measuring current draw in DCC
#1
fellas,

Is there a practical way that I can readily measure the individual current draws to each of my four power districts, all being supplied from a 5amp booster? My layout has grown and even though voltages are running at about 12.5 in each district I was wondering if one or two zones are drawing most of the current.

Thx
Eric
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#2
Your current draw should be proportional to how many "loads" (locomotives) are in each district. I would assume each decoder draws a small amount of current whether the locomotive is moving or not. But, if the loco is moving, the current draw should be much more than if it is still. In other words, your current draw for each district should be different depending on how many locos are on the track and how many are moving. An ammeter will tell you how much current you are drawing, but that will vary depending on how you are operating your layout.
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#3
Any one district doesn't "draw" anything if there is no load (locos/lamps, etc.) placed upon it. What you may have is a voltage drop and the lower voltage will place a greater load (amperage) in that district when a loco enters it. Why are you feeding all 5 districts from one booster? The idea behind districts is to isolate them from each other to prevent a total system shut down in case a greater load than a booster can handle (such as a short) is placed on it. As you have it wired, it will still shut down. To measure the draw (amperage) you will need a special DCC "enabled" ammeter placed between the booster and the track, on only one lead.
Gus (LC&P).
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#4
Ready made:

<!-- m --><a class="postlink" href="http://www.tonystrains.com/products/pop-meters.htm">http://www.tonystrains.com/products/pop-meters.htm</a><!-- m -->

or DYI:

<!-- m --><a class="postlink" href="http://hackedgadgets.com/2006/08/20/dcc-ammeter/">http://hackedgadgets.com/2006/08/20/dcc-ammeter/</a><!-- m -->

Search google for "DCC ammeter" and you'll find all kinds of ideas.
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#5
Gus, I can think of a few reasons to use one booster even if the layout is divided into districts. One reason would be cost... boosters are expensive. And if the layout normally has only one or two operators, it isn't a big deal if the whole layout shuts down on a short circuit. Now, each district should have it's own disconnecting switch, mainly for troubleshooting purposes. And of course, Digitrax makes the PM42 which takes one booster and divides it into 4 seperate districts, each with its own overcurrent protection.
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#6
Speed control in DCC is accomplished by using "duty cycle", or the varying length of time the applied voltage is "high" (the speed control voltage is effectively a "square wave", and it is the "on time" that varies as the throttle is increased or decreased). The current, therefore, is also pulsed ( which is why a straight DC motor "whines" in 00 mode), and a standard DC ammeter cannot measure it accurately. I'm sure you could find a formula that would allow you to use the average DC current, to compute the peak current of the "wave". Peak voltage is always the same, but the time period that it is,on, at peak, varies.

Yup, that's what I said.
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#7
Thank you for your comments fellas. I am not having any trouble with my layout and as stated in the original query, each district voltage is right up there. However, because I expanded my layout and now one district is larger then the others I was wondering if it also was drawing more amps. I try and place my locos all over the layout even when not in use to avoid the potential problem of them all residing in just one district (the engine service facility). As noted in your comments my system is supervised by a PM42 and it seems happy. Thanks again.

Eric
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#8
Eric: would it help if you measured the current before it gets to the booster device? This would assume identical boosters and that you could measure the prime power source somehow.
I'm assuming that the input to the booster (is that what you call the power unit for a district?) is proper AC.
David
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#9
David, that is a darned good idea and worth a try. It is possible that the AC current on the power supply side of the booster would be filled with harmonics and variations in the sine wave current due to the loading of whatever is on the other side of the booster. These things could cause a regular meter to be innaccurate, but I think it would be a means of measuring relative current.
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#10
David,

Yep I could measure the current before the booster BUT that only tells me the total amp draw to the system, not the individual draw to the separate isolated districts or zones that I have (supervised by the PM42). But in retrospect, as there is no actual problem on the layout I likely was concerned about a non issue.

Gary S Wrote:David, that is a darned good idea and worth a try. It is possible that the AC current on the power supply side of the booster would be filled with harmonics and variations in the sine wave current due to the loading of whatever is on the other side of the booster. These things could cause a regular meter to be innaccurate, but I think it would be a means of measuring relative current.
BR60103 Wrote:Eric: would it help if you measured the current before it gets to the booster device? This would assume identical boosters and that you could measure the prime power source somehow.
I'm assuming that the input to the booster (is that what you call the power unit for a district?) is proper AC.
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#11
You're right, Eric. Before the booster would be total, not per section as you mention. Still, as everyone else said, the current to any given area will depend on the number of locos running in that section plus whatever stationary current consumers in each section.
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