# Current draw



## tommyheadleycox (Oct 15, 2010)

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Greetings, 
I have some musings and questions re: Stall current, stall torque, decoder ratings of current capacity, oscilloscope vs. Rramp meter and stuff like that.

1. How many of you have ever fried a decoder by exceeding its rated current draw? (this seems to be a rare occurrence).
2. If it's so easy to fry a decoder by exceeding its rated current, why do the manufacturers tout their overload protection so much?
3. If you measured a large scale engine running under raw DC, pulling the biggest load you owned, up the highest grade you owned, with all accessories ON, and it only drew 0.9 amps, why wouldn't you put an HO decoder in it, rated at 1.2 amps for $120.00, instead of a G scale decoder rated at 5 amps for $220? ( I personally have such engines and they only draw .9 A maximum. I have re-motored them with coreless Faulhaber motors)
4. If as I said decoders have such good overload protection, why is "stall torque" emphasized so much as a determinant on what size decoder to buy instead of just "wheel slip"?
5. The nice folks at NWSL told me that determining what motor to put in a large engine is easy! Just put the biggest possible motor in that will fit! You'll be running it at a fraction of its ratings, so you won't have to worry about frying your decoder. But big motors like the Pittman draw 24-30 amps at stall. !!!!! The biggest decoder in the whole world only allows 8 amp momentary current draw. Why use a 24 amp motor?
6. If you wanted to always know how much aggregate current was being drawn by all the DCC loks on your pike at any one time, you could use a RRamp meter. But if you got lucky and could buy an oscillosope from a pal who didn't need it - for the same price as an RRamp meter, wouldn't it make sense to buy the oscope since it will do everything and more? ( I realize that this scenario is unlikely , but hope springs eternal)
7. If you were re-motoring an old large scale engine that formerly had an open frame universal motor in it, with a new coreless Maxon or Faulhaber, and you could buy the new motor in any nominal rating your chose eg. 12v, 15v, 18v, or 24v, which should you choose, assuming that DCC at the track is approx 16V?
Thanks for pondering these questions.
Tom


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## riderdan (Jan 2, 2014)

Tom,

I can't speak directly to issue number one, but I've only ever heard of decoders being fried by incorrect wiring. I've let the magic smoke out of a couple, myself.

What I can offer is a data point: I have some extremely cheap decoders, unbranded, Chinese-manufactured, HO/N that cost about $5 each--and they've never failed due current draw. I tested one of them in an LGB track cleaning loco and the overload protection kicked in about every 30 seconds when I was sending it around the layout. But it still works--it just cuts out when the draw exceeds its limits. Once they cool off, they're fine again. I suppose at some point it might die, but it's in an Eggliner now and running happily.

Since these super-cheapies don't seem to have a problem with being killed by overloading, I'd be surprised if name-brand decoders do.


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## chuck n (Jan 2, 2008)

I don't do DCC, but my question to you is what engines are you using that draw 0.9 amps. Do they have smoke, lights, and sound? As a rule of thumb I rate every engine I have at 1 amp per motor and add a little more for good measure, when thinking about a power supply.

The only engines I have that draw that is LGB Stainz without smoke and with lights.

Chuck


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## Greg Elmassian (Jan 3, 2008)

Well, I do have a bit of experience in DCC, in several scales, but I started outdoor DCC about 10 years ago.

Answers/comments inline below:


Greetings, 
I have some musings and questions re: Stall current, stall torque, decoder ratings of current capacity, oscilloscope vs. Rramp meter and stuff like that.

1. How many of you have ever fried a decoder by exceeding its rated current draw? (this seems to be a rare occurrence).
>>> sure a few times, usually shorted motor
2. If it's so easy to fry a decoder by exceeding its rated current, why do the manufacturers tout their overload protection so much?
>>> because it is easy to mess up wiring and transistors are not really fault tolerant
3. If you measured a large scale engine running under raw DC, pulling the biggest load you owned, up the highest grade you owned, with all accessories ON, and it only drew 0.9 amps, why wouldn't you put an HO decoder in it, rated at 1.2 amps for $120.00, instead of a G scale decoder rated at 5 amps for $220? ( I personally have such engines and they only draw .9 A maximum. I have re-motored them with coreless Faulhaber motors)
>>> so you need to understand that you were reading average power, the peak power could be double to quadruple that... you can try an HO decoder... remember the power is normally a total, so add in the lighting... and it's more like $120 vs $200
4. If as I said decoders have such good overload protection, why is "stall torque" emphasized so much as a determinant on what size decoder to buy instead of just "wheel slip"?
>>> never heard "stall torque", but "stall current", that is basically the worst case current. You need to also understand that when starting from zero speed, that full stall current is needed, just for a brief time until the motor starts to turn.
5. The nice folks at NWSL told me that determining what motor to put in a large engine is easy! Just put the biggest possible motor in that will fit! You'll be running it at a fraction of its ratings, so you won't have to worry about frying your decoder. But big motors like the Pittman draw 24-30 amps at stall. !!!!! The biggest decoder in the whole world only allows 8 amp momentary current draw. Why use a 24 amp motor?
>>>> don't know what pittman motor you are using, but if it is the sealed ones used for "servo" type applications, then they can have very high stall current.
6. If you wanted to always know how much aggregate current was being drawn by all the DCC loks on your pike at any one time, you could use a RRamp meter. But if you got lucky and could buy an oscillosope from a pal who didn't need it - for the same price as an RRamp meter, wouldn't it make sense to buy the oscope since it will do everything and more? ( I realize that this scenario is unlikely , but hope springs eternal)
>> I have both, RRampmeter easy to see from distance, scope has to be looked at and see the scale... scope will show you peak and variations better...
7. If you were re-motoring an old large scale engine that formerly had an open frame universal motor in it, with a new coreless Maxon or Faulhaber, and you could buy the new motor in any nominal rating your chose eg. 12v, 15v, 18v, or 24v, which should you choose, assuming that DCC at the track is approx 16V?
>>> most large scale locos and DCC systems are set for 24 volts on the rails, with voltage drop and the drop from the decoder that seems to work best... if you want to use a lower voltage, you can remotor... but why?
Thanks for pondering these questions.
Tom[/QUOTE]


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## Cougar Rock Rail (Jan 2, 2008)

Tom,
Have a look at this page which shows some information about the various motors that LGB uses. Champex-Linden gets them directly from Buhler so they are always available:

http://www.champex-linden.de/cl_pr_lgb_ersatzteile.htm

The other day I was running two LGB sound equipped locomotives, both duel motored, one pulling three lit Kiss coaches and one freight train, and my Massoth system showed a maximum of 1.4A. Most of the time it was 1.2A, and my track voltage is about 22.5V. The only decoder I have ever fried was in a propeller driven air car, which you would think would not be under much load, but I guess the high RPM had something to do with it. 
You can use HO decoders in small engines, but you will be limited with things like smoke makers and function output current that may be inadequate for large scale.


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## East Broad Top (Dec 29, 2007)

I don't know that I'd ever use a decoder rated at less than 1 amp in large scale, but you can definitely get away with using 1-amp decoders for your smaller, lighter locos and critters. I've got 1-amp Tsunamis in my two Galloping Goose style railcars. 

I know with regard to the Tsumanis, their current limitation stems from heat build-up. There were a few companies who took regular 1-amp Tsunamis and "beefed them up" for O-scale and large scale essentially by sandwiching them into a big heat sink. I don't know if that's the case for other decoders; there could be hardware limitations. 

One consideration--track voltage. Most small scalers seldom use voltages north of 18 volts, while large scalers tend to gravitate towards 20 - 24 volts. Make sure the decoder you use will be happy at those higher voltages. Some will not be, and will go into over-voltage shutdown. 

My general rules of thumb: if the locomotive is small enough to where a large scale decoder won't fit inside the locomotive, then it's probably not going to draw enough current to damage a small scale decoder and you'll be okay. Anything that's got two motors should have a higher-current decoder. Any steam locomotive with 8 wheels or more (drivers and pilot/trailing wheels combined) should get a higher-current decoder. 

The good news is that from a cost standpoint, both Soundtraxx and TCS have introduced large-scale decoders which are hitting the streets at the $130 - $150 price range.

Later,

K


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## Dan Pierce (Jan 2, 2008)

I use a .8 amp decoder (Zimo MX621 rated for 35volt) in the LGB small hand cars (gustav, Santa, Disney, etc). 
Also note that when a motor fails it can draw more than 5 amps and I have seen these melt the plastic motor blocks!!


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## tommyheadleycox (Oct 15, 2010)

chuck n said:


> I don't do DCC, but my question to you is what engines are you using that draw 0.9 amps. Do they have smoke, lights, and sound? As a rule of thumb I rate every engine I have at 1 amp per motor and add a little more for good measure, when thinking about a power supply.
> 
> The only engines I have that draw that is LGB Stainz without smoke and with lights.
> 
> Chuck


I'm using Märklin 1 scale engines from the 70's . I remotored them 'cause DCC wouldn't work with the universal open frame motors of that vintage. These particular engines, for the moment, have no lights, no smoke, no sound. Hence the low current draw.


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