AC Power Indicator

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

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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