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LED current interrupted for 1ms...would this be visually noticeable?

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treez

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Hello,
We have a 150W LED driver. The LED bank is 150V and with 1A in it.
Supposing we suddenly turn off the LED current (with a series transistor) for just 1ms (and then bring it straight back on) so that we can take an ambient light reading, -do you think that anyone would notice a flicker?
This 1ms turn-off would be done once every minute.
 

treez said:
Hello,
We have a 150W LED driver. The LED bank is 150V and with 1A in it.
Supposing we suddenly turn off the LED current (with a series transistor) for just 1ms (and then bring it straight back on) so that we can take an ambient light reading, -do you think that anyone would notice a flicker?
This 1ms turn-off would be done once every minute.
Click to expand...
Why don't you just try it?
 
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    T

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Hi,

No it is not visible.
Imagine: Many LED power supplies use PWM dimming about 2% to 100% with about 200Hz.
Then they are periodically OFF for about 5ms.

A single 5ms "OFF" is almost impossible to recognize.

A 200Hz PWM dimming is about not to recognize when everything is still.
A 200Hz PWM dimming is invisible for many people when moving, or things are moving.
(Sadly I recongize it with low duty cycle and moving eyes...it results as stroboscopic effect. Not nice, but no big problem either)

Klaus
 
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treez said:
do you think that anyone would notice a flicker?
Click to expand...

A long time ago I came accross an experimental project of distributed communication installed in a supermarket, where the price of the products was updated in small displays installed in the seel points only by a serial code in the flickering of the florescent bulbs. Timing, I have no idea, but this concept of embedding data in lighting is not new, so there should be some research published on it.
 

    V

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1mS isn't a problem but check the decay time of light from the LEDs, the phosphorescence and residual charge may still produce some luminance after 1mS. Beware of reaction time of an LDR if you are using one, they can be quite slow. If you are taking the reading with an ADC, turn the power off, wait 1mS then take the ADC reading and turn the power on again. Remember that many small ADC, especially ones in MCUs, have a sample and hold circuit so you can 'snapshot' the voltage and then let it do the digital conversion while the light is on again.

Brian.
 

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Hi,

here a scope picture of light brightness of a usual 50% PWM dimmed 5W warm while LED.
About 250Hz, very hard edges.
Lflicker.png
Measured with a high quality, high speed photodiode circuit. Very reliable.

I´d say decay is about negligible.
Other LEDs and other LED drivers may operate differently.

Klaus

- - - Updated - - -

Added:
I found another scope picture of the same light source, but with better timing resolution.
Lflicker2.png

Klaus
 

treez said:
-do you think that anyone would notice a flicker?
Click to expand...

No, no one will notice it.

Just like we do not see the flicker in the movies. They are visible only when we look at the car wheels (sometimes they turn in the opposite direction)

I presume you do not have a power capacitor in the circuit; that will hold on the voltage for 1ms (or more). Anyway, you go ahead with your experiments...
 

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