High Power LED Stroboscope Problem

[m_seifi_e]

Hello guys
Sorry for wrong topic selection and bad English

I'm working on a project with 3*10 Watt High Power LED's wich act as a frequency & duty variable stroboscope via a Micro Controller . The problem is for accurately stopping a high speed rotary device the duty should be below 2% ( as an experience ) . but with such a low Duty, brightness of LEDs are so low . I know that with higher current pumping to LEDs the problem can be solved . but the question is HOW? and is there a way that current flowing through LEDs be adjusted with selected duty in Code ? would you please help me on over current protection circuity too ?

Thanks a lot

 

[BradtheRad]

I made a stroboscope using a bright white LED. The flash is short enough and bright enough to see letters on a fan belt with the automobile engine running.

The timing circuit is quite simple. I can ajust the time between pulses with a potentiometer.

The timer output turns on a transistor. This discharges a capacitor through the LED for a tiny fraction of a second.

The timer turns off the pulse. The capacitor starts charging through a resistor in series with the 6 V battery pack.

I found there was no need for protection circuitry.

I simply adjusted the charging resistor so that the capacitor would not have time to charge too high.

I had to make sure the led would not be exposed to more than 20mA average current, no matter how slow or fast was the flash rate.

I also wanted to stop fast movement. This needs a short, bright flash. To get the brightest flash I had to adjust the bias on the discharge transistor in order to drive it to its lowest possible resistance.

The way it turned out, the capacitor only has so much time to charge. Hence the more frequent the pulse rate, the lower the voltage the capacitor can attain. So the LED is not exposed to overmuch current.

The flash is easy to see in the dark but it can barely be seen in bright light. Sometime I plan to add more LED's for more brightness.

 

[m_seifi_e]

I made a stroboscope using a bright white LED. The flash is short enough and bright enough to see letters on a fan belt with the automobile engine running.

The timing circuit is quite simple. I can ajust the time between pulses with a potentiometer.

The timer output turns on a transistor. This discharges a capacitor through the LED for a tiny fraction of a second.

The timer turns off the pulse. The capacitor starts charging through a resistor in series with the 6 V battery pack.

I found there was no need for protection circuitry.

I simply adjusted the charging resistor so that the capacitor would not have time to charge too high.

I had to make sure the led would not be exposed to more than 20mA average current, no matter how slow or fast was the flash rate.

I also wanted to stop fast movement. This needs a short, bright flash. To get the brightest flash I had to adjust the bias on the discharge transistor in order to drive it to its lowest possible resistance.

The way it turned out, the capacitor only has so much time to charge. Hence the more frequent the pulse rate, the lower the voltage the capacitor can attain. So the LED is not exposed to overmuch current.

The flash is easy to see in the dark but it can barely be seen in bright light. Sometime I plan to add more LED's for more brightness.

Thanks a lot
Very valuable information in your post

would you please tell me more about the capacitor charge-discharge section of the circuit ? so i can make it and find the appropriate values . i don't know how to use capacitor for such a circuit

and why Transistor is preferred compared to n-Channel Power MOSFET which I'm using ?

Thank you again

 

[BradtheRad]

I examined my homebrew stroboscope and looked over my schematics. I see I remembered incorrectly.

**broken link removed**

I ended up NOT using a charge capacitor. I just wired the LED in the collector leg of the transistor.

Here is the reason.

I found that a suitable pulse length is 1 millisecond.

I also found the LED could endure bursts of 100 mA. A small inexpensive transistor could be used since it had sufficiently low 'ON' resistance.

Therefore I made the control section to have a maximum pulse rate of 200 per second. This keeps the average draw under 20mA. And a 200 Hz flash rate is sufficient to observe a great many of the interesting things there are to look at with a stroboscope.

I adjust the flash rate via a potentiometer. Slowest rate is 1.4 Hz.

Power is from a 9V battery. Gives the unit portability.

A mosfet should work just as well, if you need greater current.

I'm sure I experimented with a discharge capacitor. However the LED was less bright at high flash rates. So I chose the above method because it was the simple way to go.