Need help understanding HP10544 osc oven circuit.
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From: John Larkin on 29 May 2007 11:28 On Tue, 29 May 2007 11:53:52 GMT, Fred Bloggs <nospam(a)nospam.com> wrote: > > >John Larkin wrote: > > On Sat, 26 May 2007 21:23:46 -0400, Jim Flanagan > > <jflan(a)tampabay.rr.com> wrote: > > > > > >> 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: > >> http://www.leapsecond.com/museum/10544/ > >> > >> > >> 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. > > > > > > > > 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. > >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. Then maybe you should give it another try. > > > > > 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. > >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. Roughly 7 volts. > > > > > 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. > >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. 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? John
From: Fred Bloggs on 30 May 2007 05:15 John Larkin wrote: > 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? > 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.
From: John Larkin on 30 May 2007 11:04 On Wed, 30 May 2007 09:15:43 GMT, Fred Bloggs <nospam(a)nospam.com> wrote: > > >John Larkin wrote: > >> 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? >> > >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. No, the sawtooth is not linear, but let's assume it's close enough. > 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. 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. John
From: Fred Bloggs on 31 May 2007 06:05
John Larkin wrote: > On Wed, 30 May 2007 09:15:43 GMT, Fred Bloggs <nospam(a)nospam.com> > wrote: > > >> >>John Larkin wrote: >> >> >>>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? >>> >> >>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. > > > No, the sawtooth is not linear, but let's assume it's close enough. > > >>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. > > > 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. > 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. |