Re: Oscillator running on home made thermocouple

Sep 30, 2012 4 Replies
J

On a sunny day (Sun, 30 Sep 2012 11:24:25 -0400) it happened Spehro Pefhany wrote in :


There was a Scientific American Amateur scientist column on making a
>small transistorized battery-powered RF probe that would change
>frequency with temperature. It allowed the person to map the
>temperature profile through the GI tract of his dog as well as acting
>as a beacon to retrieve the probe from a corner of the back yard.
>
>Of course a thermocouple will not produce any usable voltage in such
>an essentially isothermal situation.
>
>
>Best regards,
>Spehro Pefhany

I actually found the reason for the voltage difference, it is simply the voltage drop in the thermocouple resistance wire due to current. Also this thermocouple I am using has resistance wire taken from an old wirewound resistor. I measure about 1.5 Ohm or so (for 10 cm) so 18 or 15 Ohm / meter. Now you can calculate the current for the 10 BF254, it is quite a bit.

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J

current.

wirewound resistor.

I'll try the thermocouple idea. I have a roll of thermocouple twin lead so that's no problem here, simply twist the ends and use the gas soldering tool with silver bronze solder.

Jamie

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S

current.

wirewound resistor.

There's a thermodynamic thing going on here- the less heat you have flowing down the thermocouple wires, the less power is available for your circuit. Thicker wires will tend to suck more heat from your source and thus reduce the available temperature difference. Make them really long to reduce the heat flow, and you get more resistance again. No free lunch.

Best regards, Spehro Pefhany

"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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J

On a sunny day (Sun, 30 Sep 2012 12:05:36 -0400) it happened Spehro Pefhany wrote in :

current.

wirewound resistor.

Yes, but this seems a good compromise :-) I will probably play with it a bit more later, things are in motion.

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J

On a sunny day (Sun, 30 Sep 2012 12:00:07 -0400) it happened Jamie wrote in :

current.

wirewound resistor.

Yes that is cool. I just soldered these things, and the solder already melts at about 200 °C.

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