# DIY Keep Alive



## JoeG

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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## MichaelE

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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## wvgca

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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## D&J Railroad

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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## gregc

D&J Railroad said:


> 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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## wvgca

D&J Railroad said:


> 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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## MichaelE

Where is 500V coming from?


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## gunrunnerjohn

MichaelE said:


> Where is 500V coming from?


Lightning!


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## D&J Railroad

​


MichaelE said:


> 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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## gregc

D&J Railroad said:


> 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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## gunrunnerjohn

*Good Grief!* I would think a 2F cap at 600V would melt the battery cable, that's *360,000 Joules*!!! When I was a young lad I had an array of one hundred 200uf 1,000V capacitors that I charged up to around 800 volts, that's only *6,400 Joules*. I had a very large knife switch to dump the charge into a custom cone with a fine lead wire to start the arc, it would break coke bottles eight to ten feet! The loud crack of the arc was probably why I have poor hearing to this day, it put any gun I've ever fired to shame! Your rig would kill a herd of elephants, and I think audible crack would be a massive understatement of the sound! The shock wave coming from that would have to be been immense, hard to believe that there wasn't damage around the lab from that little experiment.

I won't even get into the wisdom of actually doing what you describe. I was a stupid high school student with no supervision when I was doing my little rig, how a college lab allowed you to do that defies any logic.


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## Shdwdrgn

gunrunnerjohn said:


> Lightning!


No no, that's not 500 volts. Lightning gives you 1.21 gigawatts!




gregc said:


> ...it can only charge to the voltages in the system...


Voltage doesn't kill you, amps kill you. Tesla coils can pump millions of volts through your body without harm, but you can stop a heart with 1/10th of an amp. If you had to use battery cables to discharge those caps, there's no way I'd risk letting that charge go through my body.


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## gregc

Shdwdrgn said:


> Voltage doesn't kill you, amps kill you. Tesla coils can pump millions of volts through your body without harm, but you can stop a heart with 1/10th of an amp.


if skin resistance is 2k Ohms, what voltage is required to drive 1/10 A through your body?


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## gunrunnerjohn

gregc said:


> if skin resistance is 2k Ohms, what voltage is required to drive 1/10 A through your body?


According to ohms law, that would be 200 volts. However, if your skin is slightly damp, you'll have a skin resistance less than 2K!

You may be asking how I know that. That's simple, tons of people have been killed by 110VAC, so they obviously had considerably less skin resistance, or it took less current to kill them. Either way, they're dead!


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## gregc

so are DCC voltages likely to give you a jolt even if the equipment is capable of delivering more than 1/10 A? (rhetorical)


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## gunrunnerjohn

You can get shocked from 12VDC in your car. If you manage to get it in open wounds, it can actually give you a decent jolt.


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## gregc

so if its an open wound, it's not going thru the skin. I don't know what the resistance is of living tissue.

or do you mean the coil, 14 kV? someone told me they got knocked across the room getting shocked by the coil.


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## JoeG

So what im gathering is that there really isn't a way to make a current keeper from scratch. ie schematic, video, forum. I don't know electronics to well so I was looking for a detailed description on how to build one.


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## Lemonhawk

Stick your tongue on a 9 volt battery, you'll git a little nip! I recall determining if 1.5v batteries were good by licking a finger and sticking it on one end and then touching my tongue to the other end, good if tasted a little sour. Unlike you two, I didn't mess with high voltage! Maybe because I spent my summers working as an electrician.


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## gregc

JoeG said:


> So what im gathering is that there really isn't a way to make a current keeper from scratch!


you certainly can build your own.


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## Lemonhawk

I think there have been several schematics posted for keep alives. Its a bigger problem if the decoder is not set up for a keep alive because the connection needs to go after the decoder signal pickoff and bridge. Using super caps may sound great but they do take some protection circuitry also. I think GRJ had something on this. Something about super caps having internal resistance.


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## JoeG

Lemonhawk said:


> I think there have been several schematics posted for keep alives. Its a bigger problem if the decoder is not set up for a keep alive because the connection needs to go after the decoder signal pickoff and bridge. Using super caps may sound great but they do take some protection circuitry also. I think GRJ had something on this. Something about super caps having internal resistance.


It may be worth it to pay the 20-some dollars to just buy it than try to save a few bucks?


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## ncrc5315

I routinely work with 24VAC at work, I can grab the two wires, one in each hand, and never give it a second thought. Last fourth of July, hospital took a lighting hit, damage several controllers on their cooling system, I'm working in the boiler room, getting hot and sweaty, grabbed a 24VAC wire to twist the wires together to land it, and got zapped, not bad, but enough to get your attention. So, yeah under the right conditions, even low voltage can shock you.
For a little capacitor fun, check out this video.


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## gregc

JoeG said:


> It may be worth it to pay the 20-some dollars to just buy it than try to save a few bucks?


too bad. were you just interested in saving money or learning something?


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## wvgca

the advantage to 'rolling your own' is that you can make it to fit some pretty 

odd shapes and sizes , not available with commercial models ..
but to each thier own , after all it's gotta make YOU happy ..


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## JoeG

gregc said:


> too bad. were you just interested in saving money or learning something?


Both actually, i want to learn more about electronics, and save money.


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## JoeG

wvgca said:


> the advantage to 'rolling your own' is that you can make it to fit some pretty
> 
> odd shapes and sizes , not available with commercial models ..
> but to each thier own , after all it's gotta make YOU happy ..


This is another advantage that I didn't think about!


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## Overkast

I have read there are some major risks with DIY keep-alives contingent on which capacitors you choose to work with. Some capacitors tout being able to store more energy but are more unreliable and thus prone to failures (literally in firey melt-down), destroying both themselves and your loco in the process. And as such, the best advice is to stick to reputable, commercially-sold products.

But I'm no expert on this topic, so just food for thought...


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## gregc

i bought some supercaps but was disappointed when I experimented with them on the bench. They initially lost voltage very quickly before it stabilized. I assumed i just had a bad batch.

i also tried build a keep alive using 16V tantalum capacitors which are smaller than equivalent electrolytics. my engine got about 2 feet before 1 of 4 cap blew and there was a pyrotechnic display. again, i assume the caps from china came from a batch that that didn't meet spec.

just need to test well before using and verify capacitance

i have a couple electrolytic keep alives that have worked fine for years.


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## Overkast

gregc said:


> i also tried build a keep alive using 16V tantalum capacitors which are smaller than equivalent electrolytics. my engine got about 2 feet before 1 of 4 cap blew and there was a pyrotechnic display.


Tantalum! Yes, those are the ones I read about that people said to use caution with. It sounded like they are desirable due to the small form factor, but extremely risky to bet on.


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## gregc

Overkast said:


> ... but extremely risky to bet on.


you need to understand what the tolerances are on the specs (10% ?)

my experience with electrolytic caps is that the simply pop. I don't know if the generate any heat that might damage surrounding components.

the tantalum created a bright flash and burned for several seconds which would definitely damages nearby components. Fortunately, it was an unpainted brass loco which wasn't damaged.

tantalum caps probably should not be used if a failure can cause damage


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## gunrunnerjohn

gregc said:


> you need to understand what the tolerances are on the specs (10% ?)
> 
> my experience with electrolytic caps is that the simply pop. I don't know if the generate any heat that might damage surrounding components.
> 
> the tantalum created a bright flash and burned for several seconds which would definitely damages nearby components. Fortunately, it was an unpainted brass loco which wasn't damaged.
> 
> tantalum caps probably should not be used if a failure can cause damage


The usual reason for failure of tantalum caps in this fashion is they were installed with reverse polarity. I've seen a number of instances where equipment was popping like firecrackers on the 4th of July, they were all reverse polarity situations. Without abuse, tantalum caps have a much better MTBF than electrolytic caps.


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## wvgca

gunrunnerjohn said:


> Without abuse,



ahhh, magic words ...
you forgot to tell those _other_ people, lol


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## gregc

gunrunnerjohn said:


> ... they were all reverse polarity situations..


i did have a hard time determining the polarity and paid particular attention to it. But that sounds like a better explanation.

I'll have to try building one again using the tantalums i have.


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## gunrunnerjohn

The little dipped ones sometimes are pretty hard to see what the polarity is. That's the type I had exploding all over in my video generators, the assembly house had assembled several hundred boards with these reversed in polarity! Some lasted for over a year before they popped. Since they were just bulk bypass caps, it didn't affect the operation of the video boards as a rule. However, we found it when several customers told us our video boxes were popping like firecrackers.  I had a couple of techs running around Wall St. after hours swapping out all the caps for several months, what a giant PITA!


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## gregc

the ones i have a surface mount and boxes. I guess I'm not sure whether the bar marks the positive or negative side


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## gunrunnerjohn

The bar on these are the positive end. However, larger round electrolytic caps have the black band as negative, go figure.


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## gregc

how do you know the mark marks the positive end? spec sheet? The positive is marked on the dipped tantalums, inconsistent with electrolytics


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## wvgca

SMD caps, the polarity indicator will be a bar at the end of the device that is the positive end for capacitors . Electolytics are usually marked on the negative end ...


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## gunrunnerjohn

gregc said:


> how do you know the mark marks the positive end? spec sheet? The positive is marked on the dipped tantalums, inconsistent with electrolytics


Because tantalum caps are marked that way.  Yes, look at the spec sheet for your caps.


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