LED question

[Tom41]

From what I understand, LEDs are digital devices and can either be on or off - there's no 'in between'. However, I have experience of putting varying voltages into LEDs and the brightness changing depending on the voltage, though it was a long time ago.

I've also seen some devices that use LEDs and vary the brightness - such as the Apple Mac standby indicator - but are these really varying the brightness? Are they just flickering the LED on and off very quickly and adjusting the length of the 'on' period?

I know that standard tungsten filament lightbulbs run on both AC and DC. With AC, the bulb dims and goes out when the current flow starts to reverse direction, then brightens up for the reverse half of the wave. With 50hz and 60hz AC, the bulb is brightening and dimming so quickly between the peaks that only the bright parts are apparent. I assume flickering LEDs would have the same effect, due to persistence of vision.

[gonzo0815]

Re: LED question

I think if you control the LED with a digital device, then PWM (puls widht modulation) as you mentioned, is the easiest way to dimm it, without mutch loss and heat.
If the pWM base frequency is high enough, you won`t see the flickering.
A simple phi filterwould reduce it further.
A lightbulb does not flicker that mutch, as the filament is way to slow.

[starfury1]

Re: LED question

Hi Tom41, Gonzo0815, all

LED is a LIGHT EMITTING DIODE

So yeah it behaves as any diode

Put a led across a 9V battery without a resistor and it will keep conducting current till it blows (in an attempt to maintain its voltage drop)

Here's a simple circuit

Throw a pot in there as well and you should be able to vari the brightness.

When you were changing voltages you were changing the amount of current.

people have even used then as Zener diodes
( the reverse bias region of break down) in circuits

reduced current can be used to reduce light output.

Although the above (what you and Gonzo have said) holds true too.

is it digital or analog device ?
well I suppose that depends on how you want to look at it

Me, would say its really more an Analog device

As to the mac I know the effect you are talking about, how they do it I dont know...never really thought about it.

Here is another link

Was really looking for something a bit more informative then these

here is a page of LED circuits....the last one might be of interest

A pdf
op amp pwm circuit.

HTH

Cheers

[starfury1]

Re: LED question

Doh page must have fully loaded its actually fading red eyes circuit

[Harvey]

Re: LED question

...
people have even used then as Zener diodes
( the reverse bias region of break down) in circuits

They (LEDs) make nice temperature and light sensors too

[gonzo0815]

Re: LED question

I do not have estimated the LED as an digital device, i refered to the dimming device.
So on the Mac i assume that the LED is driven by a kind of uControler (e.g. supper IQ chip etc.).
And in this case, it is cheaper to control the brightnes by the duty cycle (PWM) insted of putting an extra transistor in.

[999999999]

Re: LED question

No, an LED is not digital, there is not just on and off there is varying brightness vs current and a minimum forward voltage required and a maximum current allowed. Once you have at least the minimum forward voltage required, you current limit the LED to keep it from overheating, and within that ceiling you can vary the current and it has a direct effect on brightness.

Yes, they can be varying the brightness by current change or perceived brightness by frequency change.

No, with AC incandescent bulbs, the bulb does not dim and go out because it's a super-hot element that does not have time to "go out" before the wave goes through the zero crossing and opposite polarity, reheating it sufficiently again to stay hot always. The rate at which it cools down is lower than the rate at which the waveform changes such that the human eye cannot distinguish it.

No, LEDs are not the same because it's not persistence of vision that causes it. Since an LED does not produce light in the same way, it has the ability to actually turn off at that frequency, enough for your eyes to see it. It's faint but you can see the flicker from 50/60Hz in an LED (at least I can, and others can, but some people say they can't see it and have worse eyes apparently).