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Choosing the right Booster

Topics in this section"

Typical DC and DCC power ratings
Power Ratings:  DC vs DCC.
Power Scaling: DC vs DCC
Power Levels: DC vs DCC
Recommend Current Limits for a given scale
Booster Current Limit Recommendations
DCC Power Considerations:  What to include and not include.
The simple but potentially very Bad "Bigger is Better" approuch to booster power determination.
The Good "Calculate the Power" approach to booster power determination.
Calculation Procedure


Typical DC and DCC power ratings:

To help understand how to chose the correct DCC booster, it helps to compare it to how one may chose the right DC Power-Pack/Throttle.

Entry level DC PowerPacks/Throttles are sold by the manufacture with the right voltage and current ratings to run a TYPICAL train for the given scale your in.  Entry level DCC Systems boosters are sold the same way.  Stated another way, the manufacture has already figured out the power levels you need for your entry level system assuming it going to be a typical size layout.  

But what if your layout is bigger than a typical layout?  

To address that,  both DC throttles and DCC Boosters are offered with more power in support of the idea of being able to run a bigger train (DC) and/or both bigger and more trains (DCC) within a given scale. 


Power Ratings:  DC vs DCC.

Voltage: The full output voltage levels for DC and DCC are designed to be compatible with each other and fall into 3 ranges per the NMRA standards.  N  (12V), HO (14V) and G (18V).   (O scale track voltage can vary between HO and O scale depending on the type of O scale engine you running but it is technically it is the same as HO.)

Current: DC power-packs or throttles come with current level rating that are optimized to run one train for the given scale.  DCC booster come in different current level ratings depending on how many trains the system is most likely going to run for the given scale.  The higher the current rating, the more longer or more trains you can run.

Power:  Equation=>  Voltage x Current = Power 
Given the voltage for a given scale is going to be the same for DC as it is for DCC then we can be considered this value of the power equation to be a constant value.  That leaves current as the only variable left in the power equation that can change.  So when one says more power for a given scale, this means more current and visa versa.


Power Scaling: DC vs DCC

On a DC layout that can run multiple trains, you need to have a DC Power-Pack or Throttle for each train your going to run.  Each power-pack provides just enough power (current) to run just its own train.  It does not need to provide power to all the trains on the layout.  Stated another way, collectively all the DC power-packs work together and provide all the power to run all the trains.  So DC power scales with the number of throttles which corresponds with the number of running trains.

With DCC, there is no scaling of power on a per throttle bases.  Track power is global.   All the power to run all the trains on a given layout must come from one booster.  If that is not enough power, then you add more DCC boosters to add more power.  Assuming your DCC system can support more throttles than trains you can run on the layout, then the DCC power needs scales with the maximum number of trains you can run on the layout.


Power Levels: DC vs DCC

It is important to understand the level of power levels involved here for there is a big difference between DC and DCC.

A typical commercial DC power-pack or a entry level DCC system is offered between 1 and 3 Amps of track power.  For HO (14V), that means the amount of power available becomes.  These system are not targeted for G scale.

Entry Power Level DC:

P(1Amp) = 14V x 1 A = 15Watts

Advanced DC and Entry Power Level DCC Boosters:

P(2Amp) = 14V x 2 A = 28Watts
P(2.5A) = 14V x 2.5A = 35Watts
P(3.0A) = 14V x 3.0A = 42Watts

A typical soldering pencil is 25Watts.  A large/professional solder Iron is about 45 Watts.  This is also why the protection systems on a DCC boosters are so much more aggressive than the very forgiving DC throttles have.

Medium to High Power Level DCC Boosters:

P(4.5A)= 14V x 4.5A =  63Watts
P(5Amp) = 14V x 5A =  70Watts

High Power DCC Boosters:

P(8Amp) = 14V x 8A = 112Watts
P(10A)  = 14V x 10A = 140Watts

With these medium and high power level boosters, we now have some serious power levels such that they can potentially start a fire.  We are talking soldering gun levels (70W or more) of heat.  The point is that you much make sure your layout wiring is up to the job and that your using the right booster for the right scale.

For more information on good layout wiring, go here:Wiring Planing


Recommend Current Ratings Limits for a given scale:

Most people have small layouts in both size and scale and will follow the DCC system manual and connect the booster directly to the track.  There is nothing wrong with that IF the DCC system booster current ratings are appropriate for the the scale that you working in.  

What are the maximum current values for a given scale?

1) 5 Amps or less: O-Scale on down (N).
2) >5 Amps: Large O-Scale on up (G).

Why?  Locomotive electrical systems have been and still are designed ONLY to handle the current needs of the locomotive itself.  Lets look at the locomotive current needs:

Large Scale (O-Scale on up): These Locomotive have one or two very large and powerful motors that can draw lots of current.  The DC throttles where much larger to supply that power and as such both the layout and locomotive wiring is much larger.

Below O-Scale: There is less space as the scale of the engines get small.  This forces the motors to get smaller.  In the DC days where a typical HO DC throttle only put out enough current (1 to 2 Amps) to handle one train.  Hence the current is a lot less and the locomotive wiring is designed accordingly using 24/26 AWG or so wiring.  (Many people used the same small gauge wiring (telephone cable wiring very common) to wire up the layouts.) 

So where does the 5Amp limit come from?

Years of real world experience has established that 5Amps is the highest DCC booster current small scale locomotive wiring can take without burning up the wiring or decoder PCB traces.   How is that possible?  Remember that the DCC booster only puts out the full 5Amps when there is a short circuit and will do so for only a fraction of a second before it shutsdown.  Hence the overloaded wiring event only happens for a very short time.  It is NOT long enough to do any damage to the locomotive wiring eve though it starts to heat up.  If you look at many DCC booster ratings, you will see that the current ratings of the medium current boosters are some place between 4 and 5 Amps.

If you read on, there is a section below that will visually show you what can go wrong when you do not stick to these guidelines.


Booster Current Limit Recommendations

Booster Recommendations:

1) Below O Scale, only use boosters rated at 5Amps or less.

2) O Scale and above, Consider using a high current booster that is 5 Amps or Greater.

Why?  If you read on, there is a section below that will visually show you what can go wrong when you do not stick to these guidelines.

Cost Saving Option.

If you have a large layout, there is a cost saving option to reduce the number of boosters required to power the layout and get a bit more performance improvement.  Use the high current booster (> 5 Amps) with DCC circuit breaker that are set to 5A or less trip points.  At no time do you directly connect the high current booster to the track.  To learn more, go here: Boosters & DCC Circuit Breakers


DCC Power Considerations:  What to include and not include.

Let assume you have a large layout and/or a layout that can run lots of trains all at the same time.  One of the questions layout owners will then ask is: "How much booster power do it need?"   Stated another way, "How much booster current do it need need to run my layout?"  To figure that out one needs to know more about how the power will be consumed.

The goal here is it to come up with a budgetary current number. The current value will tell you how many boosters you will need to buy.   The final current value does not have to be a precise answer for in practice this is a worse case scenario which I have yet ever see happen in the real world.  Stated another way, the current number will be very conservative.  To get current number, we need to look at all of the 
various sources of Booster Power Consumption.  They are:

1) Running Locomotives.
2) Lighted Passenger Cars.
3) Accessory Power.
4) DCC Accessory Devices

RUNING LOCOMOTIVES:  The reason why you only count running locomotives is that locomotives do not consume enough current/power to worry about when calculating your booster current requirements.  This includes sound equipped engines assuming the volume is not set to full volume all the time.  

LIGHTED PASSENGER CARS:  If you have some incandescent (bulb) based lighted passenger cars, collectively they can consume as much as current as a locomotive.  You may or may not have notices that running these cars are DCC power track leads to a overly bright car and lots of heat generated by the bulbs.  I do know of plastic passenger cars who's roof as melted.  Today their are LED car lighting systems offered to replace the bulb based solutions.  They eliminate the heat problem and at the same time the power consumption.  The current is so low that again we no longer need to be concerned about the load current anymore in a budget number.  Anyway, if you have not converted the car lighting system to LED, then you need to consider a lighted passenger train the same as a single running locomotive.

ACCESSORY POWER:  Tempting as it may be, one should avoid place any non DCC related loads such as static scenic lighting (Bulbs) on the DCC track bus.  One of the best uses of your old DC Throttle or PowerPack will be to power such loads.  If your DC throttle is doing that now, keep it that way.  To keep booster power requirement and hence cost to a minimum, you want to eliminate any unnecessary loads that you can from the DCC booster.

DCC ACCESSORY DEVICES:  If you are going to use DCC accessory decoders to control devices such as track switches/turnouts, you will need to account for these load.   Normally these devices connect to and draw current  from the track and hence the DCC booster.  The load current can vary but the better DCC accessory decoder reduce the load current to a low constant value or some limited current value.   See the manual of your Accessory decoder for current draw information.  (That said, it is strongly recommended on LARGE layouts that one have a dedicated and isolated "DCC ACCESSORY BUS" run to power all the DCC ACCESSORIES.  Going one step further, using a dedicated medium power DCC booster to power this bus will eliminate the power consumption of these devices from the booster(s) that will be used to power the track.)


The simple but potentially very Bad "Bigger is Better" approuch to booster power determination.

















THE IDEA: 

Some people rationalize themselves into a bigger is better approach to booster selection.  They buy the biggest highest power booster they can get with the idea that that in doing so they will NEVER run out of power. 

THE PROBLEM:

The problem appears when you connect this big booster directly to the track and your working with scale below "O-scale".  Why?  This solution DOES NOT TAKE INTO ACCOUNT how much power/current the engines themselves can support with their internal electrical paths/wiring when there is a short circuit involved such as found in a derailment.  How?.  There are types of derailments that will cause a weak short that may NOT cause the booster to shutdown.   When that happens, the full current rating of the booster may flow through the locomotive's electrical system.  The heat buildup can cause wires to burn up and/or trucks to overheat and melt.  See the pictures above.   This is what happened on a layout that decided to use Big Booster on a relatively small scale layout.


The Good "Calculate the Power" approach to booster power determination.

To start with, we know that the engines consume the most power.   But how do you figure out the amount of power/current each engine draws especially if they are all different?   Most people do not know the answers up front but there are to ways to approach this problem.  
There are several way to figure engine current requirement using the list below in any combination as required. 

1) MEASURE: Test/measure for motor current draw.  Focus on the most common locomotive types to get a BALLPARK figure.  One locomotive that draws more current will often be offset by one that will draw far less current.   It is the AVERAGE loco current we are after.  To figure out how to measure the actual motor current, go here: DCC Decoder Ratings
Notes: This MEASUREMENT method also works with bulb lighted passenger cars.

2) ESTIMATE: Look at the decoder ratings used for the class of locomotive you got.  Many Decoder manufactures have a cross references that tell you the decoder to use for a given engine.  By working in reverse, you can estimate the average motor current rating from the recommended decoder's current rating.  Figure 50% of the decoder's current rating will be the average current draw of the motor.

3) RESEARCH: There are books and websites with information on measured "Stall Motor" current ratings.  We only need the average so again use 50% of the stall current rating as the average current.   I have some stall motor current numbers for you here:


Calculation Procedure

1) Figure out the maximum number of running engine you can have on the layout.  Think about the type of jobs being done, and how many engines will be involved.  It is not important that you now

2) Figure out the Passenger Car light situation and determine the equivalent load using 1 average locomotive load per string of cars.

3) Figure out the DCC Accessory decoder situation and determine the load per decoder.

4) Multiply the average motor current by the number of running locomotives.  This gives you the running "total" current value.

5) Add any passenger car loads to the total.

6) Multiple the DCC accessory decoder draw by the number of accessory decoder you will have.  Add the result to the total.  If you have different accessory decoder, repeat the same proceedure for each type of accessory decoder you have and add to the total.  (Skip this step if your going to have a dedicated DCC accessory bus.  Simply buy a booster for this service.)

7) Using the total, divide this current by the booster current rating.  Round up or down to the nearest whole number.  This will be the number of boosters you will need to run the track on the layout.