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On Thu, Sep 1, 2022 at 04:33 PM, Dr. David Kirkby, Kirkby Microwave Ltd wrote:

I have just ordered a couple of cheap IR photodiodes from RS

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It would be interesting to try these IR photodiodes with a very small lightbulb as the current sensor, where the measured current just brings the filament to the dull red glowing range. Wonder if the increasing resistance with temperature of the filament would be an issue. Possibility causing the filament output to go from barely readable (under range) to bright glow (and over range) over a small current range. Or if operating at the "low" (barely glowing) filament temperatures would mitigate that. Sounds interesting to experiment with.?

Tom, wb6b

I would not expect it to be necessary for the bult to be visible to the human eye before an IR photodiode could detect it. But I don't know. The issue I see with bulbs is their inductance. The other thing is I want to use a step-down transformer to drive the resistor. That probably means having one turn on the secondary. That one-turn could be a piece of nichrome. It is available as a strip, which would reduce the inductance compare to a cylindrical wire.

I use nichrome wire to cut foam inserts for VNA calibration kits my company sells. I use a fairly thick wire, and put 5.5 A in it, so it glows red hot. But I will be able to put the photodiode near that and see at what current I can detect it. I would hope the IR could be detected long before I can see the wire getting hot, but maybe not. At least it costs virtually nothing to try. I have a high-resistance meter which will read pA (maybe even fA), so knocking something up quick should be easy.