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i converted a 230v ac supply to 12v using transformer and using half wave rectifier i converted ac to dc.

The output waveform is as shown in fig. (in my book). When i connected a LED it glows continuously. But if u analyse the output waveform the current varies continuously. i expected the LED to flicker continuously. but its not happening so. can u explain me why?

Picture of i converted a 230v ac supply to 12v  using transformer and using half wave rectifier i converted ac to dc.
IMG0408A.jpg
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guyfrom7up7 years ago
since you are onle doing half wave rectification, you are getting half of the 60hz sine wave. This turns on and off the LED 60 times a second. This flashing is too fast for the eye to detect, and it just appears as a dim LED. If you put a capacitor (like 470 or 1000uF) in paralel with the led resistor combo, the led should be brighter (the capacitor smooths out this choppy power to smoother dc. This fast on and off is known as pulse width modulation (pwm), and it's used to dim leds, make motors go slower, etc. It's very efficient. Also, if you move the led relativley fast in this existing circuit in the dark, you should see it has a weird path, the same goes for plug in LED alarm clocks. This is known as persistance of vision (pov)
i thought half wave AC was 30 hz...
it's still 60hz, just the positive (or negative, if you want) side of the wave.
If you want to see the flickering, look at the led indirectly - constantly moving your vision around quickly - the 'line' from the led will be dashed - you will see the pulses with different parts of your retina over time, making a dotted line
AndyGadget7 years ago
It's because of an effect calledpersistence of vision.

You 'see' the LED because of a chemical / electrical reaction in your eye. This happens very fast, but is not instantaneous so there is a slight 'smoothing' effect and things which are flickering over forty or so times a second will be seen as a continuous light. Your mains frequency will be at 50 or 60 hertz (cycles per second) so you won't see it. If you wave the LED up and down in front of you, you will see the flickering as each time it lights up, it's in a different place.

Persistence of vision has all sorts of applications - Here is an impressive one (watch the viseo). These are made of a single strip of LEDs, but because of POV they look like they fill the wheel.