# LED Rise Time

#### brightnight1

Joined Jan 13, 2018
91
I recently posted about pulsing an LED for 1uS on at 10Hz. Now I'm trying to pick an LED for this project but I'm wondering how to understand the LED rise time and weigh different LED characteristics such as radiant intensity. If there's a 1uS pulse to turn the LED on, and the LED has a rise time of 5ns, is that more then fast enough? How slow of an LED rise time could I get away with or how can I determine that? I assume the higher the radiant intensity and the smaller the angle of half intenisty the more narrow and farther the distance the LED will go?

#### Papabravo

Joined Feb 24, 2006
20,993
I recently posted about pulsing an LED for 1uS on at 10Hz. Now I'm trying to pick an LED for this project but I'm wondering how to understand the LED rise time and weigh different LED characteristics such as radiant intensity. If there's a 1uS pulse to turn the LED on, and the LED has a rise time of 5ns, is that more then fast enough? How slow of an LED rise time could I get away with or how can I determine that? I assume the higher the radiant intensity and the smaller the angle of half intenisty the more narrow and farther the distance the LED will go?
These features of the LED may or may not be well characterized. Even if you know something about the statistical behavior of a part parameter, you cannot draw any conclusions about an individual part. This is a common problem with all semiconductor devices. That is why we design circuits in such a way that they don't depend on accurate characterization and tight process control of device parameters. If you don't adopt this approach you're going to "have to get used to disappointment."

If and when you complain to the manufacturer or the distributor of the parts, they will ask if you'd like to have the chaplain punch your "TS card".

#### Papabravo

Joined Feb 24, 2006
20,993
An LED is basically a current device, not a voltage device. Brightness is related to current. Question: how can you get the maximum amount of current into the LED in the shortest possible time; in other words how can you make it bright fast?

#### brightnight1

Joined Jan 13, 2018
91
An LED is basically a current device, not a voltage device. Brightness is related to current. Question: how can you get the maximum amount of current into the LED in the shortest possible time; in other words how can you make it bright fast?

Thank you for pointing it out. My plan was to drive it very very hard for ~1uS at 10Hz. The LED is connected to a battery and toggled on and off by a low side FET. I was going to try to pick a source resistor that would allow be to drive it at 1A. Very bright, very quickly. There aren't any LED specs for driving it like this but I think it should be alright.

#### Papabravo

Joined Feb 24, 2006
20,993
Thank you for pointing it out. My plan was to drive it very very hard for ~1uS at 10Hz. The LED is connected to a battery and toggled on and off by a low side FET. I was going to try to pick a source resistor that would allow be to drive it at 1A. Very bright, very quickly. There aren't any LED specs for driving it like this but I think it should be alright.
Look on the datasheet for "ABSOLUTE MAXIMUM RATINGS". When you exceed those values even for small increments of time you degrade the part and may turn it into an"infant mortality".

#### jpanhalt

Joined Jan 18, 2008
11,087
On the other hand, a lot of current-related ratings are actually heat ratings and don't consider pulses. For example, look at the difference between SCR's pulse versus continuous current ratings.

#### Wolframore

Joined Jan 21, 2019
2,609
According to Cree: while LED are capable of withstanding pulsed current well above the max continuous current, there is a loss in efficiency, chromatic shift and may affect long term life. Many LEDs can be pulsed in the 10 MHz range. The losses happen from the switch time just like other switching devices. They also recommend over current be limited to 300% for under 10% duty cycle.

Since you are proposing driving the LED in an unprescribed way, I suggest testing and finding the limits. A lifetime test would be important to see how this affects early failure as we think it might and what is the relationship to the variables of current, duty cycle and time to failure.

It may be a short test. Sounds like an interesting premise. Please share the results and maybe we can figure out how to make it work better.