[time-nuts] Re: HP Z3801A project update

[ed breya]

Tom Holmes wrote:
"I am curious about one part of the warmup process. At around 7 minutes, 
the power jumps up radically, which you attribute to
the outer oven kicking in. It has often been stated on this list that 
the outer oven was intended for use during really cold
starts, which I would expect should cause it to kick on almost 
immediately during a very cold start. I am assuming your start
was from room temperature. For a room temp start, I wouldn't expect it 
to kick on at all  if it's purpose was as reported. Or
do I have the oven functions reversed?"

By "cold start," I mean powering it up when the system has been off for 
some time, and everything is near ambient, regardless of temperature. 
Right now, the system is indoors, at comfortable room temperature. 
Normally, it resides in the garage, pretty much at outdoor conditions.

I have observed and studied the behavior quite often over the years, and 
especially, I learned a lot about the innards long ago, when the opamp 
that controls the inner oven had crapped out. I had to take the whole 
thing apart, deep into the guts, while not damaging anything. The 
surgery went well, but I sure would not want to do it again.

I've never worried about the startup, since the AC supply had no problem 
at all firing it up. Now that I'm finally wrapping up details like 
external DC running and starting, I've had to scrutinize the situation. 
Here's my take on what it does, and why.

To reduce startup power (besides inrush current to charge caps and get 
the DC-DC converters going), the ovens are sequenced. The priority goes 
to the inner one first, to get the system operational ASAP. If say, it 
was not a double oven system, but only a single, the OCXO would just do 
its thing and be ready to go in short order, regardless of ambient 
temperature. The outer oven is the icing on the cake, so to speak - it 
catches up later, to provide the higher ultimate stability. It's also 
necessarily slower (unless a lot more power and a bigger heater element 
was used), because of the much larger thermal mass and area involved.

The outer oven is always active in steady state conditions. I can see 
how there may be some confusion and misinterpretation about its role and 
operation, depending on when observations are made, and the initial 
conditions. If one were to be fooling around with power ups and downs, 
trying to figure out what it's doing, the actual temperatures inside can 
be all over the place, and change the apparent response. For instance, I 
found that the system had to be shut down for several hours to be sure 
of near-ambient conditions for my cold start tests. Restarting a test 
run too soon, say, while the inner oven is still near setpoint, may make 
it appear that the outer oven comes on immediately at power up. In 
reality, it just means the first step has been skipped or greatly shortened.

Regardless of the initial conditions, the operating sequence is the 
same. All the circuitry and the inner oven are immediately powered up. 
When the inner oven reaches its setpoint, it signals the brain that it's 
ready, then after a short time delay (maybe up to 15 sec or so - hard to 
tell), the outer oven driver is enabled.

The actual conditions at power up simply modify the heater load currents 
and timing. After a power dropout (assuming already in steady state) of 
a few seconds, the whole power up sequence may be all done by the time 
you can make a measurement, and barely noticeable.

One thing I've been curious about is what's going on at the very moment 
the outer oven turns on (during cold start). The power jumps 
dramatically to some peak level, then quickly (a matter of seconds) 
drops to a slightly lower level, then very slowly as it continues. I 
haven't been able to catch the actual peak value, with just DVM 
observation, but I know it's pretty big, and quick. This is most likely 
an artifact of electronic response, not thermal. I believe it's caused 
by an overshoot in the oven's PID loop during activation.

I'll have more to say next time on the thermal stuff and the power 
system in the Z3801A.

Ed