Fluke 80TK Thermocouple Module

[pigrew]

I've not seen any photos of the innards of the Fluke 80TK thermocouple module, so I figured that I should post some.

The module amplifies a type-K thermocouple voltage to a compensated voltage that can be read with a voltmeter. I bought it so that I can log temperatures using older bench DMMs which don't have temperature modes. This unit is very convenient, though I expect that a "better" one could have been built for the same cost in parts using a thermocouple amplifier IC (such as the AD8495).

My particular unit has a copyright 1984 board, perhaps rev N if I can read the stamp.

The reference compensation happens via a BJT (2N2484) sandwiched between the thermocouple terminals. It doesn't have good thermal conductance to the thermocouple terminals, but I guess it's good enough.

The unit is first calibrated by adjusting two pots (coarse and fine adjustment) to get the centigrade reading right, and then a third pot to correct the Fahrenheit temperature.

This unit is an older revision, with the schematic in the attached PDF. I'm also attaching a newer manual which contains a schematic for the surface mount revision of the board. There are minor differences (mostly selecting SMD replacements), but some other tweaks like removing a JFET current-source.

[pigrew]

Extra information:

My unit has a standby current draw (at 9V) of 4.7uA. Turned on, it draws 211 uA.

The new model seems to have changed how it splits the rails (so that it can output both positive and negative). It's likely that its standby current is different. The op-amp is being powered by -0.5 and 4.5V.

Another note is that the screws are Pozidriv #1. Using normal Phillips/cross-drivers is likely to damage both the screw and the driver.