Maybe this isn't a beginner problem, but I still feel like a beginner, so I'm putting this here. I'll link to it from the
main ltz1000 thread.
I have a test circuit with a bunch of test points and trim pots on it so I can better understand the parameters of the LTZ and its reference circuit. It's undergone a few bodge changes, but I think the attached schematic represents its current state of affairs. When doing the experiment below, I was logging both ltz collector currents, I(R14), V(Pin4 - Pin7), V(Pin6-Pin7).
One of my experiments was to adjust R4 so that pin6 was high enough that the heater would never turn on (it comes on around 603mV, so I had it set at 613mV) and then raise pin 6 voltage some more (to 626mV) I was expecting a very slight increase in the corresponding collector current and that was pretty much it. But what I got was a shift in pin 4 of about 500uV and a corresponding change in current (which is what I noticed first.) I considered a change in self heating, but since Q2 current is going up by so little with a small voltage drop, I would have expected the shift to be orders of magnitude less. As it stands, I think total power is going down, not up! I would have expected for of a "time constant" shape to the curve, too.
I built up a mock LTZ1000 which is discrete 2n3904's, a zener, and two heater resistors. I replaced the real LTZ with the mock LTZ to see if it was something about the LTZ or about my circuit. It behaved differently, but still in a way I don't understand. Instead of the near-instant, slope-matching response of the real LTZ, the mock LTZ appeared to very slowly warm up a little whenever I changed pin 6, regardless of which direction I changed it. This made me think the temp loop must oscillate on a change. So I put my scope from pin 1 to pin 2, but didn't observe any oscillation >= 1mV during a change in pin 6. When using the mock LTZ, there's some dozens of centimeters of extra wiring (cat5) between the test points and the components, but I don't expect this kind of result from that.
I've been at this a couple days, and I'm out of theories and things to try. Any ideas?
Changes from the datasheet schematic are: (the intentional ones, anyway, hehe.)
- Addition of C11, C13, C14 to the temperature loop from Andreas' earlier schematics. (he has subsequently found C14 to be a problem with higher speed opamps)
- LT1112 instead of LT1013
- Addition of R13 across the heater transistor to ensure some heater current even when the transistor is off, keeping pin 1, pin 2, and pin 4 in their proper potential order
- Addition of a second diode between pin 2 and pin 7. Because I'm running so cold, vbe pin 4 is very close to a 4148 Vf and I wanted more headroom.
- Diode between pin 7 and LT1112 V- to account for its voltage ranges and to allow me to buffer pin 7 (with a 2057 or something) later
- Addition of R14 before the LTZ inputs to limit current in the event I messed something up. 9mA through R1 would raise pin4 over pin2 even with the extra diode.
- R10 and R11 are left over from when there was a diode from that voltage divider to the ltz inputs to ensure bootstrap. But that diode is gone so I don't expect R10 and R11 to do anything but warm up the board a little.