Precision Voltage Reference; Oven Controller Turn-On Current Limiting; Figure 8-10. Oven Control Circuits - HP 10811A Operating & Service Manual

8-32. The purpose of R17 is mainly to reduce the power dissipated in the thermistor which causes it to
self-heat above the oven operating temperature.
8-33. R38 and R39 in parallel provide a means of sensing the heater current, rH. During warm-up, the
voltage across them, VH, is used in the current limit circuit (described later). During normal, linear
operation, VH is essentially the feedback point for the oven controller loop.
8-34. Q6 is necessary primarily for the condition when the oscillator has been stored at -55°C. Since U3
(at -55°C) cannot supply enough base current for Q8, Q6 gives the added current gain required.

8-35. PRECISION VOLTAGE REFERENCE

8-36. U2 is a 10.0V voltage reference. It provides a stable voltage source for the bridge and U1. A
change in the bridge reference voltage changes the voltage across the thermistor and hence, the
power it dissipates. See

8-37. OVEN CONTROLLER TURN-ON CURRENT LIMITING

Figure 8-11 shows the turn-on current limiting circuit. From an initial turn-on condition the
8-38.
thermistor senses the oven temperature to be low. To correct this situation the amplifier attempts
to drive heavy amounts of current through Q7 and Q8 heaters. If allowed to continue this way,
excessive current will flow; much more than is practical or necessary for warm-up. Amplifier U1
and associated components limit the current during warm-up to a practical value. When Vcc is
applied to the oven, U1B forces VH to equal V3 by sinking the base current from Q6. By sensing
Vcc, the circuit transforms the heater transistors into what appears to be a fixed heater resistance
of 47Ω typical.

Figure 8-10. Oven Control Circuits

Figure 8-10.
Model 10811A/B
47