ir photodiodes

[samarsingla]

hi all
i made a sensor plate for a line follower with ir led and phodiode side by side and 100 ohm resistor in series with led and 10k ohm resistor in series with photodiode. there is a lot of difference in senstivity of the ir photodiodes. i have six of them and four are nearly same, ie without any light the reading is .5 volts but two are giving 1 volt without any light. what could be the possible reason? please help.
thanx

 

[audioguru]

They are all different. Look at the min and max ratings on their datasheet.
4 of yours were probably made at the same time and are more sensitive than the others that were probably made at a different time.

 

[samarsingla]

they are all of same kind bought from same place at same time. thats whay i was wondering...

 

[audioguru]

they are all of same kind bought from same place at same time. thats whay i was wondering...

It doesn't matter where you bought them or when you bought them. It is important when they were made. If they were all made in the same "batch" then their sensitivity would probably operate closer than if they were made at different times. Look at the min and max ratings in their datasheet.

 

[ljcox]

I agree with Audio.
All electronic components have a "spread". So for a given light input, the resulting current in one photo diode will be different to another.

If you look at a transistor data sheet, parameters such as hFE have a spread, eg. the data sheet I have for the 2N2222 shows that hFE has a spread of 100 to 300 at a collector current of 150 mA. It also varies with temperature and collector current.

The art of electronic design is to obtain a workable result with imprecise components.

Even resistors have a spread, eg. +/- 5%

 

[Papabravo]

Semiconductor device parameters tend to be normally distributed random variables with a mean and a variance. When you see a min and a max specification it is likely to represent the three sigma points of the normal distribution.

I've long believed that constants were just random variables with a mean equal to the constant and a variance which approaches zero.