Power Control Circuitry
The power control circuit consists of U21 and its associated components. U21 is a D flip/flop with asynchronous
preset and clear; only the preset and clear are used.
Power is applied to U21 via CR11 whenever batteries are installed. CR11 provides protection for U21 if the
batteries are installed with reverse polarity. This error condition will reverse bias CR11, thereby disabling current
flow to U21.
The much larger RC time constant of R110, C67 compared to R78, C66 guarantees that the unit will not be
accidentally powered on when batteries are first installed.
Whenever the measure button is pressed, a low on GO SW sets the output signal PWR ON high. This condition
connects switched and battery grounds, enables the power supplies, and switches on the unit. Whenever the
CPU determines that the power should be switched off, it forces PWR DOWN low. This action clears output
PWR ON to a logic low, disconnecting ground, and switching off the power supplies (see also "Power Supply").
R79 and R81 provide current limit protection to U21 inputs. They also limit the current that will flow through
U21 inputs to the CPU when the batteries are installed backwards. In the reverse battery error condition,
massive current can flow from the inputs of U21 through the input protection diodes and/or substrate inside the
CPUs integrated circuit. These resistors limit that current path to safe levels.
Power Shut-off Circuit
Fuse F1 protects the unit from excessive current draw. CR24 protects against large voltage transients caused by
ESD, EMI, etc.
Q15 is a dual-channel FET; the drain of Q15 part 2 (pin D2) is connected to battery ground; the gate (G2) is
connected to battery plus; and R155 applies a bias to the source (S2) so it will switch on when a positive voltage
is applied to G2. When batteries are correctly installed, Q15 part 2 is switched on and conducts. If batteries are
installed backward, Q15 part 2 switches off and disables current flow. This protects the units power supply
circuitry from an accidental reversal of battery potential.
Q15 part 1 controls the power supplies. When a logic high is placed on the gate (pin G1) signal PWR ON
battery ground is connected to the circuit and switched to ground via Q15 parts 1 and 2. When the power
control circuitry pulls PWR ON low, switched ground switches to a high impedance state. This action switches
off the power supply, and therefore the unit, except for the power control circuit.
Vcc Power Supply
The Vcc power supply is a switched inductor voltage regulator operating in boost mode (U22). The power input
is provided by the batteries (VBAT). NFET (Q17) operates as a linear post regulator. The I M resistor (R77)
operates as a static bleed device across the switched regulator when the regulator is switched down. The
regulated output is Vcc (5 V ± 5%).
12-20