Power Meter

Colin Kaminski · Post by **Colin Kaminski** » Thu Aug 08, 2002 1:58 pm

wler has posted a power meter schematic to his web site! Has anyone tried it?

wler · Post by **wler** » Fri Aug 09, 2002 2:32 am

Well I use it for measuring beam ratios, absolute intensity for determining developing time, and for comparing laser power. The circuit is adapted from Sam's, and provides a low-impedance current-to-voltage converter which is important for high linearity. I made some measurements and the whole thing turned out to be very linear up to several mW.

The good thing: the photo diode UDT PIN10-AP comes with a 2% accurate wavelength sensitivity specification, ie, it has a guaranteed accuracy with respect to human eye spectral response - see data sheet on UDT web site:
http://www.udt.com/pdf/df.pdf
That makes for example possible to compare HeNe with Argon laser power with some reasonable accuracy.

It has also 100mm^2 sensitive area which is important for these applications - it averages out and also is not sensitve to precise beam location.

Also good: the diode can be obtained for $25, the seller also often has it on ebay for $20. See:
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 1756692903
http://www.mi-lasers.com/index1.html

Tom B. · Post by **Tom B.** » Sat Aug 10, 2002 2:58 pm

Nice contribution. So where's his/her web site?

Colin Kaminski · Post by **Colin Kaminski** » Sat Aug 10, 2002 4:17 pm

And I thought you had been everywhere. I pasted the link to the photo from his site and the image showed up instead of the link. I guess that is what the "enable formatted text" box is for.

Here is the root address:

http://perso.wanadoo.fr/redlum.xohp/argonlaser.html

Colin Kaminski · Post by **Colin Kaminski** » Sat Aug 10, 2002 4:25 pm

I know very little about zener diodes. Does ZD3.3 give me enough information to go to my local electronics store and order one? Is it a 3.3V zener diode and the resistance is not important?

wler · Post by **wler** » Sat Aug 10, 2002 6:13 pm

You can take any small-power 3.3V Z-Diode.

It is there only for the protection of the photodiode,
with regard to a too high reverse bias voltage.
That is, while many diodes can do 10 V or so, and some PIN diodes 30V or more, some are rated only at 5V max.

Even though the data sheet for the UDT PIN10AP is avalailabe at their web site, the max voltage is not listed in it; so I put 3.3V to be on the safe side. A higher voltage and a reduced series resistor would improve the dynamic range at the high power end.