Figure 8-4. Feedback Loop During Warmup Mode - HP 5501B Operating And Service Manual

8-31. The heater coil is used both to heat the rod and to sense its temperature. For 2.56 seconds
(nominal) out of every 25.6 seconds (nominal) of the warmup cycle (see Figure 8-5), the state
machine disables the power amplifier and the heater allowing the heater coil to stabilize to the
rod temperature. The heater coil and a resistor in the power amplifier (A1R15) form a voltage
divider. As the heater resistance increases with rising temperature, the divider voltage increases.
The difference between this voltage and the reference voltage is amplified to provide the warmup
mode error signal.
8-32. At the end of the 2.56 second disable period, the heater coil temperature has settled to the rod
temperature. The error signal is sampled and held and the power amplifier is enabled. During the
remainder of the 25.6 second period, the power amplifier, re-enabled and under control of the
sampled error signal, drives the heater.
8-33. When the error signal gets close to zero, the warmup error amplifier sends a digital signal called
HEATER OK (HTR OK) to the state machine. HTR OK is sampled by the state machine whenever
the error signal is sampled. After HTR OK is sampled true, the state machine waits 100 seconds
before switching to optical mode.
8-8

Figure 8-4. Feedback Loop during Warmup Mode

HP5501B
Service