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AC Power Indicator Schematic Circuit Diagram

Safe and Isolated AC Power Line Indicator

In this design, an AC power line indicator offers complete galvanic isolation from the grid. The indicator itself is a LED that illuminates when a current flows, although for precise measurement, an AC voltmeter set to its mV range is recommended. The detector component is a transformer repurposed from an old cell phone charger. The specific value of the transformer’s secondary winding is inconsequential since only the primary winding (typically at 115V in the US) is utilized. To ensure accurate detection, the (extension) cable carrying the current should have a minimal section of its outer insulation removed.

Careful separation of the wires is crucial. The blue wire is positioned above the transformer and the brown wire below, or vice versa. Importantly, the brown and blue insulation must not be removed to maintain safety, preventing exposure to the AC line voltage. If there’s a green/yellow wire, it can be placed on either side of the transformer. The brown and blue wires should run in parallel with the windings on the transformer. Additionally, the secondary winding(s) should be left open circuit to prevent signal attenuation during measurement.

AC Power Indicator Schematic Circuit Diagram

Precision Measurement of Induced Voltage

During our prototype testing, we discovered that connecting a 30-watt soldering iron to the extension lead induced an alternating 60 Hz voltage of approximately 2 mV. Notably, this voltage increases proportionally with higher-powered devices. Given the unlikely scenario of transformer core saturation, the relationship between the induced voltage and current flow remains fairly linear.

Amplification and LED Indication Circuit

To process the transformer output signal, a differential amplifier circuit is constructed around T1 and T2. Optionally, an AC voltmeter can be connected across the collectors of T1 and T2 to gauge the current’s magnitude. The circuit’s remaining components are dedicated to illuminating the LED when current flows through the (extension) cable. The amplified signal undergoes further amplification via T3 and is then utilized by T4 to drive the LED with a 60 Hz square wave. For power, a 9 V battery suffices.

Enhancing Circuit Stability with Capacitor Integration

The addition of a capacitor in parallel with the transformer’s primary winding can enhance the circuit’s stability, making it less susceptible to frequencies beyond 60 Hz. Ideally, the circuit should resonate precisely at 60 Hz for optimal sensitivity. Choosing the right capacitor is crucial; it should maximize the measured signal across the collectors of T1 and T2 for a specific current flow. However, while beneficial, the capacitor is not mandatory; the circuit operates effectively using only the transformer. If a low-current LED type is employed, R13 can be increased to 1.2 kΩ (approximately 5 mA max. for D1), ensuring efficient functionality.


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