Heated expo-converters

[Tony Clark]

> Check the polarity of the op-amp connections. The op-amp positive goes to
> the sense transistor connected as a diode. The negative goes to a 690 MV.
> voltage reference. That indicates VBE goes DOWN with temperature. The
> op-amp should produce a rounded square wave at around 200 Hz. depending on
> the value of the capacitor. The chip won't get very hot. The 690 MV. value
> seems to be just above room temperature. The Serge CA3046's don't get very
> warm either (same circuit). If you put your finger on the chip for a moment
> the op-amp will stop oscillating.

   Yep, this was my problem.  I didn't have the 6.9V zener and was using 
a LM336 5.0V!  Geez, how stupid!  Anyway, threw a pot in there to adjust 
the voltage to the appropriate level, now I'm seeing some action!
   The interesting thing I've found, so far, is that the op-amp doesn't 
cycle.  It initially was in the on state (15V) but then after a minute, 
the output voltage sank down to around 1V and stays.  I assume this means 
that the circuit has come to a nice equilibrium state.  Thankfully the 
current limiting resistor isn't so bloody hot now, so maybe I'll try a 
reduced resistance and see if the op-amp starts cycling more or if it 
stays at an equilibrium state.
   Should also pull out the heat gun and see how things affect it.  :)
   One last note, for anyone who wants to try this with an MAT-04, the 
transistors are layed out in a nice square pattern.  I've set it up in a 
diagonal configuration so that the heater is next to both of the current 
mirror transistors and the sensing/limiting transistor is diagonal to the 
heater.  This should make for better heat distribution on the die.
   Unfortunately the LM3046 data sheet doesn't have a die printout so I 
can't tell how it is setup for heat distribution.  Anyone know the answer 
to this?
   Thanks for all of the suggestions and knowledge.

   Tony

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I can't drive (my Moog) 55!
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Tony Clark -- clark at andrews.edu 
http://aupe.phys.andrews.edu/~clark
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