[sdiy] CA3046 Heater / Substrate Question

[Roman Sowa]

Wow 4 seconds? That's quite an improvement.

I see your point. How about potting the chip in a blob of polyurethane 
foam? Looks utterly ugly and pain to repair, but not much heat escape 
then and 300mW will be more than enough.

BTW, I think you've reversed data sources on 2nd graph ;)

Roman


W dniu 2016-05-06 o 14:01, Nils Pipenbrinck pisze:
> On 05/06/2016 10:44 AM, Roman Sowa wrote:
>> You say, Q3 base is at -1.8, but with no heating current (at start) Q3
>> emmiter is at about -0.7, determined by 2 small resistors and BE
>> junction of Q2. So the substrate is higher than Q3 base and all
>> transistors are connected now, no substrate isolation.
>> Loose the LED and it should work, but substrate will be at very low
>> negative voltage now, I'm not sure if it will not degrade expo pair
>> performance.
>
> Hi Roman,
>
> thanks, that makes sense. I'll have thoroughly think through it, but
> it's clear that I messed up the substrate somehow. I just don't know
> where exactly the s. hits the f.
>
>> BTW, "better" is an enemy of "good". What is wrong with just 1
>> transistor heater?
>
> There is nothing wrong with it, but it's not ideal because the 1
> transistor heater can only dissipate 300mW. The cooling of the package
> takes away a big chunk of the energy we put into heating. So reaching
> tuning stability is slower than it could be.
>
>
> I did some measurements yesterday, measuring the Vbe drop directly while
> heating with two different target temperatures. I faked a bit, left the
> substrate transistor connected as usual and used one of the matched pair
> transistors as my second heater:
>
>     http://torus.untergrund.net/synth/heater_65deg.png
>
> Here I aim at 65°C. You'll see that the two transistor circuit reaches
> stability after 4 seconds while the one transistor circuit needs about
> 100 seconds:
>
>    http://torus.untergrund.net/synth/heater_90deg.png
>
> Here I aim at about 90°C. The two trannie method reaches stability after
> half a minute while the single transistor never reaches it's target
> temperature and asymptotically drifts to somewhere around 70°
>
>
> I think this is a great improvement over the one transistor circuit, and
> the substrate transistor is unused, so why not use it as a second heater?
>
>
> Best,
>    Nils
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