On Wed, 23 Aug 2006 19:07:45 +0200, Steve <
alienrelics@...> wrote:
> We should probably move this to Electronics_101. ;')
> A silicon diode can be used as a temp sensor for cold junction
> compensation for thermocouples. Very linear around room temp,
> approximately -2mV/C to -2.2mV/C. Practically free, add an OpAmp
> before feeding it to your micro.
> You can build a very simple and cheap wheatstone bridge thermometer
> with a silicon diode and a DMM, without any Op Amps. A low dropout low
> power linear regulator is the only active component needed, output can
> be scaled to 1mV/C or 1mV/F.
> It does change very slightly at the extreme ends of an Si diode's
> useful range, but within the range of room temperature it is very
> linear. You can get complicated and calibrate by bringing distilled
> water to a slushy frozen and liquid state and then boil distilled
> water to get 0 and 100C and calibrate the exact mV/C, or you can stick
> it under your tongue and call that 37C and assume -2.1mV/C (split the
> difference btwn 2.2 and 2). Note that I've labeled it as negative
> because the voltage goes down as temp goes up.
Yes, this is all very well, but there is more.
First, the compartment in the oven, even with insulation, gets pretty hot,
so that makes it all worse than usual.
Second, you need to design the whole thing with temperature drift in mind.
It does no good to measure the cold junction with a diode if the
temperature drift of references, opamps, and even resistors is not taken
into consideration. A precise reference with low drift is another $5 ic.
Thinking about it the elektor thing appeals more and more.
ST