# Question on Stall Amps



## rdamurphy (Jan 3, 2008)

If I test a locomotive to see what the stall amps are, does it matter which power pack I use? If I use a power pack with, say, 2 amps, how can it measure stall amps if they're above 2 amps?

Or does it matter? I used a REA Express pack that says it has 20VA, and the results were 1.41 Amps. Would a larger power supply give a larger result? Or can I trust that number?

Robert


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## chuck n (Jan 2, 2008)

I'm not that electronically knowledgable, but what do you by stall amps? I can understand stall voltage, "what is the voltage when the engine stops moving?" But stall amps means that you are pulling more amps than the power supply can provide. 

A 2 amp power supply will cut out if the current draw exceeds 2amps. Usually, this results in no damage. If a 20 amp supply cuts out, that could cause damage. Depending on what you are running I would recommend a power supply of 5 amps or more.

We need some help, to help you.

20 VA is very minimal. If your train needs 1.5 amps to run a power supply that puts out 1.4 amps won't work. A power supply with a higher amp rating will improve your running.

Chuck

Thinking more about your question, as long as your power supply exceeds the current draw of the engine every thing will be fine. The only problem is when the engine draws more current than the power supply can provide. As a rule of thumb, I estimate the power needed by an engine/train is 1 amp per motor, 0.5 amps for smoke, lights and sound. Any lights on cars will add to the power draw. I have a passenger train with an AB F3s pulling and 6 lighted streamliner cars. That train pulls about 7 amps, 4 mothers at 1 amp each and 6 cars with lights. Each car needs 1/2 an amp for the lights.

You can use these values as an estimate of your power needs. Otherwise get an amp meter and measure your power needs directly.


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## Garratt (Sep 15, 2012)

A power pack rating of 20VA may only be able to provide 1.4 amps for the voltage used.
The VA rating is only correct at a particular voltage which is ambiguous to the user. They often cherry pick the rating from the range of tested operating voltages. The VA rating is for the designed operating range. The amp draw can be higher but things will start to get hot.
Be careful when stalling your electric motors as they will over heat and eventually fail if the power pack does not cut off first. If there is no cut off, either the power pack or the motor will over heat, melt and fail. All that current has to be spent somehow. 

Stall current is the maximum current drawn when the motor is applying its maximum torque, either because it is being prevented from moving entirely or because it can no longer accelerate given the load it is under.

Andrew


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## Treeman (Jan 6, 2008)

Stall current is measured with a locked rotor. A larger power supply would be required to supply sufficient power.


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## Greg Elmassian (Jan 3, 2008)

What Mike said.

He has the correct definition. It's pretty hard to get to this point in real life, but it does become useful if you are determining what decoder to use.

How about telling us why you want to measure this?

Greg


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## East Broad Top (Dec 29, 2007)

To measure the stall current, you need a fairly gutsy power supply, so you know you're measuring the maximum stall current of the locomotive as opposed to the maximum current output of the power supply. 

Just exercise a fair amount of caution when doing stall current test. There's a danger of damaging the drive when attempting to lock the mechanism. From a practical standpoint, a locked mechanism is a pretty rare failure. Steam locos may lock up if something on the valve gear breaks or jams up. (Twigs caught in the running gear are probably the most likely situation.) Diesels are less likely to stall because there's seldom anything on the face of the wheel which would allow it to get jammed. I'm not saying don't do it, but just be aware of the potential drawbacks. 

I find measuring the current draw at full throttle, full slip to be more "real world" in terms of what your locomotive will draw in operation. It's also safer to measure, since you're not locking anything up. Unless you have a good reason for needing to know what the stall current might be, I'd not worry about it. 

Later,

K


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## rdamurphy (Jan 3, 2008)

Basically, I'm trying to discover which decoder I should use. I don't want to risk burning it up, but I don't want to spend way too much for one that is way overrated. The locomotive is an Accucraft Davenport switcher, four wheels, with side-rods - D&RGW #50. I can get a Tsunami Economi with a 4Amp stall rating.

The thing is very heavy, and doesn't slip, and very low geared, so it's pretty slow. I have no idea how many cars it can pull, but given it's a tiny switcher, probably not more than a handful would make sense any way.

Since I only use DCC, I've never bought a power pack, the only ones I have laying around are the REA one, and several Bachmann HO/N scale ones that my son uses.

I'm using an SB3A for DCC power, which has been completely adequate for my needs, since I only own three locomotives, 2 K's and a C-19. I have triple-headed them, and since max speed on the C-10 is only about 25 mph, it's been fine. 

Robert


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## Dan Pierce (Jan 2, 2008)

For some, the stall current is not enough of a measurement as the decoder must supply current for lights and smoke units.
Many decoders are rated for full current draw, not just the motor current.
Leds have little impact but regular bulbs can add up fast along with some smoke units to increase current draw by more than 1 amp!!


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## chuck n (Jan 2, 2008)

Robert

You can estimate the drawbar pull of your Diesel engine by weighing the engine. I have found that the drawbar pull of an engine averages about 1/3 the weight of the engine (range was from 25% to about 50%). How many cars that represents depends on many factors, weight of cars, friction in the journals, grade, radius/diameter of curves come to mind.

Chuck

Note added: The fact that your diesel has only two driving axles really doesn't matter, it is the weight of the engine that matters. 

My AristoCraft Mallet weighs 14 pounds and has a pulling power (drawbar pull of 5 pounds). That is about 36% of the engine weight. It can easily pull a 34 car train: 1 battery car, 1 caboose, and 32 LGB iron ore cars. All the cars have metal wheels and I do not use ball bearing axles. In my experience weight of the engine is more important than number of driving wheels. The weight is spread out over number of drive wheels, therefore the weight per wheel is smaller with the more driving wheels you have. You say your #50 is heavy, then it should easily pull more cars than you are ever likely to have behind it.


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## Greg Elmassian (Jan 3, 2008)

Thanks Robert,

You are indeed asking the question for the **right** reasons.

When a loco starts, it does draw "locked rotor stall current" for an instant, even though it starts normally.

This is the best way to "size" your decoder, although not all manufacturer's spec their decoders "honestly"..

Since decoders are solid state devices, if you go over the specs for the decoder, it can destroy the output transistors.

Nowadays, modern decoders often have output transistors that are plenty capable, but very few have heat sinks that will let them run at full capacitry.

The bottom line, if you can measure the full locked rotor stall current (and yes add in the current for lights and smoke) that will give you a real world absolute maximum current, and properly sized, your decoders will last longer or perhaps forever.

Years ago George Schreyer investigated this and it led to much larger capacity decoders than were "thought" necessary, because people were only looking at "average" current. These short durations of very high current on certain motors (USAT and some Pittman are typical) then explained failures on a number of decoders.

On a normal diesel, normally you can press it against the rails and read (quickly!) the max current. As Kevin stated, you should be more careful with steam locos where not all axles are gear driven, then it is very important to lock the gear driven axle.

Regards, Greg



rdamurphy said:


> Basically, I'm trying to discover which decoder I should use. I don't want to risk burning it up, but I don't want to spend way too much for one that is way overrated. The locomotive is an Accucraft Davenport switcher, four wheels, with side-rods - D&RGW #50. I can get a Tsunami Economi with a 4Amp stall rating.
> 
> The thing is very heavy, and doesn't slip, and very low geared, so it's pretty slow. I have no idea how many cars it can pull, but given it's a tiny switcher, probably not more than a handful would make sense any way.
> 
> ...


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## East Broad Top (Dec 29, 2007)

I've got the 4-amp Econami in my Bachmann K-27, so I'm pretty confident it will have no issues in your Accucraft Davenport. The K is (obviously) a much larger loco with a larger motor, and I've been kinda "torture testing" it since installing it just to put it through its paces. If you're worried about exceeding the stall current, you can always install a polyfuse or similar protection. I usually use a 3 to 5 amp fuse on my installations as "cheap insurance." 

Having said that, the Econami doesn't yet come with a prime mover sound that would be prototypical for the #50. Number 50 has diesel motor with a 4-speed transmission, and the prime movers on the Econami are all diesel-electrics. (I don't know how much of a purist you are in that regard.) QSI has an RDC sound file which is a similar diesel-mechanical set-up, though someone mentioned that the RDC is likely a 2-stroke diesel vs the #50 being a 4-stroke. (I haven't checked, and YouTube videos are pretty worthless when it comes to comparing the sound of the prototypes.) Alas, you can't listen to QSI's prime mover sounds without actually installing the sound file on your QSI decoder, so you've got to spend your money before you take your chances. Zimo has a davenport sound file which sounds pretty decent, though it's also not prototypical to the #50. (I've got the Zimo decoder in my LGB Davenport.) 

Later,

K


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## Greg Elmassian (Jan 3, 2008)

Actually for failures caused by excessive stall current, a polyswitch (correct word) is not a great idea, since it has to heat up over a period of time to "trip".

When a decoder fails because the stall current exceeds the transistor rating, it's pretty **** quick.

Polyswitches are better for long term overloading or detecting the "average" current is too high.

They are good for track powered locos to protect internal wiring from burning up, the wiring will often stand a short for a while until the circuit traces or wire overheat.

Perhaps reading George Schreyers site will help understanding of this situation.

Oh, also you CAN listen to the prime movers sound without a QSI decoder, but the Q3 files won't do it, the Q2 and Q1 files can be played. (this will be fixed with a software update)

Robert: If you have a specific issue on hearing a QSI Q3 file, email me privately, I'll load up a decoder and make a youtube video for you.

Greg


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## rdamurphy (Jan 3, 2008)

That's kind of my problem. I need to know the stall current, but I really have no way to test it. Using the REA power pack, it moves along at .6 - .7 Amps, and when I stall it, it hits 1.4 amps, but if that's the capacity of the power pack, it may actually stall at 3 amps on my SB3a 5 amp.

Hence, my problem. Given my results, I should be "safe" with a 2 amp peak Tsunami Econami, but!? What happens when it stalls on a 5 amp power supply at much higher voltage? 

I'm much more inclined to go with the 4 amp peak Tsunami Econami, but again, how do I find out? I really don't want to buy a 5 amp DC power pack that I have very little use for...

Robert


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## East Broad Top (Dec 29, 2007)

Greg Elmassian said:


> Oh, also you CAN listen to the prime movers sound without a QSI decoder, but the Q3 files won't do it, the Q2 and Q1 files can be played. (this will be fixed with a software update)


You mean people still use the Q2 files? How gauche.  

Good point about the polyswitches. I rarely use them unless really pressed for space, preferring the old fashioned fast-blow glass fuses. 



rdamurphy said:


> I'm much more inclined to go with the 4 amp peak Tsunami Econami, but again, how do I find out? I really don't want to buy a 5 amp DC power pack that I have very little use for...


With regard to the Econami, it has built-in over-voltage and over-current protection built into it, as well as a thermal shut-down in case things get hot, but nothing triggers the other safeties. (Most decoders have some level of self-preservation.) Also, the board is rated at 4 amps maximum, but that doesn't mean it releases the magic smoke at 4.1 amps. It's got a safety margin built into that rating. According to the manual, over-current protection kicks in at 150% of the rating (so, 6 amps) so it would seem there's a fairly healthy safety margin. 

Again, I've been running my Econami through some fairly arduous tests, and have not had it go into failure mode yet. If TCS doesn't get on the ball with their large scale WowSound decoder, I'll likely end up putting an Econami in my next project loco as well (2-motor diesel). I'd not hesitate to put the 4-amp board in the Accucraft Davenport. While not quite prototypical, their Alco 244 sound file I think has a right good amount of character for the Davenport.

As for the 2-amp board, I'd likely bet you'd be fine with that. Again, the Econami has over-current protection which would kick in at 3 amps, and a 3-amp fuse in line with it would add an additional level of protection. It's a savings of $40 - $50 compared to the 4-amp board. Not an insignificant chunk of change, but at the same time, not all that much extra for peace of mind. 

Later,

K


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## SD90WLMT (Feb 16, 2010)

I have had great success using the mini auto plug in fuses...buy a fuse holder from an auto supply parts store...fuses come in many ratings from all stores..
I fit mine inside fuel tanks..easy n quick external access if they need replacing on the fly.


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## rdamurphy (Jan 3, 2008)

As far as the sound, I feel like the EMD 567 non-turbo would fill in nicely, if there's a way to lower the RPM's to simulate a V8 diesel. Also, if there's a way to change the decoder to four notches instead of 8.

Robert


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## Greg Elmassian (Jan 3, 2008)

Actually I should have corrected Kevin's broad statement and stated Q3 files will play on the QSI windows software, but not the Q3 emulator files.

How do you test something for max amps with an inadequate power supply? You don't. If you really want the information you need a power supply with more oomph..

But, how will you run a loco that may need brief periods of 5 amps if you only have a 1 amp supply? Something does not balance here.

Fuses work, hard to make accessable from the outside, but good safety. 

One final note: I would be interested to see what a 4 amp load does to a Econami.... my guess is that it will do what Kevin said. Why? Because the predecessors of the Econami shut down on shorts and overload, in fact often prematurely. One would reason that SoundTraxx did not radically change this circuitry from what they have used for years.

Greg


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## East Broad Top (Dec 29, 2007)

rdamurphy said:


> As far as the sound, I feel like the EMD 567 non-turbo would fill in nicely, if there's a way to lower the RPM's to simulate a V8 diesel.


Alas, the prime mover sounds are what they are. You may be able to tweak the 7-band EQ to shape the sound a bit, but it's going to be a 12-cylinder diesel. 



> Also, if there's a way to change the decoder to four notches instead of 8.


Yes and no. You can't just tell the decoder to use only 4 notches. What you can do is set the speed steps per notch to its maximum value (15 steps per notch), then set your top-ends speed (CV5) such that you're going plenty fast enough at speed step 60 and remember not to go over that. That setting is CV114. 

You may also try going into "switching mode" (F14). This overrides any momentum settings you may have programmed in (CVs 3, 4, 23, 24), and also cuts the speed of your motor in half. What I don't know is if it also adjusts the speed steps per notch. If it does, then you'll have your 4 notches through your entire speed range. If it doesn't, then you'll have 8, and you'll have to remember not to go past speed step 60 again. (That, and you're overriding your momentum, which means you can't use the prototypical braking, which is seriously fun!)

You can also use manual notching. This divorces the notches from the throttle setting, and you'd use the F5 and F6 keys to notch up and notch down as you adjust your throttle to control your speed. 

Later,

K


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## East Broad Top (Dec 29, 2007)

Greg Elmassian said:


> Actually I should have corrected Kevin's broad statement and stated Q3 files will play on the QSI windows software, but not the Q3 emulator files.


You lost me. What's the difference between the "Q3" files and the "Q3 emulator" files? That I see, there's only the .q3 files for download that are on the web page. Within those files are various horns, bells, whistles, airpumps, etc. that you can preview via the programming software, and the prime movers and chuffs which you cannot. (I've downloaded most of them and the latest programming software in hopes that you could preview the prime movers... no such luck as yet.) 



> ...I would be interested to see what a 4 amp load does to a Econami...


I know my K draws around 2.5 amps slipping, and I've likely exceeded that with added weight being applied here and there during my tests. I've not exceeded 5 amps because I've not blown a fuse. How close I've come to 4 amps, I couldn't tell you. I have (briefly) stalled the K while testing BEMF chuff rates, and no magic smoke came out. It kept doing its thing. 

Later,

K


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## Dan Pierce (Jan 2, 2008)

One other spec to look at for current is the way Zimo specs their large scale decoders. 6 amps continuous with a 10 amp peak for all outputs combined and no heat sink required.


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## Greg Elmassian (Jan 3, 2008)

Let's not derail the topic on who knows more about QSI Kevin, you and I have been down this road before.

I'm wise to this game. The fact you seem to want the last word on DCC when you are a relative newcomer and still have a lot to learn speaks volumes, so let's just stop the competition.

Let's focus on give the OP help.

I suggest everyone who wants to be an expert, or learn, start by digesting what George had to say:

http://www.trainweb.org/girr/tips/tips5/dcc_tips.html#stall

http://www.girr.org/girr/tips/tips4/tractive_effort_tests.html



Robert, I apologize, this war between Kevin and I is long standing, and it really does not help you.

If I can be of further assistance, please feel free to email me, I'm done fighting with Kevin on this thread.

Greg ... leaving the thread due to oppressive moderator.


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## East Broad Top (Dec 29, 2007)

Geeze Louise, Greg. I'm not competing. I know you know _far more _about how QSI decoders work than I do--you worked with QSI on their design! You wrote something that confused me; I followed up with a question asking for clarification. 

You mention two types of QSI Q3 files; "Q3" and "Q3 Emulator" files. What's the difference between them, _because I don't know what you're referring to_. I only know of one type--the ones on the site for download. 

If you choose to view my asking for clarification as a competition, and wish to label me an "oppressive moderator" for doing so, then I'll bear that cross. I'm not at war with you. I respect the knowledge you bring to the table in these discussions. If I'm confused by something, I ask questions. Always have, always will. That's how I learn. 

Later,

K


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## wigginsn (Jan 9, 2008)

I always thought instantaneous stall current for DC motors was simply applied voltage/resistance. 

No need to lock anything, just measure the motor resistance (overall loco) with an ohmmeter, then divide by the maximum voltage output of your intended supply.

This does ignore the inductance of the armature which may reduce the current by a tiny amount if the motor starts to turn before the current reaches its peak..

Digging up theory from old memory banks here - happy to be corrected.

Cheers
Neil


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## Randy Stone (Jan 2, 2008)

OK, All along, I'm reading about amps and current draw.

But doesn't the voltage make a difference? 

If a locomotive is designed to operate on 18 volts, shouldn't the locked rotor amps be tested not only with a high amp output, but with the correct voltage supply?

Also, if the locomotive is battery powered, might the battery's thermal switch come into play? Seems it could shut down power output, making it look like the motor went into locked rotor when it didn't


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## rdamurphy (Jan 3, 2008)

I believe that voltage isn't related to amperage. I've always heard the comparison that it's like a water pipe. Amperage is how much water can go through, voltage is how fast it goes through. 

So, you can have peak amperage at a tiny voltage, as the motor attempts to break the resistance of the drive train and the First Law of Motion. If a motor is using 10 volts and 3 amps, and starts climbing, it will draw more amps, but cannot draw more volts.

My question was directly related to the fact that if a locomotive stalls from either having too high of a load, or a direct impact with some force or object, the amperage can shoot to the max, and a DCC decoder with too small of an amperage limit will simply release the smoke they put in at the factory...

Considering the cost of the things, I was kind of trying to avoid that...

The thing about the Accucraft Davenport is that it weighs 8.0 pounds. No, that's not a typo, _EIGHT_ pounds! In other words, it isn't going to slip it's wheels. Ever.

Robert


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## SD90WLMT (Feb 16, 2010)

Robert..sit down...take a deep breathe here buddy..
An out of the box SD70mac weighs around 10 pounds..maybe a bit more.
My battery ballasted locos...weighs in @ 19.375 pounds...
Yes..this is real weight..real traction..on one loco..a monster that works!
It has spun the wheels under load..using full traction tires..everyones worst nite mare btw.
Not mine..it can pull better than 8.5 pounds..at the coupler..more than your loco weighs..this is pulling cars...

It does this..max running current over 7.15 Amps & 12.0 thru 14.0 volts under load..measured at the motor block feed...in other words... That which is in actual use with 2 motor blocks..pulling 70 cars on grades exceeding 2.5% thru 2.8%.

You need more grasp of where your trying to go..keep asking questions here..!
Dirk


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## chuck n (Jan 2, 2008)

Robert

When the pull on the rear coupler exceeds about 3 pounds, your wheels will slip. To test this just hold on the coupler and gradually increase the voltage to the engine until the wheels slip. This is how I test the pulling power of my engines. I increase the voltage and watch the fisherman's scale and note the weight when the wheels slip.










Chuck

Heavier engines will pull more, but there is a point when the wheels will slip on every engine. The only exceptions, that I know of, are the LGB cog locomotives. But those are gear and traction drive, not just traction.

An example of a the pulling power of a heavier engine is the USAT GG1. That engine weights 34 pounds and has a pulling power (tractive effort) of about 8 pounds. That is 24% of the engine weight. This is on the low side of the engines that I have measured. The reason is that there are two axle leading and trailing trucks. These are supported by fairly stiff springs, so some of the weight is distributed to these non-driving trucks.

The percentage of the engine weight that is not on the driving wheels on the USAT GG! and Hudson is 31.8 ans 35.2 respectively. The AristoCraft Mallet has only 7.1% of its weight not on the driving wheels.


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## StanleyAmes (Jan 3, 2008)

The way I determine the stall current is to measure the resistance of the motor rotating the motor by hand to get the smalest number. Then use V = I R with V =24

Note however that each of the DCC manufacturers rate their decoders amperage differently. WIth some the amp rating is the stall current, with others they provide the amp draw obtained when key components reach a certain temperture and others the expected amp draw at room temperture.

Best is to know the stall current, the max continuous load and accessary load and then talk to the manufacturer.

Hope that helps

Stan


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## rdamurphy (Jan 3, 2008)

Right, guys, but my point is that the Davenport has a very small motor, and 8 pounds of weight on only two axles. I've slipped the drivers on my K27's before, although I try hard to avoid it, and I get the concept, but we're talking four pounds per axle, on a very small locomotive without a lot of motor behind it. Plus, it has a NWSL gear box with two identical gears, meaning no gear reduction. When I bought it, I had to replace the axle gear, which gives me an idea of where the weak point is...

The problem, as I see it, is there's so much (mis)information floating around out there about how to size decoders, and I'm just trying to sort through it. In the HO/N world, it doesn't seem to really matter that much, and to a large extent, us large scale guys have been pretty much ignored by everyone except QSI. The Tsunami's, I believe, are actually more intended towards the O Scale guys, it seems...

And, of course, everyone has their own favorite "flavor" of decoders that they swear by and everything else is garbage...

My guess is that the Econami 2 Amp should do the job, but I'm using a 5 amp power supply, so, if it doesn't, I just wasted 80 bucks. So, do I go for the 4 amp for 40 bucks more? And, of course, the QSI would definitely work, but are 200 bucks.

And, of course, manufacturers aren't going to say "this will definitely work" and this is a pretty limited run model and from what I gather, there aren't a lot of them around and most of the Fn3 stuff seems to just sit on a shelf, unlike you USAT/Aristocraft guys that run your stuff hard all the time with 100 car trains!

Add to that my inexperience with DCC, since up until last week when I installed two decoders in my son's HO locos, I'm pretty much only had three QSI Plug n Play decoders in Bachmann locomotives. Not exactly much of an accomplishment, since they were made for the locomotives and just plugged into a socket...

So, I'm just seeking as much information as I can get, and much and many thanks to everyone!

Thanks,

Robert


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## East Broad Top (Dec 29, 2007)

Robert, sometimes ya just gotta go with your gut. A lot of what we do in large scale is somewhat uncharted territory in terms of what works with what. I experimented with the smaller Zimo and TCS boards in my LGB Davenport and LGB Porter, and it paid off. They're both small locos which will only be running fairly slowly pulling rather short trains while switching my yards. I wouldn't be nearly as quick to trust them in one of my mid-sized steamers, despite them averaging less than 1 amp during normal operations. 

If you're worried about the 2-amp Econami in your Davenport, then the extra $40 for the extra headroom of the 4-amp board is worth the expense. As I said in an earlier post, I've got one in my K, and it works beautifully. I have little doubt it would serve you well. 

If cost is a factor, I'd personally be tempted to have a go with the 2-amp board. Despite its weight, the Davenport is a small loco that will be running fairly slowly pulling fairly light loads. The board has built-in over-current protection, so it should protect itself from damage. (And--again--you can always fuse the inputs for added protection.) If you find the board going into protection mode more than you would like, then you can pull the board and replace it with the 4-amp version. You'll still have the 2-amp board to use in another project (one of your HO locos, maybe?)

Later,

K


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## Dan Pierce (Jan 2, 2008)

I do not believe DC stall current matters in the DCC world as DC applies constant current at low voltages and DCC sends high voltage pulses with high current pulses at startup/low speed running. Thus the mosfet rating in the decoder must handle this high current pulse!!


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