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Message ID: 5976     Entry time: Tue Nov 22 06:12:43 2011
Author: Zach 
Type: Update 
Category: RF System 
Subject: Prototype temperature controller 

Tonight I built a simpler version of what will be the new general-purpose precision temperature controller. This one is built on a breadboard and will be used for RFAM testing at the 40m until a better version is made. Some  differences between this version and the final one:

  • In the interest of time, this controller senses temperature using a DC wheatstone bridge, instead of the audio-frequency bridge of the final controller.
  • I eschewed the more complicated transistor current source in favor of a simple current buffer. In effect, using a constant-current source is not absolutely necessary, since we are not interested in constant current but rather a constant system temperature. In this sense, it doesn't matter if we have a transistor current source or a transistor voltage source or a current-buffered op amp voltage source; the loop will simply drive the heater with the proper current to keep the error signal nulled. 

So, how it works:

  1. The DC bridge drive voltage is supplied by a voltage-divided and buffered AD587 (low-noise 10-V reference).
  2. The reference resistors are just 1% metal film leaded resistors, but I have put some effort into making them quiet:
    1. Each resistor's body is wrapped in Al tape, and then all the resistors are taped together using Al tape, as well. This is to strongly couple them to each other thermally.
    2. All the reference resistors are embedded in some foam I found in the Bridge sub-B hallway. It's nothing fancy, but it keeps large advection currents from causing thermal drifts.
  3. The sensing element is a PT100 100-ohm RTD. Tempco is ~0.0037 1/K
  4. The bridge differential voltage is read out by an AD620 instrumentation amplifier with G = 100
  5. The AD620 output is fed directly to an OP27 with G = 0-20
  6. This is fed to an LF356 (FET-input op amp, to reduce the effects of bias current when the integrator is on) with a single pole at 0.1 Hz, switchable via jumper to DC for true integration
  7. This is summed with an offset via an OP27 summer (the offset determines the heater current with no signal---half the maximum current of ~120 mA is optimal)
  8. The summer output is buffered with a BUF634, which can provide well over the maximum current we can push through our heater, and the BUF634 directly drives the heater
  9. Between the BUF634 and the heater is a back-biased diode to ground. This is to prevent the current from going negative when the error signal is well below zero.

I have tested the circuit using a spare resistive heater and a potentiometer to simulate the RTD. First I tested the sensing and drive circuits separately, then I connected the sensor output to the drive input and modulated the potentiometer resistance while monitoring the current. The circuit behaved as expected.

When I got to the 40m, it struck me that the resistance I had chosen (115 ohms) corresponded to 40 C, which I realized might be above what we could reach with the current we can provide. I used the Newport 6000 via telnet to drive the heater at several current values and see what the resistance became. I found that with I = Imax/2 ~ 0.6, the resistance was around 113 ohms (it was ~111 at room temp). So, I switched the reference resistor in the leg above the PT100 from 115 -> 113.

I then plugged everything in while monitoring the heater current and AD620 output (error signal), and it seemed not to do anything. I was tired so I figured I'd leave it for tomorrow.

Here is a sketch of the schematic, as well as some pictures:

refres1.pngrefres2.pngproto_temp_ctrl.png

schematic.png

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