At 10:38 PM 12/27/01 +0000, Dave Dilatush wrote: >ashly Dearden wrote... > > >I'm thinking of doing some temperature sensing using the approach that > >Dallas and Maxim has with the current source and measuring the voltage > >across a diode-connected transistor. They don't go into alot of detail on > >what they are doing...other than around 100uA of current. I assume this must > >be a stable current source and just measure the voltage that is generated? L Stephen Woodward did a design idea several years back in Electronic Design or EDN (I don't remember which) but he credits Jim Williams of Linear Technology for the idea. Its actually pretty simple: modulate the forward current by 10:1 (100 uA, 1 mA). AC couple the resulting voltage change to an amplifier and use a synchronous detector to rectify the output. Feed to an A/D convertor. I did a simple version a few years back where a PIC pin supplied the modulation current. Calibration was simple: just adjust the gain (in software) so that the display agreed with the actual temperature. Because it was AC coupled, there is only a gain adjustment required: AC signal amplitude is a straight line from absolute zero (0K) to whatever temperature is being measured. Simple and accurate! Long term accuracy is dependent on supply voltage but I use LP2950 regulators which seem to have adequate long term accuracy. I did find that diode connected transistors (B & C tied together) worked much better as sensors than just simple diodes - I could swap in a 2n4401 or 2n4403 as the sensor without affecting calibration. I've got source code (12ce673) and a schemo somewhere - it was part of a project that got put on the back burner. dwayne Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 17 years of Engineering Innovation (1984 - 2001) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads