7-1. Introduction 7-3
7-2. Specifications 7-3
7-3. Theory of Operation 7-3
7-4. Functional Block Description 7-3
7-5. Switching Power Supply 7-5
7-6. Cycle/Float Charge Rate Switch 7-5
7-7. Low Battery Indicator Detector 7-5
7-8. Low Battery Disconnect 7-5
7-9. Constant Voltage Trickle Charger 7-6
7-10. Other Circuits 7-6
7-11. General Maintenance 7-6
7-13. Installation 7-7
7-14. Performance Testing 7-9
7-15. General Operability 7-9
7-16. Low Battery Indicator Detector and Low Battery Disconnect Test 7-10
7-17. Cycle/Float Charge Rate Switch Test 7-10
7-18. Calibration 7-12
7-19. Troubleshooting 7-13
7-20. Additional Tests 7-14
7-21. Schematic Diagram 7-14
7-22. List of Replaceable Parts 7-14
The Fluke 45-01K Battery Kit is a field installable option that may be installed in the meter. The installation of the Battery Kit facilitates portable operation of the meter for typically eight hours.
7-4. Functional Block Description
The Battery Pack Option PCA consists of the five functional blocks illustrated in Figure 7-1. Each block is described below:
• Switching Power Supply
The Switching Power Supply regulates the 7.5 V-to-35 V raw dc input provided by the main circuit board (J1-7). The supply output (9.3 V to 9.8 V, temperature-compensated) is used to charge the 8 V lead-acid battery.
• Cycle/Float Charge Rate Switch
The Cycle/Float Charge Rate Switch monitors the charging current required by the battery pack and sets the output voltage accordingly (9.35 V for trickle charging, or 9.8 V for cyclic charging).
• Low Battery Indicator Detector
The Low Battery Indicator Detector monitors the battery pack voltage, outputting a logic low to turn on the meter low voltage indicator when the battery pack voltage is below approximately 7.7 V.
• Low Battery Disconnect
The Low Battery Disconnect circuit interrupts meter loading on the battery pack when battery pack voltage drops below approximately 7.0 V. This action prevents deep discharge of the batteries.
• Constant Voltage Trickle Charger.
When the meter is operated on line power, the charge on the battery pack is maintained with the Constant Voltage Trickle Charger (9.25 V output).
Specifications for the Battery Kit Option are as follows:
• Type of Cell: Sealed Lead-Acid Rechargeable
• Battery Voltage: 8 Volts Nominal
• Charge Time: 16 Hours Typical w/meter off
7-3. Theory of Operation co a>
The switching power supply regulates the output voltage with a pulse-width modulating technique that varies the on time of the FET switch, Q5. The controller chip for this process (U4) contains the required 1 V reference, switching oscillator, regulator comparator, current limit comparators, and output switch. The controller chip is supported by several external components: C7 sets the oscillator frequency; R17, C6, and C8 are required for stability; R15, R21, and R20 set the output voltage level. The variable-width pulse from U4-5 is first level-shifted and clamped by R23, R24, and CR7 and then coupled to the FET switch circuit through inverter U3-2. Hex inverters U3-4, U3-6, and U3-10 are connected in parallel to provide sufficient drive to the FET gate. FET switch Q5 is turned on at the beginning of each square wave drive cycle and turned off at the point necessary to set the desired output voltage.
The current limit is set by the voltage drop across the 0.1-ohm resistance provided by the parallel configuration of five 0.5-ohm resistors (R29, R34, R35, R36, and R37). Current limiting occurs when the voltage drop across the ohm resistance is greater than approximately 100 mV. Current limiting is effected by reducing the duty cycle of the switching transistor, Q5, which in turn reduces the output voltage of the supply.
A bootstrap supply for the driver and coupling/clamp circuits is provided by CR8, R25, C9, C11, VR3. The output level has a negative temperature coefficient to provide the proper charging voltage level for the battery pack over a broad temperature range. This negative coefficient is provided by the Temperature Compensation circuit (Q4, R18, and R19) in the feedback path of the switching power supply.
7-6. Cycle/Float Charge Rate Switch
The Cycle/Float Charge Rate Switch monitors the charging current supplied to the battery pack during charge when the meter is de-energized. If the current is 60 mA to 80 mA or higher, the charging voltage is increased to about 9.8 V. This level of current causes a voltage drop across R28 that is sufficient to turn Q6 and Q7 on, connecting one end of R22 to common. The resulting current through R22 modifies feedback to the switching supply, causing the increased output voltage.
As the battery pack charge reaches about 90% of capacity, the charge current at 9.8 volts drops below 60 to 80 mA; Q6 and Q7 turn off, and the charging voltage decreases to 9.35 volts, the trickle charge level for the battery pack. Diode CR10 clamps the voltage across R28 to about 0.8 V maximum.
The Low Battery Indicator Detector uses comparator U2-7. Bandgap reference VR2 is connected to the inverting input of the comparator, while the divided-down battery voltage is connected to the noninverting input. Resistor R16 provides the comparator with a hysteresis of approximately 0.25 V.
The comparator output is an open collector transistor connected to pull-up resistor R45 on the Main Circuit Assembly. When the battery pack voltage falls below about 7.7 V, U2-7 goes low. When the battery pack voltage rises above about 8.0 volts, U2-7 goes high. If the battery pack option is not installed, the pull-up resistor on the Main Circuit Assembly pulls the battery line high to keep the low battery indicator turned off.
7-8. Low Battery Disconnect
The Low Battery Disconnect uses FET switch Q2. The FET gate is driven by a latching circuit comprised of Q3, Q12, Q13, and VR4.
When the power switch is first turned on, a pulse through C15 turns on Q13, which then turns on Q3. With the Q3 collector voltage now near its emitter voltage and the FET gate near the battery voltage, the FET switch is on. At the same time, Q12 is turned on; its collector pulls down the base of Q3, latching the circuit on.
When the voltage between the base of Q12 and the anode of VR4 drops below 6.9 volts (battery voltage of about 7.0 volts), Q12 cannot be maintained on due to the 6.2 V zener diode in its emitter circuit, and the circuit unlatches itself. The gate of Q3 then goes to zero and turns off, disconnecting the battery pack. If the battery pack charge is low, this circuit does not latch, and the battery pack is protected from being deeply discharged.
The Constant Voltage Trickle Charger uses a voltage regulator set at 9.25 volts. The output voltage is set by the values of R38, R40, and the setting of R39. The 30-volt supply on the Main Circuit Assembly provides the source for the regulator. Trickle charging occurs only when the meter is operated on line power.
If trickle charging occurs when the battery pack is low, R2 limits charging current to about 15 mA maximum.
When the meter is operated solely on battery pack power, the charger is disconnected from the battery pack. A logic level signal (ACON*) from the Main Circuit Assembly (J1-5) is maintained high, and the output of comparator U2-1 goes low, pulling U1-1 to common and setting the U1-2 output at 1.25 volts. CR4 is thus reversed biased, disconnecting the charger from the battery pack.
During the various modes of operation, switching diodes provide the required battery pack connections. During battery pack operation of the meter, CR3 connects the battery pack to the switching supply on the Main Circuit Assembly. During charging (line power connected), CR5 connects the battery pack to the charging circuit. If line power is disconnected, CR9 disconnects the battery pack from the charging circuit.
The 5-amp battery pack fuse opens if either the battery pack is connected with the wrong polarity or if the battery pack output is shorted on the battery pack circuit board.
7-11. General Maintenance
Use the following instructions to remove the Battery Pack Option from the meter:
1. Make sure the meter is turned off and unplugged from the power outlet.
2. Remove the screw from the bottom of the case and the two screws from the rear bezel (as shown in Figure 7-2 detail A). While holding the front panel, slide the case and rear bezel off the chassis. (See Figure 7-2 detail B.) At this point, the rear bezel is not secured to the case.
3. Locate the flat white battery pack option cable connector at the Main PCA (see Figure 7-3.) Use needle nose pliers to detach this connection; alternately pull up on each end of this connector.
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