# How many times a light bulb blinks in 60 Hz AC

#### Alchemy One

Joined Oct 5, 2019
106
I seem to have a different position in regard to answer to this question. I thought for longest time that it was 120 times that current changed direction in a 60 Hz AC system and strangely it is what you find online as well. But what is strange is that I used to find both 60 and 120 times. And also that every time current changed direction a light bulb blinked.

Then lately, just lately as of yesterday I ran in to something I had a hard time understanding. The difference between physical rotation of a rotor" and "electric cycle".
It appears that in a cycle while one leg of the rotor passes one pole of the magnet, the other leg is passing the second magnet's pole at the same time and that is one cycle. If this one full cycle implies one complete sign wave 360 degrees, then all it shows is that the second minus part of the sign wave from 180 to 360 is happening at the same time as 0 to 180 degrees. It is like drawing a sign wave with two hands. The left hand is drawing one half while at the same time the right hand is drawing the second half it. No?
Therefore for every 360 degrees of armature rotation on a two pole motor, there is two electric cycle or 720 degrees of rotation, two sine waves that is. The situation becomes even more clear if you ad two more pair poles. Yea pair poles, magnet comes in two poles, even electromagnets. With more poles you don't have to rotate the armature too many times to get the 60 HZ you want. That sheds light to that confusion. Therefore more poles of stator the slow the rpm gives you the same frequency result. No wonder an 6 pair pole motor runs with much less rotation. Someone could have really and simply said this instead of using all them big words.

That then would not mean there is change in direction of current in that full cycle. All it means that in one wire current moves one way while at the same time current moves in the opposite direction in the second wire making a loop.
Then on the second cycle the first wire is moving through the second pole of the magnet with current moving through it of course in the opposite direction, change in direction of current and the the other leg the opposite way of course.

This seem to make perfect sense and therefore light blinks 60 time and not 120 times in one second of a 60 HZ AC.
If this explanation makes crystal clear perfect sense, why no one ever told me in plain English as I just did that that sign waves that are not does not show or represent change of direction or light blinks at all. It has nothing to do with it. It is just cycle and what that cycle mean. Cycle and change of direction of current have nothing in common. No one has ever mentioned it like this anywhere?
The great blunder is that those sign waves and cycles on boards and papers have nothing to do with change of direction of current at all. They do not represent change of direction of current. It is obvious how great the confusion is. It is big. and who is at fault for god's sake.
And now that I have brought my own reason that make sense to me, why on Earth the internet all of a sudden say that direction of current change 120 times and therefore light blinks 120 times in 60 HZ. What is this a twilight zone. Google it and see it. Type, " how many times current change direction in a 60 Hz AC system?"

Now that I believe it is 60 and not 120, everyone seem to disagree with me.
So what is it going to be here now?
I have my previous post as evidence that I am not making things up. Thank god I could not delete other people's responses but my own editing them to the points of deletion after I was distraught due lots of other thick disagreements.
I find all this very interesting, bewildering, strange, crazy, bizarre. Thank god I have evidence to my claim and it is in this site. As I claimed previously last year that the current changed direction and the light blinked twice in each cycle. Unless someone comes along and say that although current changes 60 times, light blinks 120 times. That will be different.
Let me have it.

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#### Dodgydave

Joined Jun 22, 2012
9,084
60 Hz is 60 times per Second, 120 Half pulses.... QED...

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#### schmitt trigger

Joined Jul 12, 2010
201
You are severely overthinking the problem.

A sinewave crosses zero twice during a full period. At each zero crossing, the light blinks. Flicker is the correct word.
Therefore..... A light bulb will flicker 120 times in a 60 Hz supply.

#### OBW0549

Joined Mar 2, 2015
3,306
There are two voltage peaks in each cycle, one positive and the other negative. Incandescent light bulbs don't care about polarity, and so will reach a peak of brightness on each of the voltage peaks. This repeats 60 times a second, therefore there will be 120 brightness peaks per second.

You can confirm this by taking a light sensor (photodiode or phototransistor) and hooking it up to your oscilloscope, and shining an incandescent light on it.

#### wayneh

Joined Sep 9, 2010
16,384
Agreed, with the careful wording of @OBW0549. There will be peaks and valleys (corresponding to the absolute value of the current) at 120 Hz. But as a practical matter an incandescent filament glows during the valleys and the amount of light emitted is not linearly proportional to the instantaneous current. LEDs of course can show the flicker.

#### nsaspook

Joined Aug 27, 2009
7,184
Then lately, just lately as of yesterday I ran in to something I had a hard time understanding.
Someone is bored at home during lockdown.

Think about how the current is generated at the source during that 360 degree rotation.

#### OBW0549

Joined Mar 2, 2015
3,306
Agreed, with the careful wording of @OBW0549. There will be peaks and valleys (corresponding to the absolute value of the current) at 120 Hz. But as a practical matter an incandescent filament glows during the valleys and the amount of light emitted is not linearly proportional to the instantaneous current. LEDs of course can show the flicker.
I've measured the amount of ripple in the light output of various incandescent lamps; it depends on the wattage and therefore the mass of the filament, with low-wattage bulbs having more ripple than higher wattage bulbs. IIRC for a 60W bulb the ripple amounted to roughly 10% of the average light output.

Joined Jan 15, 2015
5,361
"Blink" would be a poor choice of words looking at an incandescent lamp on a 60 Hz Sine Wave. Blink sort of suits better with devices like an LED and even then I guess it depends.

So here we are again, something just seemed familiar about this post. You know like a Yogi Berra, "It's like déjà vu all over again".

As to the incandescent lamp? I have never taken a photo-transistor to one but while the current is changing direction how much illumination (glow) does the filament really lose? Does it actually "blink". Anyway, based on history, I think I'll just sit this one out.

Ron

#### Analog Ground

Joined Apr 24, 2019
364
The average incandescent bulb light output has a large "DC" or constant output with 120 Hz ripple on it. The thermal time constant of the filament acts as a low pass filter of a rectified input. A typical fluorescent bulb looks like rectified 60 Hz AC. I have seen it many times with light sensors. Perhaps the larger AC component is one reason some people find a fluorescent bulb tiring to the eyes. A lot of overthinking going on here.

Joined Jul 18, 2013
20,441
I recall in the UK when florescent fittings became popular, they emitted an almost imperceptible flicker (50Hz supply).
This led to accidents in machine shops due to the strobe effect on spindles etc appearing stationary..
The answer was to place the fittings in the shop across 3phases with each fitting on a separate phase.
Max.

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#### Glenn Holland

Joined Dec 26, 2014
705
Incandescent bulbs have a large thermal mass so the variations in voltage and current are not perceptible to your vision.

Joined Jan 15, 2015
5,361
Incandescent bulbs have a large thermal mass so the variations in voltage and current are not perceptible to your vision.
That was my thinking. I know somewhere around here I have some photo diodes (transistors) that were spares for my ballistic chronograph. I guess if I could find them I could look at a scope and see what we have.

Ron

#### MrAl

Joined Jun 17, 2014
7,500
Yes it is true that the filament has a time constant associated with it. It's like heating a piece of metal with a flame, when you remove the flame the metal stays hot for a while as it cools down. That's what happens to the filament ... it heats up and then stays somewhat hot even when the voltage goes to zero. So as one peak goes to zero the filament starts to cool but does not cool that much, and when the next peak occurs it heats back up to maximum again. What we see is a change in brightness but it's not a total on/off effect it's more like a bright to slightly less bright then back to bright again effect.
Because there are 120 peaks in a 60 Hz wave that means this happens 120 times per second. That is because the current flow in the filament can be in either direction and still produces the same level of light output.
Almost any light sensor even some LED's will pick up this small fluctuation and on the scope it would look like a filtered full wave rectified sine except with rounded valleys instead of sharp dips.
Now place a diode in series with the bulb and things change. Now there is only one peak per cycle so the bulb light output goes up and down just once per second and since it has more time to cool between peaks the light output goes lower and so the light fluctuation will be greater.

If you get a larger bulb you can see the light output die down a little slower than with a small bulb when the power is cut completely off with a switch. That's because just like a bar of metal, it stays hot for a while between peaks. You can see this effect with the naked eye.

There are spice models of bulbs around now you could experiment with one in LT Spice.

#### Tonyr1084

Joined Sep 24, 2015
4,383
AC is just DC confused. That is to say DC can't make up its mind which way it wants to move. First it moves this way, then it changes its mind and moves the opposite way. But still confused, it continues to do this.

DC is just AC rectified. There are two ways to rectify AC. Half wave and full wave. In half wave there are 60 pulses. In full wave there are 120 pulses. And with DC you can use an LED to examine it.

Many years ago a friend hooked an LED to an audio source. As the DJ spoke my friend could see the LED pulse to the amplitude of the signal. That is to say that when the DJ got loud the LED got bright. When he was quiet the LED would go out. Bored with that phenomena he put it down. But as he moved it across his field of vision he could see that not only was the LED pulsing to the amplitude it was also responding to the frequency of the sound. He showed the bunch of us this. I hooked his LED to the output of an electronic keyboard, then played a note. As the note was struck the LED was bright. But as the note decayed so did the brightness. Again, a note was struck and the LED was moved at a common rate of motion you could see the LED light up and extinguish - thus representing the frequency of the note. Higher notes would produce smaller and closer together pulses as the LED moved while lower notes would produce longer pulses and spaced further apart. The LED was reacting to the positive going sine wave and going out during the negative going wave.

So if you think of a half wave rectified AC signal, there are 60 positive pulses per second during a 60 Hz sine wave. But when you invert the negative wave into a positive current there are 120 positive pulses. In a 1 Hz signal there is one positive wave form and one negative wave form. Since incandescent light doesn't care about polarity, it sees two pulses during that one second.

The thing with the LED and the audio signal??? Try it.

#### Tonyr1084

Joined Sep 24, 2015
4,383
Because there are 120 peaks in a 60 Hz wave that means this happens 120 times per second. That is because the current flow in the filament can be in either direction and still produces the same level of light output.
Here's another thing you can try: Connect an incandescent bulb to an AC source. I'd recommend a 12 volt transformer and an automobile bulb. Notice it's full (apparent) brightness. Then add a single diode into the circuit. The bulb will be half as bright. (assuming you're using a powerful enough transformer)

I'm not entirely sure on this but I think hair dryer fans are half wave or full wave rectified. During half wave rectification the motor runs on Low. During FULL rectification the motor runs on High. As for the heat elements - I'm not sure at all how they change the heat, so I'll leave that part alone.

#### ErnieM

Joined Apr 24, 2011
8,037
The great blunder is that those sign waves and cycles on boards and papers have nothing to do with change of direction of current at all.
Actually they have everything to do with the change in direction.
They do not represent change of direction of current.
Quite the opposite.
It is obvious how great the confusion is. It is big. and who is at fault for god's sake.
It only seems so big because you are sitting inside it.

#### nsaspook

Joined Aug 27, 2009
7,184

Joined Jan 15, 2015
5,361
Bingo! Post 8 I called it but it sort of went unnoticed. In defense or trolls could be hopeless pedantic but I lean towards troll. Next week we can move along to the direction of current flow, that's always fun to have a deja vu experience.

Ron

#### OBW0549

Joined Mar 2, 2015
3,306
You are severely overthinking the problem.
...and paying entirely too much attention to the voices in his head.