On Sun, 2011-02-13 at 07:59 -0600, John Thornton wrote: > use a multiplexer to get 4 thermocouples into one MAX6675
Given the microvolt-level signals from a thermocouple, it's not clear the signal emerging from a multiplexer would bear more than a casual relation to the actual temperature. With any additional conductors / connectors in the middle of a run of thermocouple wire, each temperature gradient adds *another* thermocouple joint with its own voltage to the mix. An analog multiplexer adds several solder joints & socket pins, each with its own temperature gradient, to the input and output signals. Inside the chip, the signal faces silicon offset & leakage voltages that are about the same size as the thermocouple signals. While I'm sure multiplexing thermocouples can be done (nay, has been done), it's a nontrivial project to get *accurate* measurements out of the far end. The MAX6675 is actually a pretty good deal, considering that it goes directly from thermocouple microvolts to Celcius digital values: think of it as buying engineering expertise. DIY op-amp circuitry faces similar problems: while it can be done, *accurately* amplifying microvolt DC signals and stabilizing the result is a nontrivial project. If you do use MAX6675 chips in a situation where the thermocouple bead could contact a live wire, take this problem into account: http://softsolder.com/2011/02/06/thing-o-matic-mk5-extruder-protecting-the-thermocouple/ Sounds like you've got a fun project, though... -- Ed http://softsolder.com ------------------------------------------------------------------------------ The ultimate all-in-one performance toolkit: Intel(R) Parallel Studio XE: Pinpoint memory and threading errors before they happen. Find and fix more than 250 security defects in the development cycle. Locate bottlenecks in serial and parallel code that limit performance. http://p.sf.net/sfu/intel-dev2devfeb _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
