led persistence

[ecreek]

if a 20ma rated led is pulsed at 20ma, how often (frequency) would it have to be pulsed to maintain the illusion that it is on continuously? or do i have to pulse it harder (>20ma) to give the illusion?

 

[MacGyverS2000]

Depends upon the person... do you see your TV flicker? What about a fluorescent bulb? What frequency are they updated at? Those are good starting points...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[btrueblood]

Also depends on if there is any motion. We used to install an LED temperature readout (gage) in an IC engine installation. Problem was you couldn't read it in some installations, due to excessive vibration and the fact that the led's were flickering at some 10 or 15 hz. Lovely moire pattern, though.

 

[MacGyverS2000]

bt,

I would have expected an annoying flicker without vibration at that refresh speed ;-) If vibration or motion is expected, I'll shoot for double the standard refresh rate.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

There is a reason why TV field rates are at least 50 Hz. Motion flicker can require upwards of 100 Hz.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[IRstuff]

But, bear in mind that TVs used to use phosphors that had ~15 to 20 ms of persistence. Not the case with LEDs

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[IRstuff]

LED Christmas lights can be seen to pulsate, even at line rates.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[OperaHouse]

I had a project with a LED power on indicator. Hooked it to an extra pin of a PIC. It was sending out data all the time at 300 or 600 baud. No one ever noticed it flicker. Had a little cup pickup with a photo diode in it to place over the indicator. Easy way to get data out without turning a screw.

 

[itsmoked]

I'd aim for about 150Hz unless motion is involved.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ecreek]

let's say i go with 150Hz. any thoughts on duty cycle? if i pulsed 20ma at 5% d cycle, would the led appear as bright as it would with 20ma steady DC current?

 

[BrianG]

LEDs manufacturers publish graphs of brightness versus current and pulse currents allowed. An LED at 20mA and 5% duty cycle is not going to be as bright as one running at the same steady d.c. current. The trick is to work out what higher pulse current - within the maximum pulse current and overall power dissipation allowed - gives you the same mean power as the d.c. current for the brightness you require.

For an "ordinary" LED you may well find that you can't get the brightness at the duty cycle you require without exceeeding the maximum pulse current. Consider hyper-bright LEDs from manufacturers such as Avago - really bright on just a few milliamps. (Usual Disclaimer)

 

[MacGyverS2000]

ecreek,

What is the final application? Certain types of LEDs can be pulsed at higher than rated currents for short time periods and give a perceived increase in brightness compared to their average DC power.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[ecreek]

final app would b what is already on the market...scanning dot matrix indicator signs. this is mostly a project for therapy reasons...just as my occasionally repairing an analog tv to keep in practice. on an aside, over the years, i have noticed that if i flick my eyes from side 2 side while directing my gaze to a lit digital readout, i would see superimposed (side to side) replicas of the readout. from this i calculate that the refresh rates are ~ 60Hz (plus or minus many many % because i have a preconceived notion that refresh rates are from 30 to 120hz)

 

[IRstuff]

Refresh rates are a function of eye persistences as well as source persistence. TV phosphors used to have about 10-15 ms of persistence, so 50 to 60 Hz field rates worked well.

However, LCD displays currently have about 8 ms of persistence, so LCD frame rates need to be much high to mitigate flicker. LEDs basically have no persistence, which can often be seen on LED brake lights or traffic lights. When they're turned off, it's immediate and abrupt.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[sreid]

There's data and recomendations from LED manufacturers about this. I'd contact Avago.

 

[itsmoked]

Running short duty cycles may mean you're running much higher current than the rated maximum. This means you have to be careful that whatever you're driving them with can't crash and leave them on..

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zzzjim]

Contrast ratio of scene may also have an effect , I have witnessed Auto brake lights flickering in the dark ,with the motion of a passing vehicle ,worstened by turning your head " Giving impression of more than 1 vehicle " (NOT impressed !)

 

[WoodrowJWeen]

Agree with zzzjim. Newer Cadillacs using LED tail/brake lights seem to stobe them in tail light mode but not brake light mode. Very distracting and kind of the opposite of what you'd really want. Then again I'm rather sensitive to flicker.