[time-nuts] HP10811 current versus temperature

[Bruce Griffiths]

Didier Juges wrote:
> It took longer than I anticipated, but I have the starting point of an
> adequate data logger.
>
> For now, I simply recorded the HP10811 heater current versus ambient
> temperature in my shack over night.
>
> It was a little more involved than I anticipated because the temperature
> variations right now are not much.
> So I ended up having to use a processor with a 24 bit ADC (Texas
> Instrument/Burr-Brown MSC1210Y5), a stable voltage reference (AD580) and a
> precision op-amp (OP-27) to measure the voltage drop across a 1 ohm
> precision wire-wound resistor (type has been long forgotten).
>
> The op-amp is wired as a diff-amp to amplify the signal (gain of 5 approx)
> and eliminate ground differences between the microprocessor board and the
> HP10811 supply board.
>
> I checked the microprocessor data against my HP3456 connected directly
> across the 1 ohm resistor and the readings were within 0.1% of each other
> over a period of > 1 hour. The weak points are most likely in the 1 ohm
> resistor and to a lower extent the 4 resistors used in the diff amp (all
> RLR05C metal film type, I will try to get some RNC55). I am not sure what to
> do about the 1 ohm resistor, I don't have a lot of choices there at the
> moment. 
>   
Ideally you should use a 4 terminal current sensing resistor and a high
input impedance instrumentation amplifier.
A Thomas style 4 terminal resistor in a temperature controlled oil bath
would be nice, but failing that a low tempco 4 terminal resistor will
suffice.

Bruce