The AC power line indicator presented here has a complete galvanic isolation from the grid. The indicator is an LED that lights up when a current flows, although the current can be measured more accurately with an AC voltmeter set to its mV range. The detector is a transformer taken from an old mobile phone charger. The value of the secondary isn't important because we only make use of the primary 230 V (115 V) winding. The (extension) cable through which the current has to be detected should have an as short as possible section of its outer insulation removed. The wires should then be moved apart.
The blue wire should be placed on top of the transformer and the brown wire underneath, or the other way round. The brown and blue isolation shouldn't be removed, so there is no danger of the AC line voltage becoming exposed.lf there is a green/yellow wire as well, this can be placed on either side of the transformer. The brown and blue wires should be in parallel with the windings on the transformer. The secondary winding(s) should be left open circuit so that they don't attenuate the measured signal.
Circuit diagram :
AC Power Indicator Circuit Diagram
In our prototype we found that an alternating 50 Hz voltage of about 2 mV was induced when a 30 watt soldering iron was connected to the extension lead. With higher-powered devices the measured voltage rises proportionally. Since it is unlikely that the iron core of the transformer will ever become saturated, the relationship between the measured voltage and the currentflo should be fairly linear.
The transformer output signal is amplified bv a differential amplifier built around T1 and T2. ; you wish, you can connect an AC voltmeter across the collectors ofTl and T2 to get an 'ndication of the size of the current. The rest of the circuit takes care of lighting up the LED ',',,hen a current flows through the (extension) cable. The measured signal is amplified again by T3 and then T4 is used to drive the LED with a 50Hz square wave. A 9 V battery is suitable for the power supply.
When a capacitor is connected in parallel with the primary winding of the transformer it can make the circuit less sensitive to frequencies other than 50 Hz. ldeally, the circuit should resonate at exactly 50 Hz. This will make the circuit most sensitive. The capacitor should be chosen such that the measured signal across the collectors of T1 and T2 is at a maximum for a certain current flow. However, the capacitor isn't vital and the circuit stillworks well when just the transformer is used. When a low-currenttype is used forthe LED, Rl3 can be increased to 1.2 ka (= 5 mn max. for D1).
Author : Jacob Gestman Geradts - Copyright : Elektor
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