Current Limit Board Configuration

The Power switch (S901 ) connects the line voltage to the instrument through line fuse F901 and transient suppressor VR901. Suppressor VR901 protects the instrument from large voltage transients. High-frequency line noise is attenuated by C901.

The Preregulator circuit converts the ac-power-source input voltage to a regulated dc voltage. A triac is used as a switch to conduct current during a controlled period of the inpuMine-voltage cycle so that energy to be used by the Inverter circuit is stored in capacitor C937.

Current from one side of the ac-power-source input will go through L925 (a current-limiting impedance) and triac Q925. Diodes CR931 and CR933 (on the Main board) and CR932 and CR934 (on the Current Limit board) form a full-wave bridge rectifier circuit. The rectifier converts the ac-input voltage into dc pulses that charge C937. Surge arrestor VR938, connected in parallel with C937, conducts to protect the following circuitry should the Preregulator output voltage become too high.

The two-transistor circuit composed of Q933, Q938, and associated components provides over-current protection in the event of triac misfiring or ac-power-source transients. Transistor Q938 is an insulatd-gate FET used as a switch in the charging path of C937. Transistor Q933 controls the FET bias to limit the current under abnormal firing conditions of Q925. In normal power-supply operation, the voltage developed across R937 is not sufficient to bias Q933 into conduction. The gate-to-source voltage of Q938 is set to 10 V by VR934 and R938, so the FET presents a low resistance to the charging current to C937. If triac Q925 should misfire to cause excessive current, Q933 becomes forward biased and Q938 is switched off to reduce the current. When Q938 switches off, the current that was flowing through Q938 flows through R939. The voltage drop developed across R939 causes current to flow through VR933 and R933, which holds Q933 on for most of the remainder of the ac-power-source input cycle. Resistor R939 limits the rate of collapse of the field around L925 to prevent damage to Q938. Thermistor RT935 adjusts the bias of Q933 over varying ambient temperatures.

PREREGULATOR CONTROL. The ac-source voltage is full-wave rectified by CR903 through CR906 and applied to a voltage divider composed of R911, R912, and R915. Output from this divider serves as a reference voltage for a ramp-and-pedestal comparator utilizing a programmable unijunction transistor (PUT), Q921. Capacitor C912 filters the line noise to prevent false triggering of the PUT. Voltage-dropping resistor R914 provides current for zener diodes VR914 and VR915 to produce constant voltages during each half of the ac-power-source cycle.

When the instrument is first turned on, C917 is not charged. Capacitor C915 charges through CR917 to the voltage of VR915 minus the diode drop of CR917. When the anode voltage of Q921 is greater than the gate voltage, Q921 will fire and C915 will discharge through the primary of T925. This event will happen after the peak of the voltage waveform. Pulse transformer T925 is connected to the gate of Q925, and the discharge of C915 through the T925 primary winding is coupled to the secondary to cause triac Q925 to conduct. After firing, the triac will turn off again when the sinusoidal source voltage crosses through zero. As C917 charges through R917, Q918 current increases proportionally to charge C915 more rapidly. When C915 charges at a faster rate, the anode voltage of Q921 rises above the gate voltage earlier in the ac-source cycle and thereby causes Q925 to conduct for a longer period of time.

The portion of the cycle preceding the zero-crossing point over which the triac is conducting is called the conduction angle. The conduction angle will increase from nearly zero (at turn on) to an angle sufficient to supply the energy needed by the inverter. Feedback from the inverter through optical isolator U931 holds the correct conduction angle by shunting current from R917. This shunting action controls the voltage on C917, thereby controlling the increase in base voltage on Q918. This action controls the charging rate of C915 and therefore the conduction angle of Q925.

The Preregulator circuit can handle a wide range of input voltages by changing the conduction angle of the triac as the input voltage changes. As the input voltage increases, the conduction angle will decrease to maintain the Preregulator output voltage at a constant level. The voltage divider composed of R911, R912, and R915 produces an output voltage proportional to the input line voltage that is applied to the gate of Q921. Since VR914 and VR915 hold bias levels on Q918 constant regardless of input voltage, the point on the cycle at which Q921 fires will vary with changes in the ac-source votlage. This feed-forward, together with the feedback from the Inverter through optical isolator U931, ensures a constant Preregulator output to the Inverter.

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