


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## gregc (Apr 25, 2015)

JeffHurl said:


> In DCC, where does a capacitor go? Seems like if you put it somewhere between the track and the decoder, the digital signals would get "muffled" by the capacitor and wouldn't even get to the decoder. And if you put it between the decoder and the motor, how would you control the speed?
> 
> Seems like the capacitor would need to be inserted somewhere downstream of where the digital signal is read, but upstream of where the current to the motor is controlled.


between the bridge rectifier and the power input to the regulator and h-bridge providing power to the motors. not shown is a where the processor connects to one of the rails to see the command packet


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