One technique , I think it was an HP RF power meter. Used a thermistor in a bridge configuration, which had a bias supply to set the thermistor to a particular resistance. When the RF was applied, it unbalanced the bridge. The bias, was (automatically) controlled, and reduced until the bridge was in balance. And this control loop indicated the applied RF power.
On Fri, Sep 2, 2022 at 03:34 AM, Dr. David Kirkby, Kirkby Microwave Ltd wrote:
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.
This will be very interesting to find out at what point the temperature of the wire gets to the point that it can be detected by a near infrared diode. Definitely has a possibility of being good low cost method. While I was thinking about light bulbs because the filament may be thermally isolated inside the bulb, the possible inductance issue is well taken. Plus nichrome wire, if ?remember correctly, has a fairly low change in resistance with temperature, unlike the light bulb.?
Will be interested in watching for your results. And if the temperature that the nichrome wire may need to operate at is fairly high, this may lessen the need to compensate for ambient temperature.
