214 Power Supply Circuit Description

The Power Supply consists of the following three functional sections:

The Raw DC Supply converts line voltage (90 V to 264 V ac) to a dc output of 7.5 V

• 5 V Switching Supply

The 5 V Switching Supply converts the Raw DC Supply output to 5.1 V ± 0.25 V dc.

Using the 5 V Switching Supply output, the Inverter generates the -30 V dc, -5 V dc, and 5 V ac supply levels needed for the vacuum-fluorescent display. Also, the to 5 V.

Inverter provides isolated positive and negative 5.25 V outputs for the in-guard circuitry.

2-15. Raw DC Supply

The Raw DC Supply uses a power transformer (A1T3) that operates on input line voltages ranging from 90 V to 264 V ac. Since there is no power switch in the transformer input circuit, the Raw DC Supply is energized whenever the meter is connected to line power. The transformer uses an internal 275 V ac MOV (metal-oxide varistor) to clamp line transients. This MOV normally acts as an open circuit; when the peak voltage exceeds approximately 400 V, the MOV turns on and, working with the line impedance in series with the line fuse, limits the transient peak voltage to 400 to 500 V. All line voltages use a T 0.125 A, 250 V (slow blow) fuse.

On the secondary side of the transformer, the output is rectified by diodes A1CR2 and A1CR3 and filtered by capacitor A1C27. In addition, A1C26 reduces rectifier diode switching emi emissions from the meter. The meter power switch (A1S1) is also connected in the output of the Raw DC Supply; it connects the Raw DC Supply either to the 5-V Switching Supply (when the meter is ON) or to the Battery Charger Switching Supply through A1J1-7 (when the meter is OFF.)

2-16. 5-Volt Switching Supply

The 5-Volt Switching Supply incorporates the A1U11 controller device and several external components. Operating on an input of 7.5 V dc to 35 V dc, the 5-volt Switching Supply uses a pulse-width modulation technique to regulate its output at 5.1 V dc. The nominal switching frequency is 40 kHz.

With the controller, the output voltage is controlled by varying the duty cycle (ON time) of the switch transistor in A1U11. (Controller device A1U11 contains the supply reference, oscillator, switch transistor, pulse-width modulator comparator, switch drive circuit, current-limit comparator, and current-limit reference.) Resistors A1R41 and A1R42, in conjunction with the reference circuit, set the input levels to the pulse-width modulating comparator. Resistors A1R35 (in parallel with A1R41) and A1R36 (in parallel with A1R42) are used in production to adjust the 5.1 V supply output. Removing A1R35 decreases the output by approximately 5%. Removing A1R36 increases the output by approximately 7%.

Within the controller, the output of the comparator is combined with the oscillator signal to form the drive signal for the switch transistor. Diode A1CR6 operates as a complementary switch with the switch transistor. Dual inductor A1T1 is a magnetic device that regulates current pulses as the switch transistor is turned on and off. A current shunt (A1R47, A1R48, and A1R49) senses the overall current flowing through diode A1CR5 and the switch transistor of A1U11. If this current rises too high, the duty cycle (ON time) of the switch transistor is reduced to current-limit the supply. Capacitor A1C33 serves as a filter capacitor and energy storage device, and A1C34 and A1C35 are the output filter capacitors.

The boost circuit (A1CR4 and A1C32) supplies the controller A1U11 with sufficient supply voltage when the switching supply input is low. For example, this condition may occur when the meter is operating on low line voltage.

Resistor A1R40 and capacitors A1C28 and A1C29 are needed for proper dynamic performance of the switching supply. Capacitor A1C30 sets the operating frequency of the supply.

2-17. Inverter

The inverter uses transistors A1Q10, A1Q11, A1Q12, and A1Q13 connected to form an astable 30-kHz multivibrator. The operating frequency and drive to the transistors are determined by the values of the interconnecting resistors and capacitors. The inverter transformer (A1T2) primary is connected across the collectors of the multivibrator transistors; the primary winding is thereby driven by a symmetrical square wave. Resistor A1R46 and capacitor A1C41 form a filter network to reduce the amplitude of current pulses generated by the switching of the inverter transistors.

The secondary windings of A1T2 are used for three sets of supply voltages. The first winding of the A1T2 secondary provides out-guard outputs using the following rectifier diodes and filter capacitors to provide the required voltages for the display and the battery trickle charger circuit.

• A1CR10 and A1C46: +30 V source for the battery trickle charger.

Next, an isolated secondary winding provides the various in-guard supplies. Dual diodes A1CR8 and A1CR9 and capacitors A1C43 and A1C44 are the rectifiers and filters for the in-guard +5.25 V and -5.25 V supplies. Dual diode A1CR7 and capacitor A1C42 are the rectifier and capacitor for the relay (5.25 V dc) supply.

A third transformer winding provides the heater power (FIL1, FIL2) for the vacuum-fluorescent display. Zener diode A1VR3 and resistor A1R55 bias this winding at approximately 5 volts more positive than the -30 V supply.

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