[Re: thermal tips re expo converters:]

[Bissell, Harry]

Probably the truth lies somewhere inbetween. First, I don't know whether the
tecniques of "thermal isolation" will make the cooling rate to slow to be
practical. And there is something to be said for minimizing the error term.
The problem (as i see it?) is not that the cooling rate is slow but that the
chip is exposed to Ambient, and that is not only a variable sink of heat,
but it is the source of the error. I'll try to whip up a test unit and see
what happens. Wish me luck! :-) Harry Bissell

> -----Original Message-----
> From:	Ian Fritz [SMTP:ijfritz at earthlink.net]
> Sent:	Tuesday, February 09, 1999 9:58 PM
> To:	Bissell, Harry
> Cc:	martin.czech at intermetall.de; synth-diy at mailhost.bpa.nl
> Subject:	Re: [Re: thermal tips re expo converters:]
> 
> Harry --
> 
> I have many years of experience in laboratory temperature control
> systems. I can assure you that Martin is correct, and for the reasons he
> states. A system with an extremely slow cooling rate will *not*
> generally achieve steady state, as you claim. Instead, it will
> oscillate. The only way to stop the oscillation would be to use a very
> low loop gain, which would result in poor regulation.
> 
> It seems to me that it is not easy to get comparable heating and cooling
> rates using a plastic IC package. This is undoubtedly why the commercial
> VCOs use such a high temperature, e.g., 85deg C for the SSM2033, 60deg C
> for some Moog VCO I remember seeing, etc.
> 
>   Ian
> 
> "Bissell, Harry" wrote:
> > 
> > Correct, no doubt. The techniques I'm suggesting serve to decouple the
> > external temp changes from reaching or disturbing the expo converter. If
> we
> > change ambient and the converter takes a half hour to notice, we win.
> With
> > high heat loss, or low heat loss the circuits will achieve steady state.
> You
> > could even turn the heater power down and still stabilize the loop.
> Which
> > factor is more significant, good thermal isolation, or ability to bleed
> off
> > excess heat ?  Beats me... what does anybody think ? :-)  Harry Bissell
> > 
> > > -----Original Message-----
> > > From: Martin Czech [SMTP:martin.czech at intermetall.de]
> > > Sent: Tuesday, February 09, 1999 2:21 AM
> > > To:   varner at k-online.com; harrybissell at netscape.net
> > > Cc:   synth-diy at mailhost.bpa.nl
> > > Subject:      Re: [Re: thermal tips re expo converters:]
> > >
> > > I'd just like to mention that you need some thermal loss in order to
> > > cool the device down again. If it would be perfectly isolated, this
> > > would mean that heating is very fast, but cooling very very slow. The
> > > slightest regulation loop overshoot, or self heating of the expo pair
> > > would need very long to settle down.
> > >
> > > A peak detector is an electrical analogy to this behaviour.
> > > Without bleeder the control loop goes nuts.
> > >
> > > m.c.