AC Line Powered Pilot Light Circuit Schematic

The Circuit Schematic

On circuit analyzed today, we will analyze AC Line Powered Pilot Light circuit designed by David Johnson. Using the circuit, you don’t need a transformer to power a standard LED. As always, David Johnson makes his design simple and smart, but he doesn’t talk much about his design. Let’s discuss his design, the formula behind the design.

Main Components Functions

The capacitor act as a current limiter, equal to resistor in many LED design. Yes you can replace the cap by a resistor, but the current limiter will dissipate power and that’s not good. Using capacitor, you can limit the current without wasting the power.

The bridge diode function is for rectification, so the current will flow through the LED on both positive and negative cycles of the main voltage source. Without this bridge diode, the current won’t flow in both direction, and even won’t flow at all because the current limiter is a capacitor that block a DC current. You can use only a resistor and a LED in series to make a pilot light powered directly to line voltage, but you will see a rapid blinking light because the the LED will light only on the half cycle of the power supply.

How to Choose The Component Values

To choose the appropriate values for each components, you have understand how it works: the formula (sorry for those who don’t agree with the proposition that understanding the formula means understanding how the circuit works and vice versa).

The whole circuit can be modeled as a series of resistance/reactance. The formula of the capacitor’s reactance is

For 0.22uF, at 50Hz line frequency, the reactance will be -14469i ohm. That’s a complex number, and must treated with complex calculation. The current will be the source voltage divided by the total reactance of the capacitor, diode, and the LED. By assuming that the resistances of the LED and the bridge diode are much lower than the capacitor’s reactance, the current will approximate 110/14469A = 7.6 mA. Because the voltage source is normally stated in RMS (root mean square) value, the peak current will approximate sqr(2) 7.6 mA = 10.8 mA. Standard LED normally rated for 15-20 mA maximum current (mostly depends on its color), running the LED for 10.8 peak current is wise to keep the LED running for its specified life time.

Design Guide for Modification: Multiple Series LEDs

The circuit can be modified to make much brighter light by high power LED circuit as shown below:

Design Step:

1. Look at the LED’s data sheet, find voltage versus current graph. Choose a point where it will be used for the circuit, for example 2.6 Volt-18 mA (Vd-Id)
2. Find the total LED voltage by multiplying the Led’s operating voltage (Vd) with the total number of series LED (all LED are from the same type).
3. Find the voltage drop of the bridge diode (look at the current versus voltage at the point Id), if you can’t find its data then you can just simply measure it with a multimeter (the value would be slightly different because it use the meter’s operating forward current, but it’s OK).
4. Find the total voltage drop by the bridge diode and the LEDs by adding up 2-3.
5. Find the voltage that must be dropped by the capacitor Vc, Vc= Vs-(Vdtotal).
6. Find the capacitor’s reactance Xc, Xc=Vc/Id
7. Find the capacitor’s value C=1/(2.Pi.F.Xc)

In the step 5, try to use RMS value for Vs (110/220), then check if the 1.41*Id doesn’t exceed the maximum pulsed current, if it exceeded then use peak value of Vs at step 5. Any comments will be appreciated.

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