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Old 02-24-2019, 06:40 PM   #1
JoeG
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DIY Keep Alive

Im not sure if this has been asked before. I did some research, but haven't found anything. Is there a way to make a current keeper/keep alive yourself? Or is there too much involved with the electronics to make it worth your while?
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Old 02-24-2019, 06:47 PM   #2
MichaelE
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It's just a capacitor. Many brands of decoders have solder pads to install a capacitor as a keep-alive device.

Check your decoder operating instructions. There may be useful information for you there
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Old 02-24-2019, 09:20 PM   #3
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sometimes it's just one, sometimes several, usually they are super caps ...
some decoders have both pads, even labeled, most you have to find the negative pad connections, depends on the decoder ..
it's not hard, just putsy ..
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Old 02-24-2019, 09:36 PM   #4
D&J Railroad
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Sure there's not a resistor in the circuit so the cap doesn't unload its entire 500volts all at once?
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Old 02-25-2019, 04:09 AM   #5
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Sure there's not a resistor in the circuit so the cap doesn't unload its entire 500volts all at once?
don't know where 500V comes from. The resistor (and diode) limits the inrush current to prevent the booster/command station from thinking there is a short when power is first applied if there are a large number of keep alives on the layout.

there's a circuit diagram on another site that won't appear here (.gif) showing how a keep alive is connected to the diode bridge of a decoder

super caps have large capacitance values but low voltage ratings. They need to be stacked in series to handle the voltage, but when in series, their capacitance is reduced by 1/N where N is the number of capacitors.
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Old 02-25-2019, 05:01 AM   #6
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Sure there's not a resistor in the circuit so the cap doesn't unload its entire 500volts all at once?

well, the cap -may- be rated up to 35 volt .. usually no more than that, and a diode takes care of unload duties ...
a resistor [ try 100 ohm for a starter] takes care of loading the cap up when the system powers up, etcetra ..
it's pretty easy to do, especially if the bridge negative is marked on the decoder
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Old 02-25-2019, 05:32 AM   #7
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Where is 500V coming from?
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Old 02-25-2019, 09:36 AM   #8
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Where is 500V coming from?
Lightning!
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Old 02-25-2019, 12:12 PM   #9
D&J Railroad
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Where is 500V coming from?
I noted 500 volts as a point. Some capacitors can store up a heafty jolt.
The real point was, just a cap isn't all there is to it. Some electronic wizzes presume everybody knows electronics well enough to include the additional components in a circuit.
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Old 02-25-2019, 01:39 PM   #10
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I noted 500 volts as a point. Some capacitors can store up a heafty jolt.
just so that nobody walks away thinking they're going to get jolted by a keep alive ...

no matter what the voltage rating of the cap, it can only charge to the voltages in the system, which in this case is ~14V. A couple 1000 uF caps aren't going to give you much a of jolt.

in college i worked on a pump laser using a couple 1F caps charged to ~600V. we discharged it with a battery cable and there was a very audible crack when we did.
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