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True-RMS Ammeter

Since DCC is a “variable-duty-cycle, non-sinusoidal” signal, any attempt to read voltage or current using conventional panel meters or low-cost multimeters will produce only incorrect and frustrating readings. Accurate readings require “True-RMS” meters. While the track voltage is not a very interesting value (it’s set by the booster, closely regulated, and never varies), the current drawn by the DCC layout can be useful information. The circuit shown below converts a conventional DC panel meter into an accurate “true-RMS” ammeter.

This circuit is the original work of North Coast Engineering.
I’ve modified the circuit to use a more-available NPN transistor. This is not your garden- variety junk box NPN, however; it has an emitter-to-base breakdown voltage (BVebo) of greater than 15 volts (most NPNs are in the 5-7 range, and won’t do the job). The NTE2505 device shown is a “universal replacement” type of critter, and available by special order thru Mouser (800-346-6873, ) or Allied Electronics (800-433-5700). The circuit is simple and well-tested. All the passive components are widely available (Radio Shack, Jameco, etc.), but the meter may take a bit more searching — a 0-1 DC voltmeter will do nicely, as will a 0-50 DC microammeter with a 10 Kohm series resistor. A digital panel meter could also be used (very classy!); just ensure that the meter module has an input resistance of at least 100 Kohms. You could also use your handy multimeter on a low-DC volts range for occasional measurements.



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