Hi

İ am trying to see effect of illumination on Silicon Photodiode. For that i have tried to set up setup in dark, in ambient light, while shining led light as well as under lamp. However İ am just getting simple diode curve. No downward shift which is mentioned in theory.

The setup i am using is simple variable power supply ( positive and negative) connected in series with photodiode and a 100 ohm resistor

Can anyone suggest how to observe that downward shift?

BR

Hamza

 

Photo diode currents are measured in MICRO-amps, you need a more sensitive detector than a 100 ohm resistor and a meter.

 

...However İ am just getting simple diode curve.

Sounds like you're running the photodiode foward biased. They are intended to be run reverse bias.

 

Sounds like you're running the photodiode foward biased. They are intended to be run reverse bias.

İ am applying + 5 v. So yes you are right İ am operating it in forward bias but should n't there still be negative current?

 

İ am applying + 5 v. So yes you are right İ am operating it in forward bias but should n't there still be negative current?

How can there be a "negative" current, if the diode is forward biased? There may be a small change in overall forward current, but it will be completely swamped by the much, much larger forward current itself. In fact, the small change may be less than the shot noise (and other sources) generated by the forward current.

Generally, you want to operate the photodiode either reversed biased, or zero biased (with active feedback).

 

The photo diode’s voltage vs. current curve is almost like a regular diode, with the exception that the entire curve will shift up or down as the light level changes.

 

Photo diode currents are measured in MICRO-amps, you need a more sensitive detector than a 100 ohm resistor and a meter.

A commercial light sensor I traced out put the photo-current direct to an op-amp voltage follower to accommodate the high resistor that fed the PD reverse bias.

A simple JFET buffer can be used - there are some good examples in the Sharp optoelectronics appnotes manual, another good source of appnotes is Hammamatsu. Those and other relevant PDFs are not hard to find online.