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* If anyone would like to play along, I could use some help with a
combination heater/temperature sensor using an up to 16-inch (406.4mm)
nichrome wire threaded through ceramic cores (inside a 1/4x4-inch copper
tube that's closed on one end) with one side connected to ground and the
other to a SPST relay so that when the relay is powered, the wire will
be heated with AC power; and when the relay is unpowered, the resistance
of the wire can be sensed to determine its temperature. The sensing
circuit needs to produce an output voltage that can be fed to an Arduino
A/D converter with a 5VDC Vref and sharing the ground connection. Heater
power needs to be something that can be reasonably derived from 120VAC
60Hz mains to provide power in the 250W neighborhood. I just uploaded a
photo of some ceramic cores to
http://www.iedu.com/Solar/Electricity/Fusion/Core4(500x400).jpg
Hi Morris,
Not sure I clearly understand what you want, but I'll stick my neck out!
After you have heated the wire then you want to measure the resistance
of the wire and from that deduce the temperature. I would want to
totally isolate the AC from the resistance measurement. A DPDT relay
would do that. (isolate both ends of the wire. I don't know about
connecting AC neutral to 5v ground, might be ok, but...)
Unless you cycle the relay for heat/measure modes, I don't see how you
will know when to stop heating. So, you have the relay on some clock
cycle, (heat 5 seconds measure temperature 1 second) then when the
proper temp is reached the relay measures temp only until temp drops
below some threshold then your heat/measure cycle starts again.
To measure the resistance of the wire, you will drive some DC current
through the wire and a series (sense) resistor and measure the voltage
across the sense resistor. The voltage will probably need to be
amplified before going to the Arduino.
There my be a way to sense the AC current to the wire and know the
temp of the wire but any AC voltage fluctuations to you home may cause
errors. I'm sure there is a way to compensate for that though.
Ok, have I got the basic idea?
Fill me in.
Thanks, Mikek