Lionel 71-4002-250 Add-On Meter for Postwar ZW Transformers

The Lionel 71-4002-250 Add-On Meter for Postwar ZW transformers provides digital readouts of voltages and currents for all four ZW channels. The unit attaches to the back of the ZW and utilizes the four “U” terminal posts as electrical returns.

This metering unit has a serious flaw related to the current measurement technique. (There is no mention of this problem in the Operator’s Manual.) Instead of reading the individual currents flowing out of the A,B,C and D terminals, the Add-On reads the currents returning through the four “U” terminals. This would be just fine if all of the outputs fed totally independent and isolated loops, but that is not the case for most layouts. The Common or outer rails of all of the layout areas are generally tied together into one massive Common point, and that mixes up all of the currents from the various outputs. We normally do not have separate returns, and that means that there is really only one point to hook up to the “U” terminals. If only one “U” terminal is used, all of the current for all four outputs of the transformer will be read on just the ammeter for the selected “U” terminal.


The problem is related to the current sensing technique that is employed in the Add-On. Each “U” terminal on the Add-On feeds a .01 ohm current-sensing resistor (the four large black cylindrical components in the photo) that dumps the current onto the “U” bus of the actual transformer. An analog-to-digital converter reads the voltage developed across the current-sensing resistor to calculate the current through the resistor. The “U” bus is attached to the (single) zero-volts end of the transformer’s secondary winding.

Assume for a moment that we are using the A and D handles to feed Train A and Train D, respectively, and Aux handle B is used to supply a group of isolated accessories (not units like the cattle or horse corrals that physically and electrically connect to the track Common.) The currents to Trains A and D pass through separate power blocks to the trains, and the currents return through the outer rails to the central Common point for the layout. The currents are no longer separate, and they cannot be separately fed into the “U” terminals for A and D on the Add-On.  The A and D current meters will each read part of the returning current, but the portion that each reads is not necessarily the portion that went out the respective A and D terminals.  The sum of the two currents will equal the total current from A and D combined.

The solution to this problem is to read the currents on the A-D terminals by inserting small current-sensing transformers on the A-D leads.  I have tested this technique and found that it works well, overcoming the “current distribution” problem.

Other components on the board in the photo above include:

The voltage regulator and connection for the 9V battery on the left,

An LM324 quad opamp for boosting the voltages across the current-sensing resistors,

The LED display driver transistors in the upper right, and

The large PIC microcontroller that includes the A/D converters and outputs the LED display information.

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