Model ACM-2

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ACM-2 AC Current Monitor with sensing transformer

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AC Current Monitor

The ACM-2 AC Current Monitor provides the RFC-1 remote control system with a means to passively monitor AC current. A common application is to verify the proper operation of obstruction lighting on communications towers. More generally, it may be used to meter the AC current consumption of a 120 or 240 volt device drawing from 1 to 70 amperes.

The ACM-2 AC Current Monitor package contains:

  • one AC current monitor PC board with mounting bracket
  • one AC current pickup transformer
  • product documentation

The ACM-2 consists of a current transformer that samples the AC signal and a small circuit that converts the output of the current transformer to a DC voltage suitable for monitoring. The current transformer is mounted at a location that is convenient to the AC wiring to be monitored. The hot AC lead is passed through the insulation tubing in the current transformer and this becomes the source for the sample.

!The installer must supply any enclosures and junction boxes that are required by national or local electrical codes.


The ACM-2 PC board can be mounted in any suitable dry location. Applicable national and local electrical codes should be followed when installing and wiring of the current transformer.

Under normal conditions, the output of the current transformer is low voltage AC (less than 20 volts RMS). However, during lightning strikes or other transient voltage conditions the voltage may be very high. Appropriate precautions should be taken to avoid injury.

Tower Lighting

The ACM-2 is typically used to monitor a tower lighting circuit. The DC conversion circuit smooths the AC voltage sampled by the pickup transformer to a nearly constant value even if flashing beacons are used. This makes it easy to monitor a lighting system with a telemetry system like the RFC-1.

When setup properly, very small changes in lighting current can be observed. Typically a current change of 1% is easily resolved. In most cases this is sufficient to detect the failure of a single bulb in the lighting system.

Alarm Monitoring

Telemetry alarm systems typically monitor for alert conditions 24 hours a day. This can become an issue for devices that are only active during a portion of day or night.

One way to monitor the tower lights only during dark hours is to automatically disable the ACM-2 telemetry during daylight with a photosensor circuit. The ACM-2 comes with connections that allow such a circuit to be added. The manual includes a sample circuit that can be built from easy to locate parts.