Subject: Re: photodiodes
From: [email-masked] (P Aphalo)
Date: Wed, 25 May 1994 07:26:43 GMT
Newsgroups: bionet.photosynthesis

John Cheeseman ([email-masked]) wrote:
: I am looking for a source of appropriate photodiodes to make a micro
: quantum sensor.  The smallest I have found so far are about 5 mm from
: Hamamatsu.  I would like them to be smaller if possible.  So far, I have
: not been able to find out where Walz gets the micro units they use with the
: PAM2000, but that is the idea.
I have not seen the photodiodes in the PAM2000, but what is what matters
in your design: the diameter or the thickness of the sensors? The
sensitivity is related to the area, so do not make it smaller than
needed.

I have used BPW21 silicon photodiodes, but they are probably bigger than
what you want (size 9mm diam, 3mm thick).
You may find small diodes if you search for "surface mount" style of
package.

International Rectifier makes the "spectra band" range of photocells.
Part VB02505EPL is visible-blue sensitive (size 2.5 x 5.0 mm). They are
extremely thin (<1mm) and have flexible leads. Photocells give a bigger
signal than photodiodes, and are more expensive....

TI (Texas Instruments) has photodiodes with integral amplifiers (size
4.6 x 4.6 x 2.5 mm) types TSL250, TSL251, TSL252. I have not used them,
I am just looking in a catalogue of electronic components, but I will
probably give them a try soon. 

You do not need an amplifier as you can probably connect almost any
photodiode directly (Li-Cor quantum sensors do not have any
amplification, they are just photodiodes in a fancy housing).  Older
quantum meters used a "calconector" which had a couple of resistors
inside. So if you connect the photodiode to the BNC socket instead of
the quantum sensor you probably do not need an amplifier. However, if
you want to use them in addition to the quantum sensor you may need an
amplifier, but more likely just a single high quality resistor 
to convert the current from the diode into a voltage.
A simple circuit with an Op Amp can be used if you need very high
sensitivity, or a high voltage output (V vs mV).
Photodiodes normally do not need temperature compensation.

: I would also be very interested in sources for the electronics and 
: compensation end...  how to make them quantum sensors rather than 
: radiometers, how to make them cosine corrected, how to improve the flatness
: of the wavelength response, if that is necessary.  To whatever degree the
: electronics of the LI6200 can be used, I would like to use that

I will post later some references (if I manage to find them ;-) 

: I wish to use these in a leaf chamber that will connect to the LI6200 irga,
: and I will use it in the field where temperatures are likely to be higher
: than 45C.
If they are going to be used in the field with natural light you probably
do not need a perfect spectral response, as long as you calibrate the
photodiode using sunlight.

You may try to look in the service manual of Waltz to identify the part
they use ;-)

: Any help you can provide will be sincerely appreciated!

: **************************************************************************
: John Cheeseman                                 "We haven't the money,
: Department of Plant Biology                     so we've got to think."
: University of Illinois                          -- Lord Rutherford (1962)
: Urbana IL 61801 USA

--

=============================================================================
 Pedro J. Aphalo

Formerly at:
 Intitute of Ecology and Resource Management
 University of Edinburgh
Now at:
 Finnish Forest Research Institute
 Suonenjoki Research Station
 SF-77600 Suonenjoki, Finland

 Janet: [email-masked]
 World: [email-masked]
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