led response time

Hello all,

How can I measure rise and fall time of a LED ?
LED: ~650nm, 30 degree, 1000mCd, Red Led.

What should I do to decrease rise and fall time of the led?

10x

Because LED are build on fast semicondutors compounds, i think they are very fast.
Perhaps the lead inductance dominate switching time while the intrinsic diode may be very fast.

In any case you may measure spar of a biased LED and, whit the help of a post-processing, generate an RLC equivalent circuit , in order to find time constant.

Hi, how do you measure the rise and fall time of a simple diod ?
You put on an Ac-coupled square wave, and you analyze the voltage over a series resistor. there you'll see as you increase the frequency, you'll have a forward current for a while, though you've switched it off already, due to the diffusion of the carriers to the pn-junction. Those of the carriers that are close enough.
This will increase the off time.

I guess you should forget the light emission part, because it seems that the diode lights up instantaneously as soon as the voltage has passed a certain treshold, so analyze it as an ordinary diode.

Kindest regards,
StoppTidigare

Your need fast Si photodiode with 50 ohms oscilloscope load. If expected rise and fall time is in nanosecond range PiN photodiode is useful. If rise and fall time is in picosecond range ( laser sources) your need avalanche photodiode.
Look at Hamamatsu photodiode catalog and application notes- response time for fast photodiodes is given in datasheet. Of cource, DC bias voltage is nessesery for fast photodiode responce ( high DC bias reduce internal photodiode capacitance).

Hi again,

I have found a phototransistor that response time is shorter than 3us. I measured my led with a simple R and phototransistor circuit via oscilloscope and I saw that rise time of it is 10us and fall time is 35us. Both rise and fall time of the led must be shorter than 20us for my application. The rise time is ok but How can I decrease fall time of the led? Should I change the led that I use?

10x

Do I need photodiode or phototransistor ?
What happen if I measure it with phototransistor?

Photodiode of cource.
Phototransistor is much slower than photodiode, its work in kHz range. And phototransistor detectivity (A/W) is more nonlinear then photodiode. Usually, phototransistors used for detect on/off switching light and direct drive logic IC input.

Hi,

How can I measure rise and fall time of a red led correctly with a photodiode?
I have found SIEMENS BPW34B coded photodiode and prepare a circuit contains a photodiode and RL (10Kohm). But every time I saw different rise and fall times in digital scope by changing RL. Is this normal ? My measurement circuit is ok? If I use 10K as RL, rise and fall time are being shorter than 1us, If 100K, rise: 5us, fall time: 13us, If I use 50ohm, I can't see anytnig. If I look directly pins of the photodiode without any DC supply and RL with scope, I measure rise time as 35us, and fall time as 53us. Which of them is correct ?

Photodiode outputs current (each photon X efficiency = 1 electron-hole pair created). So you can see a bigger voltage with a high load resistance, but you will have significant R-C time constant issues. You might want to build a current input op amp circuit to change current to voltage, with some gain. The circuit will have low input impedance, so you will have a fast response time.

After you have built the proper meauring circuitry, you will likely find that your DC driver circuitry is limiting your LED response time.