PWM lamp dimmer
This article explores a straightforward and effective lamp dimming method employing a timer IC NE555. Traditional dimmers based on linear regulators, as used in the past, could only achieve a maximum efficiency of 50%. They significantly lag behind PWM-based dimmers, which can achieve efficiency levels well above 90%. PWM dimmers are superior because they waste less power as heat, necessitating smaller heat sinks for the switching elements. This, in turn, results in considerable savings in terms of size and weight. In essence, the standout attributes of PWM-based lamp dimmers are their remarkable efficiency and compact physical dimensions. You can observe the circuit diagram of a 12V PWM lamp dimmer below.
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The core of this circuit is the NE555 timer IC, configured as an astable multivibrator operating at a frequency of 2.8KHz. The timing components consist of resistors R1, R2, POT R3, and capacitor C1. You can adjust the IC’s output duty cycle using POT R3, where a higher duty cycle corresponds to greater lamp brightness, and a lower duty cycle results in reduced lamp brightness.
To maintain a consistent output frequency regardless of the duty cycle, diode D1 bypasses the lower portion of POT R3 during the charging phase of the astable multivibrator. Transistors Q1 and Q2 combine to create a Darlington driver stage for the 12V lamp, while resistor R4 restricts the base current of transistor Q1.
The POT R3 is divided into two sections, referred to as Rx and Ry for the upper and lower halves, respectively. Let’s consider the initial moment when the output of the astable multivibrator is high. At this point, capacitor C1 undergoes charging through the path involving R1, Rx, and R2. The lower half of POT R3, namely Ry, is not involved because diode D1 effectively bypasses it. When the voltage across the capacitor reaches 2/3 of Vcc, the internal upper comparator changes its output state, causing the internal flip-flop to toggle its output. Consequently, the output of the astable multivibrator goes low. In simpler terms, the astable multivibrator output remains high until the charge across C1 equals 2/3 Vcc, and this aligns with the equation Ton = 0.67(R1 + Rx + R2)C1.
Now, with the internal flip-flop set, the capacitor begins discharging through the pathway R2, Ry, and into the discharge pin. When the voltage across C1 reaches 1/3 of Vcc, the lower comparator changes its output, subsequently prompting the internal flip-flop to toggle its output once more. This action returns the output of the astable multivibrator to a high state. To simplify, the astable multivibrator output remains low until the voltage across C1 becomes 1/3 of Vcc, in line with the equation Toff = 0.67(R2 + Ry)C1. For a more comprehensive understanding, refer to the internal block diagram of the NE555 timer displayed below.
How does the frequency remain constant irrespective of the position of POT3 knob?.
Irrespective of the position of the POT3 knob, the overall resistance across it remains constant at 50K. If there’s a decrease in the upper side (Rx), an equivalent increase occurs in the lower side (Ry). This principle also applies to the upper (Ton) and lower (Toff) time periods. The derivation presented below will provide a clearer understanding of this concept.
With reference to Fig 2, we have:
Ton = 0.67(R1+Rx+R2)C1
Total time period of the output waveform “T” is according to the equation :
T = Ton + Toff
There fore, T = 0.67(R1+Rx+R2+R2+Ry)C1
We know that Rx+Ry = R3
There fore T = 0.67(R1+2R2+R3)C1
Therefore frequency F = 1/(0.67(R1+2R2+R3)C1)
From the above equation its is clear that the frequency depends only on the value of the components C1, R1, R2 and the over all value of R3 and it has nothing to do with the position of R3 knob.