Circuit Labs - Electronic Circuits Collection

Author: Karel Walraven

It is not always immediately obvious whether a power-consuming appliance is switched on or not. Examples are the lamp in the attic or the shed, or electric heating in an awkward place. A nice solution would be to connect an LED directly in series with the appliance, unfortunately you’d better duck for cover if you tried…

The obvious solution would be to place a (power-) resistor in series with the load and connect an LED with series resistor across it. However, this solution has significant disadvantages, for instance, the power loss is relatively large (easily a few watts). In addition, the value of the resistor should be adjusted depending on the magnitude of the current.

It would be better to insert two anti-parallel diodes in the power lead. Unfortunately, the voltage drop is too low to power an LED. It does work with 6 diodes, for that matter, but the power loss is then also 3 times greater.

We therefore chose a solution with two diodes, followed by a 4 times voltage multiplier in the form of a cascade rectifier. That is an energy friendly solution. The current through the LED is automatically limited by the internal impedance of the cascade rectifier. The impedance isn’t that small, despite the large electrolytic capacitors. Use a low-current LED, otherwise the LED will probably not be bright enough. The 1N5404 used here can handle up to 3 A

(3 A × 230 V = 690 W).

If the power is less than 200 W, you could use two 1N4004s instead. The voltage across the diodes is a square wave with an amplitude of about 1.3 Vpp. The voltage multipliers are used to turn this into the LED voltage. This will only work if the voltage drop across the diodes in the multipliers isn’t too large. That is why these diodes are Schottky diodes. These only have about a 0.35 V voltage drop. Exactly which type of Schottky diode that you use is not too important.

You are free to experiment with the value of the electrolytic capacitors. The larger their value, the greater is the amount of current that can be delivered.

Keep in mind that working with mains voltage can be fatal. Build the circuit in such away that there is no risk that live parts can be touched and maintain isolation distances of 6 mm (also in air). For the same reason, use a 5 mm LED (not a 3 mm one!) and fit it as far into the enclosure as possible. Mount the PCB in the enclosure with nylon bolts.

 (Elektor Electronics Magazine – 07/2006)

Download this article (#044029-uk.pdf)