# Probably a dumb question - current limiting decoders



## jonathanj (Jan 24, 2008)

Hi,

I'm basically new to DCC as regards installing and setting it up instead of just playing with someone else's, and I wanted to run a couple of ideas past people who knew better...


I'm looking at building a traction line, so about half the stock will be self-powered - flip side of that is that none of them need to be very powerful, so they'll probably generally get just one power block each - and most of those motors are going to be one of a couple of types, just because of what's readily available and easy to use.

So the question is - say all my locos are actually mechanically identical, using USAt power trucks or similar, is there any type of decoder on the market that allows me to set voluntary current limits, and limit motor torque that way? That way I could make one unit weaker/stronger than another to represent their 'real' characteristics, so that the yard master/scheduler needs to take care in matching units to train weight. What I've got in mind in industrial terms is classic DC drive behaviour, with the drive folding back the voltage when it hits the current limit setting.


I looked up the NMRA standards http://www.nmra.org/standards/DCC/standards_rps/RP-9.2.2 2007 July.pdf and there is CV892, but my reading is that that gives a signed report of present current consumption - ie its a dynamic reporter, not a control point. Correct?


Help?

Jonathan


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## Cougar Rock Rail (Jan 2, 2008)

Hello Jonathan, 

With DCC (at least with the Massoth decoders I'm used to), you can set the various parameters like starting voltage, maximum speed (which limits voltage) and accel/deceleration curves for each decoder so their speeds can be matched and fine tuned. For what you are trying to accomplish (adjusting the maximum current) I believe you would adjust the various parameters of the back-emf adjustment, which is designed to keep an engine at a steady speed on curves, inclines, declines etc. For the Massoth decoders there are three adjustments you can make: how fast the back-emf reacts, how strongly it reacts, and the maximum amount of 'assist' it is allowed to give. So between the speed adjustments, and the three back-emf adjustments I think you can do exactly what you are trying to do. 

Keith


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## Greg Elmassian (Jan 3, 2008)

I've played quite a lot with BEMF parameters, (PID, Proportional, Integral, Differential to use their proper names). The QSI allows setting the 3 parameters in 4 different speed ranges. 

From my experience, you can get your loco to run better, but trying to tune it to "have less power" is very tricky, and makes the loco run sort of funny. That said, I have a loco tuned exactly that way right now. 

Many DCC decoders that have BEMF allow you to adjust at least 2 of the 3 parameters. You will have to consult the manual on each specific decoder to see if you can do what you want, i.e. there is this level of control. 

Since all manufacturers have online / downloadable manuals, you can do this research for free first. 

Regards, Greg


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## George Schreyer (Jan 16, 2009)

if you WANT to make a loco speed load dependent, then use any decoder (with BEMF turned off it is has it) and install a resistor in series with the motor. 

I have never seen anybody support CV892 

USA blocks have WAY to much torque, use an Aristo diesel replacement block OR the block from a Center Cab if you need a smaller one. 

The regular Aristo diesel block is good for 4 wheel locos as it has a rocking axle and it is more likely that all 4 wheels will actually ride on the track than rigid frame blocks.


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