ESR Meter – Useful For Finding Short Circuits

This is not something for reading a DCC voltage, but useful for finding the location of a short circuit. It is an "ESR" meter shown in Fig 1, and this is a model MESR-100 by Signstek (and also sold under other names).

Fig 1 An MESR-100 ESR meter

"ESR" is related to measuring capacitors and means "Equivalent Series Resistance". But we don’t care about this in relation to a capacitor. Instead, we care about its sensitive ohmmeter capability. It can read to 0.001 Ω resolution or THOUSANDTHS of an ohm.

Track rails and wiring all have resistance. Not much resistance, but some. You can try to read this with a regular multimeter, but the typical 0.1 Ω resolution is too low to reliably read subtle differences. Some better multimeters, like the Fluke 87V in "high resolution" mode or the OWON B41T+ can read to 0.01 Ω resolution which might be adequate, but 0.001 Ω resolution is even better for finding shorts.

You take resistance readings up and down the track (power MUST be disconnected) to locate where the lowest resistance is. And that is where the short will be. Or if the lowest resistance isn’t very low, you know you are getting closer to the short which may be at the other end of some wiring.

Fig 2 Reading the ESR meter

Fig. 2 shows the capability by reading the resistance of a short test lead with alligator clips at each end. It reads 0.453 ohms and not "zero". That is because there is the resistance of the test lead wire and possible resistance in connecting to ESR meter test lead alligator clips. The ESR meter’s test leads and connections to the meter don’t apply because those were "zeroed out" before taking the reading.

Example 1: An obvious short has been placed on the track. A quarter was placed on the track to make a "short", but it didn’t make good enough contact just under its own weight for an extremely low resistance short. So, a large steel weight was placed on the quarter to hold it tight to the rails so the "short" would be close to zero ohms. A series of resistance readings were taken between the rails up and down the track.

Fig 3 Taking the resistance readings

Example 2: One power block on the Central Coast Model Railroad at the San Luis Obispo Railroad Museum seemed to be shorted. A test similar to the above was done. However, it did not find a track short. But it did find a point of lowest resistance which was about 0.5 ohm. This suggests that if there is a short, it is still some distance away, but not via the track.

That point on the track was a place with track feeders soldered to the rails. The assumption was made that the short would be at the other end of the track feed wiring. The resistance being measured was the resistance of those lengths of hookup wire from the power source to the track. The short was at the other end of those wires. It turned out that an error was made in some temporary re-wiring and a short existed where the wires were connected at the power booster. See Fig.4 (and as a troubleshooting hint: Also, be suspect of the last thing that was worked on, tampered with, installed, or touched).

Using the ESR meter, this problem was located in just a minute or two.

Fig 4

Thanks to Charles Kinzer on the digitrax-users.groups.io forum for this information. His entire paper "DCC Voltage Testing Equipment Overview" of which this article is only a small part is located here.

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