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.

What do you think when you read the title of this post? Normally, in any measurement, noise addition to the signal acquired from a measuring transducer is the source of imprecision, then how can I say that we can also use a noise to improve the measurement precision? Yes, we can ad a noise to the acquired measurement signal to improve its precision.

Have you ever made repetitive measurement to obtain a measured value? You take the measurements for many times and then compute the averages of them. Usually, such multiple measurement to get more confident result is applied when a measurement is not precisely reproducible, so you’ll get confuse deciding whether you have to choose the value from the first or the second measurement. Just make multiple measurement and average it. If you get a same result from multiple measurement because you have a good enough instrument then it waste your time to repeat your measurement.

In analog-to-digital converter (ADC) , the precision of the reading depends on the bit resolution of the ADC design. Oversampling technique has been a common method to obtain higher resolution by averaging multiple reading. In the absence of noise, multiple reading will result in same values, making it useless to average them. The solution is by adding some noise to the signal that need to be measured by the ADC. Example of practical implementation of this technique, triangular dithering, is presented by Dave Van Ess in his article: Squeeze 10-Bit Performance From An 8-Bit ADC published in Electronic Design Magazine.

Triangle wave oscillator normally designed with a constant current source/sink to charge and discharge a capacitor. Reading an article on how to get 10 bit reading precision on 8 bit ADC on electronic design magazine today, I got something interesting on how to build a triangle wave oscillator: using two XOR-ed square wave oscillator. The picture below describe the concept:

Using this method, you can get a triangle wave oscillator with few logic gates plus some capacitors and resistors for square wave oscillators and simple RC-filters.

Source: Electronic Design Magazine

Condenser microphone is a sound pick-up transducer. The mechanism is based on the characteristic of a capacitor. A capacitor is constructed by two conductor plates separated by a dielectric material (isolator). Air is a common dielectric used in condenser microphone. One plate will be stationary and the other plate will be vibrated by the received sound wave. Because a capacitor is always keep its charge (Q=C.V) to be constant constant, then the vibration will variate the capacitance (C) since it depends on the distance between the two plates. As the result, the voltage (V) will vary according to the vibration pattern to keep the charge (Q) constant.

Before a condenser microphone can be used, it should be charged by applying a voltage on its electrodes. Theoretically, after the plates are charged, the microphone will operate although the voltage supply has been removed, but practically it’s not. First because the microphone has a current leakage (between two plates), and the second, is that the load connected to the microphone output (the pre-amplifier) draw current from it. You need a bias current via a resistor an a voltage source to keep the microphone working.