Pulse/constant?

[Flow]

Reading some of the laser related threads here and poking around, I've come to wonder what the diffrence between pulse and constant is. And why. I don't think it quite means the light flickers... someone enlighten me?

 

[B@rt]

AFAIK it is flickering... /ubbthreads/images/graemlins/rolleyes.gif
The trick is it flickers so fast you won't notice it. /ubbthreads/images/graemlins/wink.gif
Changing the duty cycle allows for dimming.

Hope this helps,

 

[Flow]

Ahhh ok, I get it. The names do make sence then. The pulse being fast hadn't occured to me /ubbthreads/images/graemlins/ohgeez.gif

 

[BigMac]

Ya, you can tell when a light is flickering (at least with leds, and I presume it works with lasers) by tying it to a string and swinging it in a circle. Not only is it usefull, but that technique is also kinda fun /ubbthreads/images/graemlins/wink.gif

 

[eluminator]

Do you need the string? I have a garage door opener with a little green LED on it. When I'm in the garage at night and turn my head, I see a line of little green dots in the corner of my eye. I assume the LED is powered by AC, possibly rectified, but not filtered.

 

[BF Hammer]

Specifically, it's called Pulse Code Modulation (PCM). You power a light with a waveform (typically a square-wave) at a frequency that is high enough that your eye cannot see the on/off cycle of the light. You then vary the on-time/off-time ratio (called a duty-cycle) of each wave cycle to make the apparent light output brighter or dimmer. Common household dimmer switches work this way, except that the duty cycle of the 60 Hz sine wave of the line voltage is what is controlled. Motor speed controls often use this also.

 

[Abe Furburger]

[ QUOTE ]
BF Hammer said:
Specifically, it's called Pulse Code Modulation (PCM). You power a light with a waveform (typically a square-wave) at a frequency that is high enough that your eye cannot see the on/off cycle of the light. You then vary the on-time/off-time ratio (called a duty-cycle) of each wave cycle to make the apparent light output brighter or dimmer. Common household dimmer switches work this way, except that the duty cycle of the 60 Hz sine wave of the line voltage is what is controlled. Motor speed controls often use this also.

[/ QUOTE ]

More correctly - it is called PWM - Pulse Width Modulation - where the pulse widths are changed to vary power to the load or brightness.

(PCM - is used mainly for serial commumications systems)

Abe.

 

[BigMac]

[ QUOTE ]
eluminator said:
Do you need the string? I have a garage door opener with a little green LED on it. When I'm in the garage at night and turn my head, I see a line of little green dots in the corner of my eye. I assume the LED is powered by AC, possibly rectified, but not filtered.

[/ QUOTE ]

No, you don't need the string, but in my experience it makes it more entertaining. Also, you don't get dizzy from spinning your head to fast. /ubbthreads/images/graemlins/cool.gif

 

[Canuke]

[ QUOTE ]
BF Hammer said:
Common household dimmer switches work this way, except that the duty cycle of the 60 Hz sine wave of the line voltage is what is controlled. Motor speed controls often use this also.

[/ QUOTE ]

I didn't think the concept of a duty cycle was applicable to a sine wave. I'd like to see what a sine wave with a 20% duty cycle would look like. Moreover, I thought those dimmers worked by using a transformer with a variable center tap or somesuch.

 

[BF Hammer]

[ QUOTE ]
Canuke said:


I didn't think the concept of a duty cycle was applicable to a sine wave. I'd like to see what a sine wave with a 20% duty cycle would look like. Moreover, I thought those dimmers worked by using a transformer with a variable center tap or somesuch.

[/ QUOTE ]

I don't have a graphic I can post showing PWM (I'll accept the correction) but AC dimmer controls use a semiconductor device - a triac. The triac has an input, output, and control. The control sets the peak voltage trigger level at which the output is shunted to reference (usually called 0 volts). Visualize each half-cycle of a sine where the the voltage rises to a control level then is shunted to 0 until the next half-cycle, rises to the same level (in the negative side) then is shunted to 0....You might be able to visualize how you can change the duty cycle this way.