Hi Folks,
Bear with me a second – when I was at work they had a 100 Amp. PSU with a brass/manganin ammeter shunt (200 Amps. 200mV). When the power was switched off the meter would read up to 0.3 A. this would fall to zero when the shunt cooled. The more positive side of the shunt would be the hottest; if the heat sink was cooled with compressed air the meter would fall rapidly to zero, and if continued the reading would change polarity and go the other way as the other side became the hottest. This movement of energy/electrons could not form part of the main current because this would violate Kirchhoff’s Law; all it did was contaminate the meter reading. This was without doubt the Seebeck effect. This makes it hard for me to grasp why it affects the op-amp circuit, although I know it does because I have experienced the phenomena. Unclejed suggested reading the data sheet; the only thing there, that may help, was the insertion of unnecessary joints, strange since originally I had to reduce the joints to a minimum. Having made six joints (three pair) thinking they could swap electrons; this didn’t make a scrap of difference. I came across a board in my junk box with copper strips made up of three hole segments. If an IC socket is placed on it, only one hole is left and if this is used for the supply wires it might isolate the supply from the rest of the board, suspecting that these high initial readings are from the power supply, how I haven’t a clue. At the input and output this one hole is not enough to accommodate wires and resistors, so a link was put in to next segment, this made 7 joints at the input, counting adjacent holes soldered together as two joints.
The initial reading was then 1.2 mV, this is where I came in. Adding another joint to the left of the strip to #2 pin caused the reading to go to 350mV. So the joint didn’t draw the electrons from the op-amp so I removed it.
THE INITIAL READING WAS THEN 0.1 mV; I can live with that!! I have been switching it on and off periodically this initial reading seems to be sticking and amplifying to 99.5 mV. So I think the board and the joints may have done the trick. I am not yet opening the champagne or taking any credit because without you guys I would have still been thinking:” Well it worked before.”
See what tomorrow brings before handing out the electronic forum equivalent of an Oscar. - Thanks