Model Railroading > Making your own Keep Alive Circuit

I've come to the conclusion that all sound equipped locomotives *MUST* have a "Keep Alive" type of circuit.  So I've spent several hours this morning studying how to make your own "Keep Alive" circuit for decoders. The reason is not only cost, but the ability to mold the package of capacitors to fit the available space. There are a couple videos out there showing how to do it, but they tend to be lacking in some key details. Anyways I've learned that most commercial and home made KA type circuits rely on multiples of 2.7volt, 1farad capacity cylindrical type capacitors. Now the trick is to combine enough to get above the typical 12volt motor usage to avoid the hassle of fancy voltage regulator circuit. So most KA circuits have either 5 (13.5v) or 6 (16.2v) capacitors wired together in series.  The next critical component is a zener diode that acts like a circuit protector. It needs to have a rating below the combined voltage of the capacitors (typically either 13v or 15v) to keep from overcharging the capacitors and causing them to explode. Finally a 100 ohm resistor and a basic diode wired in parallel makes for a simple regulating circuit. The positive end would connect to the positive common on the decoder (blue wire), but the negative end needs to be connected to the negative ground connection on the decoder. Only newer decoders will have this,  you will have to identify the negative connection on the rectifier of older decoders (can be tricky). Now this circuit can cause issues with programming tracks on older DCC systems (especially Digitrax). You will need to add a choke  to the negative side of the circuit (100uh 0.5amp was suggested). Some also suggest a 100k ohm resistor in parallel with the zener diode to discharge the capacitors slowly after the track power has been turned off.
I was disappointed to discover these capacitors are not as cheap as most other electronic components and typically run well over $1 each (hence why the commercial products are expensive as well). I looked at using a single high voltage capacitor, but discovered capacitance quickly falls as voltage goes up. The best I could find in a 16v capacitor was 0.01 farad versus the 0.2 farad of the circuit below. Each farad will typical give you about 2~4 seconds of "life" in a sound equipped locomotive. Anyone have any experience making these kinds of circuits?

Ted Curphey
Ontario, OR



Edited 5 time(s). Last edit at 07/23/19 07:02 by funnelfan.

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- Funnelfan 

Well researched!  Thanks for posting the KA circuit diagram.  ;-)

Though it is not exactly the same, my experience is a bit parallel.  Prior to the release of SoundCar, I was adding sound to a reefer car.  (The circuit board for an iPod knockoff was hot-wired so it was always ON, and the output was fed to a RailMaster speaker.)  It needed a Keep Alive, since the circuit would bring the volume to 0 when it lost power for a fraction of a second.  I found that the smaller the form-factor of the capacitor, the higher the price.  Luckily the interior of a HO reefer is massive, so size wasn't much of a problem.  It also did not require the power reserve to feed a can motor or other hungry loads found in an engine, so it could have less capacity. 

However, to be able to match the size and capacity of the commercially available units, and keep the price point sane, was not possible (5 years ago).  I haven't researched current pricing.  I bet it has decreased, but does a trainload need to be purchased in order to get the price break per unit needed to beat what is commercially available?  Unless the application requires a specific shape, that is generally the Deal Breaker (for me anyway).

I will be following this thread as this topic is of great interest to me.  I agree that a DCC sound unit (in an engine or car) is best when protected by a Keep Alive.  It would be nice to be able to build a cost-effective KA that rivals what is on the market. 

Take care and God bless!
Dave

I've made plenty for a larger club and they've worked great for a couple years now.  I've never used a zener diode, only the resistor/diode combo to slow the uptake charge and keep from tripping the circuit breaker on DCC.
It was definitely worth the extra work in order to fit them into smaller switchers, etc. 

Don't work too hard on this though.  The new smaller TCS KA3 and KA4 get by with only 4 capacitors. This product takes all the worry out. 

99% of everyone on TO should just buy these and keep rollin!

Considering that the commerical KA circuits cost well over $20, and you can buy the componets for this circuit for around $6~8, there is a quite a savings. Manufactuers typically only get about 1/3rd of the retail price, with the rest going to the wholesalers and retailers. So the high cost of componets gets multipled by 3 at the retail level, and the reason why they are expensive.

tmotor Wrote:
-------------------------------------------------------
> - Funnelfan 
>
> Well researched!  Thanks for posting the KA
> circuit diagram.  ;-)
>
> Though it is not exactly the same, my experience
> is a bit parallel.  Prior to the release of
> SoundCar, I was adding sound to a reefer car. 
> (The circuit board for an iPod knockoff was
> hot-wired so it was always ON, and the output was
> fed to a RailMaster speaker.)  It needed a Keep
> Alive, since the circuit would bring the volume to
> 0 when it lost power for a fraction of a second. 
> I found that the smaller the form-factor of the
> capacitor, the higher the price.  Luckily the
> interior of a HO reefer is massive, so size wasn't
> much of a problem.  It also did not require the
> power reserve to feed a can motor or other hungry
> loads found in an engine, so it could have less
> capacity. 
>
> However, to be able to match the size and capacity
> of the commercially available units, and keep the
> price point sane, was not possible (5 years
> ago).  I haven't researched current pricing.  I
> bet it has decreased, but does a trainload need to
> be purchased in order to get the price break per
> unit needed to beat what is commercially
> available?  Unless the application requires a
> specific shape, that is generally the Deal Breaker
> (for me anyway).
>
> I will be following this thread as this topic is
> of great interest to me.  I agree that a DCC
> sound unit (in an engine or car) is best when
> protected by a Keep Alive.  It would be nice to
> be able to build a cost-effective KA that rivals
> what is on the market. 
>
> Take care and God bless!
> Dave

Ted Curphey
Ontario, OR

funnelfan Wrote:
-------------------------------------------------------

> Anyways I've learned that most commercial and home
> made KA type circuits rely on multiples of
> 2.7volt, 1farad capacity cylindrical type
> capacitors.

>So most KA circuits have either 5 (13.5v) or 6
> (16.2v) capacitors wired together in series.  

> I was disappointed to discover these capacitors
> are not as cheap as most other electronic
> components and typically run well over $1 each
> (hence why the commercial products are expensive
> as well). I looked at using a single high voltage
> capacitor, but discovered capacitance quickly
> falls as voltage goes up. The best I could find in
> a 16v capacitor was 0.01 farad versus the 5 farad
> of the circuit below. Each farad will typical give
> you about 2~4 seconds of "life" in a sound
> equipped locomotive. Anyone have any experience
> making these kinds of circuits?

Two technical notes...  
The capacity of series capacitors needs to be adjusted for the number, so 6  series 1 farad capacitors would be 0.167 farad at 16 volts.

You need identical units, and may need to select for capacitor values and leakage current to operate correctly,
(Unequal values in series is "complicated")

Bill

Thanks Ted, not sure I’ll get around to building a KA, but appreciate the info.

Nathan

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