PWM in cell-phone torch

[tuxerman]

There's this white LED flashlight in my cellphone (a basic Nokia model). One fine day during power failure, i was flashing it about when I noticed that the streak of light it left in the darkness consisted of little line segments, in other words, the LED was switchin on and off at a very high rate.

Could anyone enlighten me as to why this PWM-kinda-drive for the LED is used in the phone?

 

[Hero999]

There are plenty of reasons, one is that it could be using some kind of boost converter which doesn't power the LED continuously but as a stream of pulses.

 

[tuxerman]

Thanks, but a white LED requires only 3.3 V, and the phone battery gives out 3.7 V doesnt it? I'm afraid I dont see the point of having a boost converter there..

 

[PCBWING]

I'll upload a photo about it.It's really twinkling.

 

[Hero999]

Thanks, but a white LED requires only 3.3 V, and the phone battery gives out 3.7 V doesnt it? I'm afraid I dont see the point of having a boost converter there..

White is more often than not 2.8V to 3.8V, depending on the LED and the forward current and the phone battery will vary between 3V and 4V depending on its state of charge.
**broken link removed**

 

[PCBWING]

I'll upload a photo about it.It's really twinkling.

See these, i took them a moment ago.

 

[Sceadwian]

tuxer, you fail to understand diode mechanics 101.
A diode rated at 20ma's at 3.3 volts powered by a 3.7 volt supply could try to pass 10 amps (if it were allowed) This current they allow to pass is also based on temperature, to control it some feedback of total power delivered to the unit is typically used along with a PWM module to control it. Hero is correct, for proper brightness there is most definitely some kind of buck/boost or sepic converter involved, what you're seeing is the human eye noticing the flicker. The low end of a Lithium is 3.1 volts (for stock low discharge) which is barely noticeable in average lighting. 3.7 will blind a user and burn the LED out in more than a handful of seconds.

 

[Hero999]

Are tuxer and PCBWING both the same person?

 

[Sceadwian]

That perhaps would be a good thing to ask them in private rather than in the thread.

 

[PCBWING]

Are tuxer and PCBWING both the same person?

No. How can you think so?

 

[Hero999]

Because you both asked the same question and your posts followed on from one another.

 

[PCBWING]

just for interesing.

 

[tuxerman]

Thanks, guys

PS: PCBWING and I are not the same person

PPS: Just noticed the reply above, AFTER I posted this too :O Eerie, isnt it

 

[PCBWING]

I'll upload a photo about it.It's really twinkling.

That's true. I am a Chinese.

 

[PCBWING]

Thanks, guys

PS: PCBWING and I are not the same person

PPS: Just noticed the reply above, AFTER I posted this too :O Eerie, isnt it

You're welcome. That's true. I am a Chinese.

 

[RCinFLA]

PWM'g is easier and more efficient then creating a controlled analog current source.

 

[Sceadwian]

It's not so much easier as it is more efficient. Controlled analog current sources are actually pretty basic, and can be made from most inexpensive variable voltage regulators, or an opamp and a pass transistor/mosfet, but they can't boost, they can only buck.

 

[tuxerman]

Read up similar stuff on a couple of websites, and from what I gathered, driving the LED via pulses actually gives u the freedom to send current as high as 500mA(depends on the diode) in short-duty-cycle pulses. Thus, the average current would be much less, and within safety limits, while higher brightness is ensured. Also, seems some research has found out that the human eye *perceives* pulsed light to be brighter than continuous light of the same luminous intensity.

 

[Sceadwian]

LED brightness does not increase linearly with increased current after it's rated current is passed, brightness increase drops. Also LEDs have pretty high bulk resistance and you lose power to I²R heating more with the higher pulsed current than you would with a proper average current at it's rated value. Keep in mind those pulses are spot heating the PN Junction which will over time guarantee that the LED fails sooner through junction diffusion even if the perceived overall module temperature remains the same (which even there it can't because of increased I²R loss) I'm not sure what you read but your conclusions don't reflect the reality.

As far as the human eye perceiving pulsed bright light PWM'd being brighter this is just a perception it doesn't actually increase the average light. I've never seen any study that has quantified the effect in useful numbers, it's only ever been done as 'I see that one as brighter' and as far as I know it's never been tested to see if this perception of increased brightness will actually allow you to see any better in a room lighting situation.

 

[tuxerman]

That's a big point, buddy. Perception vs actual luminescence.