[sdiy] lessons learned from temperature chamber design II

[Czech Martin]

hi,

some may laugh about it, but here are some practical
experiences from my temperature chamber and servo
experiments.

-1/f noise: I started the first servo circuit on a Sunday.
 So I used what I had, jfet op amps. For a DC servo loop,
 this causes serious 1/f-noise problems. So now the
 high gain part uses OP07, lower 1/f, cheap, available
-I made a mistake with the sensor, then in haste fixed
 that, but building another mistake: circuit supply
 inluences sensor output. Since the sensor output is only
 3mV/K and needs very high amplification, this can cause
 trouble. So I went back and built a bridge circuit with
 instrumentation amplifier, virtually eliminating any
 CM errors. Needs three OP07, but is cheaper then a
 high precision voltage source (burried zener type).
-the thermo chamber behaviour is of course unknown,
 until you start playing with it. So some components
 of the circuit (gain, integrator, differentiator)
 will need adjustment in servall orders of magnitude,
 i.e. you have to replace caps and resistors.
 This will damage the circuit board. Therefore matching
 or unknown value resistors are now soldered on pins
 a few mm above the board (solder nails (sp?)). I can
 exchange them whenever I like.
-the supply rails for all active components are now
 parallel silver/copper wires, a few mm apart.
 Inductance is thereby minimised. I soldered
 some 100nF blocking capacitor for each op amp,
 but did not connect them. Until now, I could see
 no ringing problems (well, the OP07 is not 
 particulary fast).
-I have inserted jumpers, in order to be able
 to seperate circuit parts from other parts.
 This makes triming, CM rejection trimming
 much easier.
-the 12V regulator (for the fans) was thrown
 out, since it will dissipate much heat and thus
 influence the circuit

so far, so good...
m.c.