[sdiy] Tempco vs. "Substrate Oven" in VCO and VCF

[sleepy_dog at gmx.de]

Ah, the original dude himself appears :-)
Thanks for describing the procedure! I'm sure I would have fried at 
least one IC otherwise - well I might still, but maybe fewer :-D

Thanks also to the others who replied with some interesting comments.


Steve

Am 29.02.2016 um 15:05 schrieb Rick Jansen:
> Well, talking about a blast from the past :-)
>
> It's hardly my circuit though, it's from the AN-299 application note.
>
> Heating up the 3046 does not cost much power, it's mass is just a few 
> grams, and the chip is not hot to the touch.
>
> This is what I wrote in my documentation:
>
> 1. Heater adjustment
>
> First make sure the heater is switched off by shorting D2. This way 
> the base of the heater is grounded and Q5 switched off. Now measure 
> the voltage present at the +-input of A1. This must be done with a 
> digital voltmeter (DVM) or another voltmeter with high input 
> impedance. A regular multimeter will not do. The voltage measured must 
> be about 650 to 700mV, being the Ube of the sensor Q3.
>
> Do not apply power unless the heater is adjusted or switched off!
>
> With every degree temperature rise this be drops 1.9mV. In operation 
> A1 will make Q5 heat up the 3046 array until the voltage at the 
> plus-input is the same as the (set) voltage at the minus input of A1. 
> So, you now need to adjust P1 so that the minus-input of A1 is fed 
> with the voltage that goes with the elevated temperature. should with 
> the elevated temperature. To compute this set voltage subtract the 
> temperature rise (in Celcius) times 1.9mV from the measured voltage.
> Example: you want a 30 degrees temperature rise and the measured Ube 
> is 695mV. The desired drop in Ube therefore is 30x1.9=57mV. The 
> minus-input of A1 must be set (with P1) to 695-57=638mV.
>
> rick
>
> On 29/02/2016 01:33, sleepy_dog at gmx.de wrote:
>> Hey there,
>>
>> first a question about thermistors (this is what "tempco" are, correct?)
>> In circuits for voltage control of a VCO or VCF which use a tempco, how
>> much could one deviate with the coefficient, say for instance in Thomay
>> Henry's newest VCO based on the TLC555 timer? I think it was spec'd at
>> 3500 - what if I could get one with 3470 or 3600 more easily?
>>
>> Also, I find Rick Jansen's Moog VCF schematic interesting, which uses 3
>> remaining transistors in a 3046 transistor array as a controlled "oven"
>> for the NPN matched pair, to get rid of temperature sensitivity.
>> How come I don't see this kind of solution more often? Is the tempco
>> solution "better"?
>> How much current will the "oven" draw, and does this reduce the lifetime
>> of the IC?
>>
>>
>> Regards,
>> Steve
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>

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