Hi all! Wanted to know something about household AC. i learnt recently that household ac is 60 hz, that would mean the current is changing polarity at 60 times a second. if this is so why are the lamps in my house not blinking 60 times a second?
thank you for the replies.

 

The fluorescent lights do blink 60 times a second. Can your eyes respond that fast?

Incandescent lamps rely on heating effect to produce light. They don't cool down fast enough to make them blink.

 

Actually the fluorescent blink would be 120 times a second since it blinks each time the voltage goes through zero. An LED powered from the line through a half-wave rectifier will blink 60 times a second and that is often noticeable.

 

Yeah, you're right. Take a photo diode or transistor and wire it up with a power supply and load resistor. Take a look at the signal on the oscilloscope while the photo diode is under fluorescent lighting.

 

...and that is often noticeable.

Yeah, my wife hates the look of LED holiday lights because of the pulsing. It gives a harshness to the light. It's much easier to notice in your peripheral vision or while moving your field of view. I see it but it doesn't bother me as much.

 

It is changing polarity 120 times per second!

 

My guess is that if it were a full wave rectified 30Hz resulting in a 60Hz flashing that it would far less noticeable and annoying than the 60Hz flashing that results from a 60Hz half-wave rectified signal since that one is off fully half the time.

Might be an interesting experiment to do.

 

Actually the fluorescent blink would be 120 times a second since it blinks each time the voltage goes through zero. An LED powered from the line through a half-wave rectifier will blink 60 times a second and that is often noticeable.

I had a turntable with indexing marks on the side. If there was a fluorescent light nearby, you could see the 'apparently' stationary marks.

 

That's called the strobe effect.

The turntable would have a little neon lamp that is powered from AC line frequency and would shine on the index marks on the turntable.

 

Modern high efficiency flourescent lamps have an internal oscillator that drives them at around 20kHz, as do some modern LED lamps.

Both still 'blink' at this frequency.

 

I had a turntable with indexing marks on the side. If there was a fluorescent light nearby, you could see the 'apparently' stationary marks.

And turntables that had those marks also had a nearby light that did shine on them and a speed adjustment so that you could get the turntable speed "exactly" right. There were a different set of marks for each speed, usually 33-1/3, 45, and 78. Because circumference has to be an integer multiple of the distance travelled by the rim in one powerline half cycle, the rim of the turntable was stepped to accommodate this. Most people that had such tables just thought they looked cool and never understood what they were for or what that knob did.

 

And turntables that had those marks also had a nearby light that did shine on them and a speed adjustment so that you could get the turntable speed "exactly" right. There were a different set of marks for each speed, usually 33-1/3, 45, and 78. Because circumference has to be an integer multiple of the distance travelled by the rim in one powerline half cycle, the rim of the turntable was stepped to accommodate this. Most people that had such tables just thought they looked cool and never understood what they were for or what that knob did.

One of my friends had 2 turntables that glowed. It was always a treat to jam out at his house and watch that glowing effect.

Some say the glowing, was brought about by a Doobie or two.
And, a few beers. That was in the early 70s, what can I say?

 

My guess is that if it were a full wave rectified 30Hz resulting in a 60Hz flashing that it would far less noticeable and annoying than the 60Hz flashing that results from a 60Hz half-wave rectified signal since that one is off fully half the time.

For LED strings I'm pretty sure it wouldn't matter. The string of LEDs only turns on when the voltage exceeds the threshold and goes off again when it falls below that. Since their only current "control" is by threshold voltage, it has to be a narrow range right at the AC peak voltage. So the LEDs are off most of the time and only flash at the peaks. If you send the same number of peaks at the same amplitude, you should get identical flashing. The profile of the rest of the waveform shouldn't matter.

Might be an interesting experiment to do.

True. Could make a challenging exam question as well.

 

I don't follow your reasoning. What you are saying would be the case only if the peak voltage barely exceeds the threshold voltage. But that will give you very poor regulation. The more voltage that is dropped across the current limiting resistor, the better the regulation. So it's a tradeoff between efficiency and stability.

Let's say you have a string of 5 LEDs that each have 2V forward drop and you power it with a 20V amplitude half-rectified sine wave, then they will start turning on at 30° and finish turning off at 150°, so the pulse will last 2/3 of that half cycle.

A good question to ask here is to find the ratio of total LED forward voltage to waveform amplitude that results in half the power being dissipated in the resistor.

 

Modern high efficiency flourescent lamps have an internal oscillator that drives them at around 20kHz, as do some modern LED lamps.

Both still 'blink' at this frequency.

CFLs have a reservoir electrolytic after the bridge rectifier - 400V and upto 22uF for the UK 230V system, the US ones probably use a voltage doubling rectifier with a series pair of 200V caps.

I have my suspicions that the cap isn't big enough to stop ripple, but there's phosphor persistence in the tube and persistence of vision does the rest.

The OP questioned why the lights don't seem to flicker - I've met American visitors to the UK who complain that the lights (and TV) flicker because our mains is 50Hz instead of 60Hz - so it must be something we're all used to and don't notice anymore.

 

What you are saying would be the case only if the peak voltage barely exceeds the threshold voltage. But that will give you very poor regulation.

That's exactly how a string of cheap holiday lights is wired. There may be resistors built into the LEDs but most of the voltage drop across the string is the Vf of the LEDs themselves. There is no "regulation".

 

i would like to say funny reply to you ! pls go to see household 50Hz ac supply.and also you want lamps blinking choose LED lamps connect with micro controller and set the timer option to its..your lamps will be blinking like your self. HA HA HA HA!!!!



BacklinksVault

 

If you say so. The humor sailed right over my head.

 

If there was a fluorescent light nearby, you could see the 'apparently' stationary marks.

When Fluorescent lights were first introduced in the UK (on 50hz) in machine shops, there were reports of Machinist losing fingers due to spindles appearing stationary due to strobe effect, the answer was to put the fluorescent's in banks each fed from 3 phase to counteract the effect.

 

I'd like to see some actual reports on that. I've worked in numerous shops that only had single phase lighting and have never seen a machine spindle look remotely like it was stationary. Not to mention the noise and vibration that a running machine produces.