Re: Flicker issues; plz help

On Thu, 7 May 2009 11:31:31 -0700 (PDT), ilcraque
<ilcraque@xxxxxxxxx> wrote:

hey, hey! hello again.

i'm bringing this topic back to life; i need some help; as mentioned
above, i would like to measure flicker of some light sources in my
surroundings; what would be the easyest and precise way to do this?

i was told that that using photodiode like BPW21 and connecting it to
a resistor of 50ohm and then using osciloscope on this resistor to
measure the drop of the current would do the trick. is this correct?
(btw: i also found photdiode OPTO BPW 24- would this do the trick? i'm
not quite sure what is the difference between 21 and 24...)

best regards;

i dont have any real knowlege of electrical devices so any help would
be real helpful. :)


These are generally the right type of devices, but the
answer to your question would depend upon what type of light
sources you were looking at and what type of accuracy you
were hoping to obtain.

Both are silicon diodes have reasonable response time. The
wavelength sensitivity of the BWP21 is corrected somewhat to
match the human eye, while the BWP24 is uncorrected. For
precise flicker measurements, the wavelength sensitivity
match to the human eye of the BWP21 would not be good
enough, and you would need a much more expensive detector,
such as the Hamamatsu S9219, but for most hobby applications
the BWP21 should be good enough - unless you happen to be
measuring incandescent lamps.

Also, while these photo diodes will work when connected in
series with a resistor, that signal will be very small. They
are usually used by connecting the photo diode in series
with a resistor and then applying a reverse bias to the
series combination of diode and resistor. When the photo
diode detects light, it will pass current in the reverse
direction, which you can then detect as a voltage across the
resistor. The current will be small, so if you are using a
50-Ohm resistor, you will need a means to measure small
voltages.

The BWP24, for example, has a reverse breakdown voltage of
50 volts according to the data***, so you could safely use
a 9 volt battery to provide the reverse bias.

--
Vic Roberts
http://www.RobertsResearchInc.com
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