[time-nuts] Mini ovens packaging

Attila Kinali attila at kinali.ch
Tue Aug 16 15:28:33 EDT 2016

Hoi Guillermo,

On Mon, 15 Aug 2016 11:07:41 +0000
Guillermo Sobreviela Falces <Guillermo.Sobreviela at uab.cat> wrote:

> -        Temperature stability of 0.1ºC, what means temperature control 
> included.

As Bob already wrote, this value has little meaning without the temperature
range at which you want to operate the oven. From what you wrote, i guess
it will be some office environment or a place with relatively low temperature
swing? Then it should be easy to get to that level.

> -        Inner cavity of 2cm x 2 cm x 1 cm.

That is nice and small. How much space do you have on the outside?
Does it need to fit into something else?

> -        In addition to the connectors needed for the oven I need 3 pins 
> reserved for the system.

That's easy to acheive. Mechanically, the easiest construction would be
to use a PCB with cut-outs and put some CNC'ed aluminium case through
these cut-outs. The PCB then acts as mounting system and electrical connection.
The thermal conductance of a PCB (without copper) is moderately low.
Maximum temperature for standard PCB's is 80°C, for higher
temperatures you need special base material with higher Tg (at least 20°C
higher than the maximum expected temperature).

> -        The system must be included into the oven via a chip socket, or 
> pasted inside with silver paint and wire bonded to the oven connections.

I do not get what you mean with this.

> -        PCB compatible oven
> I have been looking for a Peltier junction in order to reduce the oven
> temperature to 50ºC (Reducing the temperature is good for my system).
> But the temperature stability is a must and I am not sure of the precision
> level I can reach with a Peltier junction.

For a classical oven, the inner temperature is defined by the maximum ambient
temperature and the minimum required temperature delta. A minimal temperature
difference is needed to keep the loop stable.

Peltier elements have the nice advantage that you can cool directly and
thus are not bound by the temperature difference to ambient. But you need
to be aware that Peltier elements are not symmetric: They are much better
heating elements than cooling elements. This means that you need to design
your loop carefully to prevent oscillation.

> Also, I have been designing a temperature detector based in a power divider
> using a resistor and a RTD that will trigger an OPAMP (comparator). This
> OPAMP will activate/deactivate a high power BJT or FET transistor that will
> warm the oven. In order to reduce the thermal inertia I thought to make an
> escalated temperature detector as the shown in the attached picture. The
> main idea is have a fast heating and reduce the warming current when the
> temperature is approaching the final equilibrium temperature (120ºC).

Instead of using a bang-bang controller as you have shown, it is better to
use a linear controller, that controls how much current is flowing
through the FET. This eliminates temperature cycling and current spikes
that may disturb your oscillator.

> Another problem is related to the shell of the oven. It should be metallic
> in order to equilibrate the temperature, but, would it be interesting to use
> an external shell made of an isolator material in order to reduce thermal
> loses or will it difficult the thermal stability?

Depends. In theory, it's good to have the oven well isolated. But you need
some way to get the heat out of the oven. And this thermal resistance
must be low enough to keep the loop stable. You need to calculate the
thermal parameters for this. If you are not 100% confident with this,
start with an unisolated oven, see how it performs on a step response,
add isolation (can be something as simple as some pieces of cardboard),
tweak the loop parameters a bit and check the step response again.

> After this discussion, would it be possible to buy a system that meet
> the characteristics described above? I am not sure about I would be able
> to design and test this system on time, but I need this kind of oven with
> this temperature stability as soon as possible (This is why I am looking for
> a commercial solution).

It is very unlikely that you will find an off the shelf system that does
what you need. But you can find a company to design you one. Most people
with a decent electrical engineering knowledge should be able to do so.
It is also relatively easy to do it yourself, if you are not out for optimal
performance over an wide range of temperature and are ready to measure
and tweak the system parameters a bit after production.

The simplest is probably to start with a PI controller, which are fairly
easy to keep stable and only have two knobs to turn. Use that as a basis
with some simple mechanical setup. That should get you already to the 0.1°C
in an office environment. 

				Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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