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response time

kdg007

New Member
i have a 5mm white LED with 3v forward voltage .I want to figure out how to determine the response time..
 
Last edited:

ronsimpson

Well-Known Member
Most Helpful Member
You need a way to see the light with a very fast response.
The LED probably has 10nS response time so you need a detector with 2nS response time.
No photo transistor will do that.
Some photo diode will do that fast.

Do you have a driver that will drive the LED from 0 to 10mA in 2nS?
Do you have a 500mhz oscilloscope?

I included a data sheet on a 2nS photo diode.

Why all the speed questions. 10nS is fast!
 

kdg007

New Member
i have MRD500 photodiode but its for 1nS...i have oscilloscope.
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i am a newbie..so what do u mean by driver ? for 10mA current ,i can simply keep a resistor in series with the LED.
 

ronsimpson

Well-Known Member
Most Helpful Member
Do you have a signal source? Can it make fast edges? Will it drive the LED direct? or you may have to make a amplifier.

What are you doing?

Look at fiber optics transmitters and receivers.
 

kdg007

New Member
i am trying to understand the behaviour of the waveforms..
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i drive the LED directly with the function generator(with 85 ohm series resistance for 20mA current).The photodiode is connected to the oscilloscope with the BNC connector cable.
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for 1khz square wave frequency i can see the waveforms across LED is same as the waveform across the photodiode.
but when the increase the frequency to 10Khz,the (electrical signal)waveform across LED is square but the electrical signal from the photodiode is kind of looks like sawtooth waveform..i wonder why..maybe the photodiode is not fast enough when compared tot the LED ?
 

RCinFLA

Well-Known Member
White LED's are really UV LED's that cause a phosphor dome to excite. This causes them to be much slower in light output response time. Light output may degrade some over lifetime as the phosphor degrades.
 

kdg007

New Member
u mean to say that light signal is different than the electrical signal we give to the led ? can i get any material related to this issue?i want to read :)
 

ronsimpson

Well-Known Member
Most Helpful Member
Can you use a red LED for a test. Red should be much faster.

Depending on what type of white LED you have.....The current becomes UV light with 10nS delay. The UV light charges up a mixture of phosphors that glows white. (each type of phosphor has a different color and by mixing you can get about any color (of white)) The phosphor has a response time that I don't know. Probably about the same as phosphor used in CRT monitors.

You might be testing the phosphor decay time and not the LED response time.
 

crutschow

Well-Known Member
Most Helpful Member
The LED semiconductor emitter likely has a response time in the ns region, but the white phosphor likely has a response time in the ms region.
 

RCinFLA

Well-Known Member
White LED's are in the range of 20 to 50 usecs compared to less then 1 usec for red LED's. Still pretty fast although 20-60 times slower then red LED.
 

ronsimpson

Well-Known Member
Most Helpful Member
Because you chose white, you are measuring the response time of phosphors not the LED. So your tests are not what you thought.
Back when I designed CRT monitors and TV sets I could find out the response time of each type of phosphors.
In LEDs (white) there are many types of white. Blue-white, Red-white, warm, natural, etc. Each have a different mix of phosphors. There is more than one way to mix up "warm white". The LED companies will not tell me what mix they are using. In a CRT the response time was important. Because no one is using white LEDs 'fast' to send data, the response time is not important and not measured and certainty not in the data sheet.

Phosphors materials turn on pretty fast, but they turn off much much slower. For radar displays we used some that had a decay time of 6 seconds. For data terminals I have used as slow at 100mS to typically 5mS. CRT phosphors is refreshed 60 times a second. It has been a long time scene I thought about phosphors response time, I know I have used fast phosphors I do not remember their time but I can't believe they are much faster than 0.1mS.
 

kdg007

New Member
can you give the materials related to this issue please ? i really want to read them to get better understanding :)
 

alec_t

Well-Known Member
Most Helpful Member
the (electrical signal)waveform across LED is square
Where are you measuring? Directly next to the LED, or at the input end of a length of wire feeding the LED? If the latter, then the current through the LED (hence the brightness) may not be a square-wave because of the wire inductance/capacitance.
 

unclejed613

Well-Known Member
Most Helpful Member
some white LEDs use a UV diode and a white phosphor. the phosphors introduce some lag in the response of the LED. ideally, you want a white LED that consists of a combination of red, green, and blue LEDs on the same chip. since they use no phosphors, their response time will be faster. the limiting factor of an LED is it's junction capacitance. on most LED spec sheets, their response time isn't usually listed. you might want to look at TOSLINK transmitters and receivers which can operate up to 8Mb/s.
 

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