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Need help understanding HP10544 osc oven circuit.

Discussion in 'Electronic Design' started by Jim Flanagan, May 27, 2007.

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  1. Jim Flanagan

    Jim Flanagan Guest

    Hi -

    Recently, I acquired an HP ovenized oscillator (HP10544A)which was
    missing the proportional oven controller. I found the schematic for
    the oscillator at the following link:

    After building the oven controller on a small PCB, I purposely disabled
    the unijunction oscillator stage initially, in order to test the
    controller. My thought was that the switching supply (if you want to
    call it that) section was just to make the the oven controller more
    efficient. With the oscillator disabled, I found that the oven would
    oscillate itself. From a room temp start, the oscillator oven would get
    to temp then shut itself off, then repeat at about a .1hz rate. It
    wasn't until the oscillator section was enabled, that the oven would
    work correctly. By correctly, I mean that as the oven approaches it's
    set point temperature, the op amp section would go into its linear mode.
    This is apparent as the heater current begins to decrease from about
    500mA to about 180mA.

    I would appreciate if someone would take a look at this circuit, in
    terms of the oven controller, and help me understand exactly how the UJT
    oscillator is functioning in this closed thermal system. Also, I would
    appreciate some input as to how an simulate a thermal system like this
    in spice. I simply don't understand how to simulate the thermal
    feedback portion.

    Your insight and expertise is appreciated... Thanks
  2. John Larkin

    John Larkin Guest

    If the ujt is off, the loop gain is very high and the thing pretty
    much works in bang-bang mode. A very small change in opamp output will
    slam the heater full on or full off.

    The Q1-Q2 differential pair compares the opamp output to the swatooth
    created by the ujt. So the amp output has to span a roughly 7 volt
    range to move the heater from full off to full on, which is
    effectively a much lower gain.

    One less obvious advantage of pwm, as compared to a linear system, it
    that it makes heater power linear on error. A linear voltage or
    current drive into a heater is a square function.

    You can model the thermal stuff as a group of resistors (thermal
    resistance) and capacitors (thermal masses). The approximate (within
    5%) mapping is

    1 ohm == 1 degc/watt

    1 amp == 1 watt of heat

    1 volt == 1 deg C

    1 farad == 1 gram of aluminum

    the catch being that the components tend to be distributed, not
    lumped, so nasty diffusion math applies.

  3. MooseFET

    MooseFET Guest

    Also R11 and Q4 seems to be drawn wrong.
  4. Jim Flanagan

    Jim Flanagan Guest

    Hi John -
    Thanks for the explanation. Seems obvious now. Another case of not
    seeing the forest through the trees.

    One last thing, do you have a suggestion as to a literature reference or
    tutorial in regards to the thermal modeling?

    Again - Thanks a bunch...
    Take care - Jim
  5. John Larkin

    John Larkin Guest

    Yeah. The PNP should point the other way. Weird.

    Maybe Jim can check and see how it's actually built.

    SRS sells some clones of the old HP ocxo boxes, with nice SC-cut
    rocks. Their stability and phase noise are impressive, but their
    thermal design is bizarre. They use TO-220 voltage regulators as
    heaters, and it looks to me like whoever designed the loop didn't
    really understand the dynamics, so they used a proportional-only loop
    with fairly low gain, then added feedforward compensation from an
    ambient temp sensor to improve temperature regulation.

  6. Jim Thompson

    Jim Thompson Guest

    I can't seem to retrieve this thread's references, but it sounds like
    an inverted bipolar device used for AGC.

    Can someone repost the schematic?

    ...Jim Thompson
  7. John Larkin

    John Larkin Guest

    I actually meant that Jim Flanagan might check the actual oscillator
    so see how the heater driver PNP works.

  8. MooseFET

    MooseFET Guest

    That is very strange. SRS seems to know what they are doing in the
    other stuff they do. I can even see an argument for using a 3 pin
    regulator as the heater. They give a huge current gain, are self
    protecting and don't have their own tempco.

    The components needed for a PID controller wouldn't add much to the
    size of the circuit. These days op-amps with very low bias currents
    can be obtained easily so the capacitors don't have to be huge.
  9. Jim Flanagan

    Jim Flanagan Guest

    I caught that also. My unit didn't come with a controller board.
    It had been removed for some odd reason. That is why I built a replica
    and the subsequent initial posting concerning the thermal feedback
    question. I built my unit the way we believe it should be. All is
    well now, functionally.
  10. Fred Bloggs

    Fred Bloggs Guest

    Ummmm, looks like U1 is integrating the thermistor bridge error with a
    0.001Hz time constant and ~40dB gain, so hard to visualize a "slam"
    on/off action in that circuit.
    Ummm, since the Q4 current gain is so high, U1 is required to inject
    just a few uA into Q1 base for full-on of the Darlington. There is not
    going to be much of a "span" about this Q1 base voltage threshold.
    The UJT will be oscillating at something around 4KHz, and its purpose is
    to chop the Q4 drive to minimize power dissipation, nothing exotic there
    in the way of control loop processing.

    The funny labeling on the heater wires and the diode in Q4 lead me to
    suspect the heater drive may be AC...but then the color coding makes me
    think it should not be.
  11. MooseFET

    MooseFET Guest

    How do you get 0.001Hz?

    51.1K * 2 uF -> 1.55Hz

    5.6M * 2 uF -> 0.014Hz

    The latter being the zero in the PI controller, I'd expect the gain
    cross over to be near that point.

    It is the voltage on the base of Q1 that matters here.
    Q2 is fed with a ramp from C1.
  12. John Larkin

    John Larkin Guest

    Then maybe you should give it another try.
    Roughly 7 volts.

    Turning on the ujt cuts loop gain by a factor several hundred, and
    changes it from super-nonlinear to mostly linear. Do you think that
    might affect loop dynamics?

  13. Fred Bloggs

    Fred Bloggs Guest

    Looks like someone needs to review their basic arithmetic. In the
    simplest case of linear PWM, the change in output power per unit of
    error voltage drive into the UJT modulator remains the same over the
    span (7V) of modulator input voltage. This is not a gain reduction of
    several hundred. The circuit is a simple dominant pole regulator with
    temperature->voltage->power->temperature forming the loop states.
  14. John Larkin

    John Larkin Guest

    No, the sawtooth is not linear, but let's assume it's close enough.
    The gain reduction is relative to when the ujt oscillator is stopped.
    In that case, the system becomes continuous/nonlinear with very high
    gain, and becomes unstable as the op noted.

  15. Fred Bloggs

    Fred Bloggs Guest

    The gain reduction is relative to a completely different circuit
    topology when the ujt is stopped, so the comparison is not meaningful.
    Also, I'm not sure it even makes sense to use the word "gain" in that
    context because the loop becomes a simple two-state in each of which the
    incremental gain is zero. The experiment may be used to infer the
    thermal time constant of the heater system and that's about it.
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