Need help understanding HP10544 osc oven circuit.
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From: Jim Flanagan on 26 May 2007 21:23 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. Your insight and expertise is appreciated... Thanks Jim WB5KYE
From: John Larkin on 26 May 2007 23:21 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. 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. John
From: MooseFET on 27 May 2007 00:09 On May 26, 8:21 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sat, 26 May 2007 21:23:46 -0400, Jim Flanagan Also R11 and Q4 seems to be drawn wrong.
From: Jim Flanagan on 27 May 2007 09:40 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. > > 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. > > John > > > 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
From: John Larkin on 27 May 2007 13:16
On 26 May 2007 21:09:33 -0700, MooseFET <kensmith(a)rahul.net> wrote: >On May 26, 8:21 pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sat, 26 May 2007 21:23:46 -0400, Jim Flanagan > >Also R11 and Q4 seems to be drawn wrong. 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. John |