Physics based head torch question

Hi,
A quick series of questions
Am I correct that LED head torches flicker the light?
If so, is this flicker simply to do with the way an LED functions, or is it there to save energy?
Do mains LEDs found in the home do the same?

I'm trying to find the efficiency of a modern head torch and of common home LEDs. I know the max will be 683 lm/W, and in practice 30%, but I can't find out if this is assuming no flicker. If the lamp was off half the time, but the eye didn't spot it, might this give an equivalent of 60%. If so, is there a word for luminous efficiency that takes this into account.

Thanks.

david ...

some lights use PWM for brightness control

read this ...

candlepowerforums.com/vb/sh…

for home applications the AC power supply can cause flickering as well ...

read this ...

digikey.ca/en/articles/tech…

every cree LED has a data sheet that gives the given efficiency ... the one below is for the XML2, the current one used in "best" headlamps right now i believe

your best bet is to ask these quetions on a flashlight forum

;)

I would assume that the typical LED headlamp is driven using a constant-current source, so there should be no flicker at all (unlike LED lightbulbs in your house which use an AC source with a driver that includes a rectifier and high frequency ripple).

bearbreeder wrote:some lights use PWM for brightness control

Using PWM in a headlamp makes no sense though...a constant-current driver (with multiple levels for dimming) is much simpler and cheaper to implement...and more efficient since you don't lose energy through reactive components of the circuit.

ubu wrote: Using PWM in a headlamp makes no sense though...a constant-current driver (with multiple levels for dimming) is much simpler and cheaper to implement...and more efficient since you don't lose energy through reactive components of the circuit.

read the first link i posted

theres also an easy test to find out if yr light ihas shietty PWM

WHY PWM?

Using PWM to control a light’s output is simply cheaper, because so many existing Integrated Circuits sold off-the-shelves can do it and all that’s left for the manufacturer is to add a small circuit that controls the Duty Cycle or frequency. There is very little measuring, calculating, tweaking and experimenting needed from the manufacturer’s engineering department, so money is saved on manufacturing, parts AND research.

again the best place to discuss these issues is a flashlight forum ... us discussing it here is like folks yapping about technical rock self rescue on a yoga forum

;)

Current driver ICs are dirt cheap...much less expensive to implement than a PWM circuit unless you need to have full dimmer functionality. If you only want 1 or 2 discrete brightness levels, it's current drivers for the win.

This isn't to say that headlamp mfgs don't drive their LEDs using PWM...it just seems like an odd approach if you only need single or even dual brightness levels.

ubu wrote:Current driver ICs are dirt cheap...much less expensive to implement than a PWM circuit unless you need to have full dimmer functionality. If you only want 1 or 2 discrete brightness levels, it's current drivers for the win. This isn't to say that headlamp mfgs don't drive their LEDs using PWM...it just seems like an odd approach if you only need single or even dual brightness levels.

If i take a modern petzl and wave my hand in front of it it produces a strobe effect, so I guess PWM is being used

David Coley wrote: If i take a modern petzl and wave my hand in front of it it produces a strobe effect, so I guess PWM is being used

In that case BB is right...just tested a 4 year old Mammut headlamp that also strobes at it's low brightness setting (1/2 brightness, so 50% duty cycle on the PWM), but a newer Princeton tec lamp that doesn't. Both are 2-level lamps.

ubu wrote:Current driver ICs are dirt cheap...much less expensive to implement than a PWM circuit unless you need to have full dimmer functionality. If you only want 1 or 2 discrete brightness levels, it's current drivers for the win. This isn't to say that headlamp mfgs don't drive their LEDs using PWM...it just seems like an odd approach if you only need single or even dual brightness levels.

a "modern" flashlight will often have many levels .... for example i think the zebralight has 9 or so ... and some zebras use PWM at the lowest settings

whether PWM is used or not is fairly irrelevant unless on can see the stobe effect when the light is fairly stilll ....

this usually happens on the lowest settings and some folks can be sensitive to it for stuff like reading

a lack of visible strobbing may or may not indicate control using the current control, it might also indicate that the PWM is implemented very well, ie high frequency

;)

ubu wrote: ...and more efficient since you don't lose energy through reactive components of the circuit.

Switching converters are popular precisely because reactive components (ideal inductors and capacitors) store energy instead of dissipating it like resistors. This application note: ti.com/lit/an/slyt084/slyt0… shows that practical LED driver circuits may combine a constant-current source with PWM.