# Photodiode and photodetector help needed

Discussion in 'The Projects Forum' started by mishra87, Sep 1, 2016.

1. ### mishra87 Thread Starter Member

Jan 17, 2016
197
2
Hi all,

I have 5x2 led configuration.

The Vf of LEDs are 1.9V-2.4V at 20mA.

I want to drive it by 12V@30mA.

I have a dedicated constant current led driver STP16CPC05 which shink the current up to 100mA.

I have set the resistance value such a manner it shinks current only 30mA.

Now u can see total cureent is 30mA. Current divided in both branches is 16mA and 14mA.

So I want to know that why current is equally not devided in both branches.

I have 10 phodetector to detect the signal and connected alngwith LED.

So my questions are now.

1. Why current in parallel branches are not same.
2. How do we calculate photocurrent of LEDs.
3. Does 2mA difference really makes sense.
4. How to determine the output of photodetector.

What if the led glow the intensity of 16mA and what if the led glow with 14mA.

5. How much impact on detector output.

Please consider a idle condition ignoring distance , mounting angle etc.

Please do explain as per my doubts.

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2. ### DickCappels Moderator

Aug 21, 2008
2,746
665
Quite likely that is because all of the LEDs do not have the same voltage-current curve. The string with the lower voltage at a given current will tend to hog the current being shared.

The way to minimize this current hogging is to put resistors in series with the LEDs so that the resistors dominate the current sharing above the small differences in the LEDs.

Sensacell likes this.
3. ### mishra87 Thread Starter Member

Jan 17, 2016
197
2
Thanks a lot.

Actually I wanted to know that what is the variation in photdetector with these two current. How do one calculate detectors output.

I know that putting series resistor we can compensate the current.

4. ### hp1729 Well-Known Member

Nov 23, 2015
2,067
230
Which LED(s) have the odd voltage across it? Do you have more LEDs so you can select a better match?