[sdiy] expo pair "heat shield?"

Wed Nov 2 11:41:48 CET 2022

Sounds like you went on a very informative journey! :-)

A method I have used in the past to stabilise the temperature of 
electronics during some critical measurements is to put the circuit in 
the middle of its own plastic box, and then fill the box full of little 
polystyrene spheres.  Think of the little polystyrene balls you get if 
you smash up a polystyrene ceiling tile or some polystyrene packaging.  
The polystyrene is a good thermal insulator to stop thermal conduction 
and filling the box with these polystyrene balls breaks up the air into 
lots of tiny pockets in order to hinder air circulation due to 
convection.

-Richie,

On 2022-11-02 09:56, Spiros Makris via Synth-diy wrote:
> I have built two different VCO prototypes some years ago, using
> different methods of thermal stabilisation.
> The first was two 3906 transistors that I strapped together, putting a
> resistor between them as a heater. I added copious amounts of thermal
> paste, since they are supposed to be at the same temperature. The
> second used a monolithic transistor array from THAT in a thermal oven
> type of configuration; two of the transistors formed the pair while
> the other two worked as a heater and temperature sensor, creating a
> thermostat. The first one kinda worked but was very sensitive; the
> second version is fairly stable as long as you don't place it on
> strong air currents.
> 
> According to the analysis for the expo pair, the two transistors need
> to be at the same temperature and the input voltage is scaled for that
> temperature. Change the temperature and you have to change the
> scaling; the two most common methods are to either keep the
> temperature constant or observe its variations and actively scale the
> voltage in order to keep the tuning stable.
> 
> My first prototype was an open-loop version of the constant
> temperature method. The idea is that by dissipating enough heat on the
> pair, you can heat it some degrees above ambient, and that will
> eventually reach an equilibrium temperature. This final temperature is
> strongly dependent on ambient temperature as well as variations. A
> strong enough heater will mitigate some of that, but this eventually
> means that your tuning is almost seasonal, as I painfully found out.
> Another interesting effect was self-heating. I was using LM13700 for
> the oscillation core, so I had 2mA as my upper limit. In order to get
> the whole range of the IC I used 2mA as my maximum output current,
> which apparently is enough to heat up the transistors. The result was
> that my tuning would change if I played long enough on the higher
> registers. Now that was crazy!
> 
> My version did not use any kind of thermal shielding, I think that if
> you encapsulate the whole thing in a heat-shielding compound, you will
> make this process more predictable and improve stability. I'm not well
> versed in heat transfer but I think that eventually after enough time
> has passed, the encapsulated pair will stabilize on a different
> temperature that is still dependent on ambient, but this process will
> happen so slowly that your jam session would be finished before that.
> 
> The second version was a closed-loop version of this idea. It required
> 20-30 minutes to reach the final temperature (the heater transistor
> could not go above a few mA) but could do so reliably in all seasons
> and would even resist light air currents. The target temperature you
> pick should be above the highest ambient temperature you might ever
> encounter since your system can only add heat, ie increase its
> temperature. I am located in Greece, so I assumed 40C as the highest
> ambient temperature, and set my pair to 55C.
> 
> The temco solution is also closed-loop and as such, can correct for
> various ambient changes. Its weakness is that there is a lot of
> hysteresis between the actual thing whose temperature you care about
> (expo pair) and your sensor, meaning that it will adjust for average
> temperature, but will not fair well with transients. Of course, most
> synths are inside a case so that's usually not a problem. Positioning
> the circuit away from heat-generating parts, such as power regulators,
> and placing the tempco close (stepped on?) the pair would make the
> performance more predictable.
> 
> The biggest weakness of my monolithic solution is that these kinds of
> arrays are rare in the market. I needed 4 PNP transistors and THAT was
> the only option I had, however, there's lots of monolithic transistor
> pairs that could be used, if the thermal loop becomes external. It
> would be interesting to encapsulate the pair, heater and sensor and
> see what kind of performance you can get from it.
> 
> Good luck!
> 
> On Fri, 14 Oct 2022 at 05:30, drheqx via Synth-diy
> <synth-diy at synth-diy.org> wrote:
> 
>> We use a thermal potting compound to encapsulate military modules.
>> The silicone is not type one
>> ( acetic acid ) and has minimal odor. The key ingredient is
>> something like sand or glass spheres
>> that conduct the heat away from the devices. The thing is in this
>> case do you really want the heat to go away
>> or do you want it to become a constant?  Old Tektronix vector scopes
>> that I used to repair by the dozens used a
>> brass looking "oven" to encapsulate parts in several areas. These
>> things had 25", I think, CRT displays and they were mostly
>> analog. They definitely had items that needed to stay thermally
>> consistent, but not necessarily cool.
>> 
>> Just a thought about the oven idea.
>> 
>> Mike
>> 
>> On Tue, Oct 11, 2022 at 1:28 AM Neil Harper via Synth-diy
>> <synth-diy at synth-diy.org> wrote:
>> 
>>> hey all,
>>> 
>>> just wondering if anybody thinks it's worth encasing an
>>> exponential pair
>>> for stability. while this is in my modular case, there must be
>>> some heat
>>> currents moving around from the other stuff that share the same
>>> space.
>>> 
>>> Maybe a dab of hot-glue-gun (hot snot), or a silicone or
>>> something?
>>> 
>>> I'd hate for it to bite me in the arse in the future, maybe
>>> corroding
>>> the copper traces or the tin legs or becoming impossible to
>>> remove. What
>>> do the pros use?
>>> 
>>> --
>>> /// Neil Harper
>>> /// Every Wave is New Until it Breaks
>>> 
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