# QSI Decoder for Aristo loco - current rating?



## krs (Feb 29, 2008)

Anyone have some words of wisdom on the true output current capability of the QSI decoder for Aristocraft locos?

The QSI current rating in their spec is shown as 2.7+ amps continuous and 3.0+ amps stall.


Not sure what QSI means by including a "+" sign - to me it means "more"...but how much more?
I have never seen that in a maximum current rating which this is supposed to be.


But putting that aside for a minute - this decoder is meant for Aristo locos with a plug-in decoder interface like the SD-45.
Now, on George Schreyer's website, the current he measured for the SD-45 under load exceeded the 2.7 amp rating as specified QSI which leads to the question - is the QSI decoder adequate to power an SD-45 under worst case conditions.

Looking at the 2.7 amp QSI spec, the answer seems to be "no", but when QSI was queried about that, they came back with an email that there is no problem up to 5 amps.

So why specify a maximum rating of 2.7 amps which is almost half of what QSI claims the decoder can handle - this doesn't make any sense to me at all.

Can anyone shed some light on any of this?

Thanks, Knut


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## George Schreyer (Jan 16, 2009)

QSI has been pretty circumspect about the actual ratings of their decoder. I have one in a Pacific (which has only one motor but its a big 'un) and it handles it fine. It draws 2.5 amps at full slip. the SD45 draws 3.5 amps under the same condition. Stall current used to be a really important rating for decoders without active overcurrent protection. Newer decoders, like the QSI, will current limit to protect themselves in a stall condition. Further, the short pulses generated by the silent operation of the decoder don't reach stall currents even in a stalled motor because the inductance of the motor tends to limit the peaks as well. 

Ask QSI the direct question, "will it work in an SD45?"


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## krs (Feb 29, 2008)

Posted By George Schreyer on 03/24/2009 5:58 PM

Ask QSI the direct question, "will it work in an SD45?"




We did - that's what prompted me to post on MLS.

Their verbatim reply when asked if the decoder is adequate for the SD-45 since the decoder is only rated at 2.7 amps and the engine draws more than that worst case, was:


"The amperage is not a problem actually good to 5 AMPS" 



I haven't gone back yet and asked......if the decoder is good to 5 amps why do you spec it out at 2.7 amps max.


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## Greg Elmassian (Jan 3, 2008)

The output transistors are good for 5 amps. 

With the polyswitch removed (latest production) you are limited by heating on the board, there is a thermal sensing system. 

The board can handle 5 amps but cannot shed the heat generated by this current forever. 

The board seems to be able to handle close to 3 amps in 70-80 degree weather all day in an enclosed loco.. 

Without knowing what the ventilation is and the ambient is, they take a conservative approach. 

Run your loco, stop worrying... If you are concerned, calibrate and use the temperature readback function... 

Regards, Greg


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## aceinspp (Jan 2, 2008)

I run -9s and SD45 all day long with the newer boards in 100 degree heat no problem. Just get one and have fun running trains. Later RJD


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## krs (Feb 29, 2008)

Posted By Greg Elmassian on 03/24/2009 10:28 PM
The output transistors are good for 5 amps. 



Thanks Greg -

Which specific output transistors are they using?


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## Greg Elmassian (Jan 3, 2008)

I have not looked at the part numbers, but I have had this data confirmed by not only QSI solutions... but QSI industries, and was talking to Fred Severson, the "S" in QSI. 

I think this is not in question, it's cooling. I've done some extensive testing, and was one of the first people to have to figure out the old polyswitch at 2.5 amps had to go, and the unit will handle 3 amps all day. 

Maybe someone will look on their QSI for you if still curious. 

Regards, Greg


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## Stan Cedarleaf (Jan 2, 2008)

Knut... I had some "issues" with the QSI decoder shutting down in very hot weather. I don't know if this will help, but you might check this link. I added cooling fans which completely took care of the problem.

Cooling Fans in a GP40


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## krs (Feb 29, 2008)

I don't really understand QSI's rationale for all this - even the way they specify the maximum continuous current rating with a "+" symbol behind the value and a peak current of "3+" amps, barely more than the continuous current rating. 
None of this makes any sense from an Electrical Engineering standpoint. 

Cooling is definitely an issue, but that is true for all DCC decoders and it's something the decoder manufacture has little control over - the best he can do is to protect the decoder against self-destruction. 

But in general - the message about selecting an appropriate decoder has always been: 

1. Determine the dc current your engine draws under worst case operating conditions 
2. Select a decoder with a current rating equal or larger (preferably larger) than the worst case current draw. 

Years ago the focus was on stall current. Then later, that shifted to just normal worst case operating current. The assumption was stall current requirements would be covered that way automatically. 

So if I measure the operating current of an SD-45 for instance and I measure 3 amps, I would not even consider the QSI decoder as an otion since it's only rated at 2.7 amps - not something I would expectto be in QSI's best interest. 

Regards, Knut


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## Greg Elmassian (Jan 3, 2008)

It's your choice... 

I may not agree with + signs or how they rate it, etc. (I think it's weird also) 

But they work fine on these locos. 

Suit yourself... 

I use them, they work fine... no problem. 


Regards, Greg


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