Keep alive device, worth it?

[7j43k]

snjroy

A four-wheel loco is prone to power hiccups in long switches. All you need is a bit of dirt on the track and/or wheels and you will get stalls. 

 

 

I think 2-3 seconds is plenty of time to get over "a bit of dirt", and resume contact.  I suppose if there's a lot of those bits, it could be a problem, because the capacitor may not have fully recharged from the first bit or two.

 

 

Ed

 

[snjroy]

7j43k

I wonder how you have such crappy track that your locos lose contact for "2-3 sec".

Ed

 

A four-wheel loco is prone to power hiccups in long switches. All you need is a bit of dirt on the track and/or wheels and you will get stalls. 

Simon

[7j43k]

I wonder how you have such crappy track that your locos lose contact for "2-3 sec".

Ed

[know2go]

Keep alive are a good thing. Only the standard Soundtraxx one only gives about 2-3 sec on 13.5 V track voltage which at realistic speeds may not be enough. I had to use my own design and I jacked up the track voltage, too. Gives me 7 sec, which is good enough even for slow speeds. I also use keep alive for lighted cars, just a bit smaller capacitors. This prevents from flickering and provides a realistic feel.

[ndbprr]

There is another option and that is to attach a car to the locomotive ,say a caboose, and stuff all the stuff in there and use a set of plugs to the engine

[richhotrain]

Thanks, guys, for the info on using Keep Alive for lighting.

Rich

[gregc]

richhotrain

Does a Keep Alive maintain the lights on a loco or just the sound?

as much as is keeps the motor running

the circuit below shows that the keep alive is typically wired between the bridge and the decoder.   the decoder typically doesn't know where power is coming from.   (it could detect the loss of DCC).

a regulator on the decoder maintains proper voltage (5V, 3.3V) to the processor as long as the capacitor voltage is above the regulator input voltage (~6V for low-dropout 5V regulator).   but as the capacitor discharges, voltage will drop to the lights (LEDs) and motor.   current flows into the motor as long as the capacitor voltage > BEMF

[7j43k]

Contrary to Mike's assertion, "keep alive" is designed to power a locomotive through an interuption of power from the track.

Some locomotives come with it.  ScaleTrains Rivet Counter series, for example.  Of particular note is the little Walthers Plymouth ML-8, which has a "keep alive" option, but no sound.

It also can work with lights, without sound.  I have "keep alive" on two passenger cars, just to supply the "tail lights".  No sound involved.

However.  Some DCC boards have a small capacitor mounted on them.  That capacitor is there either to keep the decoder from going through start-up mode again, and/or to keep the sound from crackling.  It does not supply power for the motor.  Or the lights.

 

I place "keep alive" in quotes, as that is a brand name, and I am referring to the concept.

 

 

Ed

[Overmod]

richhotrain

Does a Keep Alive maintain the lights on a loco or just the sound?

Not dumb at all.

The correct answer is "it depends".

The general design of keepalive as I currently think best practice calls for is to maintain decoder power 'as if from the rails' -- in that setup, the motor and its decoder 'command'; the sound chip and its control; and the lighting through its control are all maintained.  (This for DCC of course.)

Naturally, a smaller capacitor arrangement could be used only on the 'sound' (for example by connecting to a Sound Value board) or to the decoder to keep the lighting from losing programming as well as power.  On a DC engine you might have separate caps for lighting and some sort of sound, or combine feeds.

But note how much simpler it is just to 'keep' the decoder 'alive' with its own little "emergency generator".  And to me that is also true for 'momentary interruption' or 'undesired reset' applications with near-momentary power reserve, too.

I confess that I've always been partial to the 'uninterruptible power supply' idea, now that supercaps with large reliable number of charge/discharge cycles are being produced.  This powers the locomotive off the supercap, the way a computer runs off the isolated battery power in a true UPS, with the DCC power serving as glorified float charging (and of course a source of command data).

That is 'by default' a starting point for dead rail, too, should your interests go that way.

[richhotrain]

Dumb question, but here goes.

Does a Keep Alive maintain the lights on a loco or just the sound?

Rich

[gregc]

Lastspikemike

Keepalive only needs to supply power for long enough to deal with dirty track sections and frog issues so less than half a second between discharges. The main purpose is to prevent sound drop out.

is that so hard to design?   don't they already exist?

[hon30critter]

Hi everyone,

There is one detail that nobody has addressed that will cause problems if it is not taken into consideration. If you hardwire a capacitor to your locomotive you will encounter programming problems if the capacitor can't be isolated while the programming is taking place. The simple solution is to include a shut off switch in line with the capacitor.

Loksound power packs (keep alives) include an automatic isolation function so there are no complications when reprogramming a decoder. I don't know if a Loksound power pack can be used with other brands of decoders.

I will also suggest that keep alives are unnecessary with anything with more than two axles. If your larger locomotives are stalling then I would suggest that you spend your time improving power pickup rather than covering up the problem with keep alives. That may require just a simple wheel and/or track cleaning, or it may involve adding more track feeders to the area where the locomotives are stalling.

On the other hand, if you are talking about two axle critters, then keep alives are well worth installing. Here is an example of a box cab negotiating a turnout. The boxcab has Code 88 wheels (since replaced with Code 110 wheels). The Code 88 wheels are clearly not capable of navigating a standard Peco Code 100 turnout but the little critter pulls through none the less:

Dave

[gregc]

Lastspikemike

Maybe the tiny keepalive we actually need is hard to design and build.

not sure what you think we "actually need".

a keep alive circuit is simple: a diode and a resistor.   no need for a charge pump to maintain a higher voltage.

the issue is size.   super-caps are smaller but space is limited in models.

[Overmod]

I thought any of these keepalives used voltage-to-voltage conversion (at some very high frequency) to manage voltage drop, same as phones do to get long life out of little batteries.

[gregc]

it's not for everybody (but presumably better than guessing), but working with capacitor value in Farads (1,000,000 uF) makes a big difference

i know any comparison of a capacitor to a battery is awkward because  battery voltage is relatively constant, slightly higher when fully charged and slighly lower when depleted.

but capacitor voltage depends on the charge and visa versa.   so as current drawn by the decoder and passed to the motor drains the charge on the capacitor, the voltage drops.   at some point, the voltage (after a regulator) is insufficient for the decoder.

the questions is how long does this take and the diminishing motor voltage is a secondary effect.    while the decoder may be fully functional as long as the regulator input voltage is exceeded, the motor voltage will drop.    as pointed out, the motor will see approximately 1/2 the voltage for the time

 

[JDawg]

Not gonna lie Greg, my eyes gazed over once I got to the second line!

[gregc]

Overmod

But tell them how far a typical HO  engine will go with sound and lights on the result...

here's an attempt

assuming 4 1F 5.5V caps in series. resulting capacitance in 1/4 F (250,000 uF) and 22V series voltage. 

Farad = coulomb / Volt.  track voltage of 14.5 results is < full charge: 14.5 * Capacitance (F) = 3.625 C

assuming current drain of 100 ma.   charge depleted by 0.01 coulombs per 0.1 sec iteration.  resulting operating voltage is the current charge (coulomb / Farad).

takes ~23 sec for capacitor voltage to drop from 14.5 to 5V, below which the decoder stops operating.   loco speed will coorespongingly drop, averaging half speed during that time.   a loco traveling at 25 scale mph (5 in/sec) would cover ~57 in

time would be half for 200 ma or half the capacitance.   time would be ~30 sec using just 3 super caps: 1/3 F and 16.5V

a simpler approach is sec = capacitance * (Vtrack - Vmin) / i (e.g. 23.8 = 0.25F * (14.5 - 5V) / 0.1)

        sec    col    cur   volt    spd
        0.0  3.625  0.100 14.500  100.0 %
        0.1  3.615  0.100 14.460   99.7 %
        0.2  3.605  0.100 14.420   99.4 %
        0.3  3.595  0.100 14.380   99.2 %
        0.4  3.585  0.100 14.340   98.9 %
        0.5  3.575  0.100 14.300   98.6 %
        0.6  3.565  0.100 14.260   98.3 %
        0.7  3.555  0.100 14.220   98.1 %
        0.8  3.545  0.100 14.180   97.8 %
        0.9  3.535  0.100 14.140   97.5 %
        1.0  3.525  0.100 14.100   97.2 %
        1.1  3.515  0.100 14.060   97.0 %
        1.2  3.505  0.100 14.020   96.7 %
        1.3  3.495  0.100 13.980   96.4 %
...
       22.4  1.385  0.100  5.540   38.2 %
       22.5  1.375  0.100  5.500   37.9 %
       22.6  1.365  0.100  5.460   37.7 %
       22.7  1.355  0.100  5.420   37.4 %
       22.8  1.345  0.100  5.380   37.1 %
       22.9  1.335  0.100  5.340   36.8 %
       23.0  1.325  0.100  5.300   36.6 %
       23.1  1.315  0.100  5.260   36.3 %
       23.2  1.305  0.100  5.220   36.0 %
       23.3  1.295  0.100  5.180   35.7 %
       23.4  1.285  0.100  5.140   35.4 %
       23.5  1.275  0.100  5.100   35.2 %
       23.6  1.265  0.100  5.060   34.9 %
       23.7  1.255  0.100  5.020   34.6 %

 584

[JDawg]

7j43k

 

 

JDawg

To everyone: Does anyone know what the UF value of a soundtraxx keep alive is?

 

 

 

 

 

200,000, according to this article:

 

https://sites.google.com/site/markgurries/home/decoders/keep-alive-compatibility

 

I DEFINITELY am looking forward to seeing your progress on this project.  Very interesting, for sure.

In my opinion, flywheels are unnecessary if you have DCC.  And a "keep alive".

 

Ed

 

 

Like I say, I'll probably be posting a photo diary of the progect. Then you can applaude, boo, criticize or say whatever you want about the progect. It will be interesting to see what happens with it. I fear it may be a painful lesson in paitence, but the result should be worth it!

Thanks everyone for the great replies, I think after re-examining the locomotive, I will have room for a KA2 after all! Yay!

[Overmod]

gregc

you're asking for a quantitative answer ... you?

It was more a rhetorical question with hyperbole overtones -- an answer in feet within a couple of orders of magnitude would do it.

I remember when I first saw these little components with 'farad' on them, and I thought back to the capacitor banks I helped build for exploding-wire experiments in high school.  A farad is a LOT of charge to be released electrostatically!

[gregc]

Overmod

 But tell them how far a typical HO  engine will go with sound and lights on the result... 

you're asking for a quantitative answer ... you?

[7j43k]

JDawg

To everyone: Does anyone know what the UF value of a soundtraxx keep alive is?

 

 

200,000, according to this article:

 

https://sites.google.com/site/markgurries/home/decoders/keep-alive-compatibility

 

I DEFINITELY am looking forward to seeing your progress on this project.  Very interesting, for sure.

In my opinion, flywheels are unnecessary if you have DCC.  And a "keep alive".

 

Ed

[Overmod]

You better go 4 just to be sure.  But tell them how far a typical HO  engine will go with sound and lights on the result... 

[gregc]

i see 1F 5.5V super caps.    3 in series would provide 1/3F at 16.5V

[Overmod]

Modern capacitors work a bit like the amp ratings on power supplies.  Think of them as a tank of electrons under pressure, but electrostatic pressure at charging voltage, just as a battery can be thought of as a tank of electrons under chemical pressure.

The keepalive circuit 'meters' the flow of electrons, as current, so even a ginormous-size capacitor that would scar your screwdriver if shorted just provides 'extended run time' in even a miniature circuit.

The voltage range is for breakdown, not 'output'.  Since DCC is not sinusoidal alternating current, there is no RMS peak correction necessary, and the actual applied voltage never really exceeds the nominal 14-15V, so I suspect a standard 25V rating would be adequate, but nothing troublesome will happen if you use a cap with nominal 100V rating...

As far as I know, modern supercaps are still made with 'nanotechnology' and have comparatively small individual-device voltage ratings, no more than around the 1.5V of a typical AA 'battery' cell, that must never be exceeded.  Commercial devices with higher rating are a kind of 'battery' of individual elements in series so the voltage across any one element stays safely below what we, as well as pilots, might term Vne.

[JDawg]

snjroy

You could also double-up with another DC version. I think the problem will be space  in the engine for anything else.

 

If you search up on YouTube, there is a video that shows someone has made a version with dcc, sound and a keepalive. He had to remove the rear flywhee, but it all fit.

[snjroy]

You could also double-up with another DC version. I think the problem will be space  in the engine for anything else. But to answer the question. Keep alives do work well...

Simon

[JDawg]

gmpullman

 

 

JDawg

Could I just sub out the 220uf for, say, a 10k+ uf? I can't think of a reason that wouldn't work.

 

 

Go for it!

Capacitor by Edmund, on Flickr

A 10K, of course, would fall somewhere in between these two. Closest I could find on short notice 

Cheers, Ed

 

Thanks Ed. Those are some big capacitors! I have a 10k that is significantly smaller. It is designed for 12-24 VDC. 

To everyone: Does anyone know what the UF value of a soundtraxx keep alive is?

[JDawg]

7j43k

 

 

JDawg

Actualy, my progect involves that teeny little Switcher. I know that i will need some sort of power capacitor for the model, it's just that the KA2 takes up valuable space in the unit. Could I just sub out the 220uf for, say, a 10k+ uf? I can't think of a reason that wouldn't work. It would take some time to charge up, but other than that...

 

 

 

 

 

Here's the loco I'm talking about:

 

 

 

Then I guess you got the DC version.  I think you're up a crick.  My best advice is to find a DCC version, and buy it.  It comes with "keep alive" already installed.

 

 

Ed

 

 

I respectfully disagree. I have the locomotive and the decoder, speaker, LEDs, ect and I have layed it out in such a way that everything fits. I will probably be posting a photo-diary/tutorial on the forum when I am finished. 

[gmpullman]

JDawg

Could I just sub out the 220uf for, say, a 10k+ uf? I can't think of a reason that wouldn't work.

Go for it!

Capacitor by Edmund, on Flickr

A 10K, of course, would fall somewhere in between these two. Closest I could find on short notice 

Cheers, Ed