HP 3457A Service Manual page 172

Over Voltage/Over Current Protection
Input Switching
Autozero
Offset DAC
Pre-charge Stage and Pre-charge Offset Adjust
Input Amplifier
8-20. Over Voltage/Over Current Protection
This circuitry has four parts; high voltage protection, low voltage protection, current protection and
ohms over voltage protection. The input terminals are protected from low energy static zap voltages to
greater than ISK V, and high energy line transient impulses of greater than 3K V . The circuits operate as
follows,
High Voltage Protection
The input terminals are protected from over voltage by a 450V Surge Voltage Protector (E101).
Voltages greater than 450V peak cause E101 to breakdown thus clamping the terminals at approximate-
ly the value of RVIOI (230V) plus the holdover voltage of E101 (approximately 100 volts),
Low Voltage Protection
The Input Hybrid, ULOI is protected by diodes connected to the individual input nodes. The diodes on
each node are connected internally in UI01 to +3.5V and -3.5V power supplies. These diodes along
with R101, R102, C101, RLO3, R408 and C401 maintain a current limited maximum input to the hybrid
of approximately 3.5 volts. The capacitors C101 and C401 absorb the transient energy before the clamp
diodes turn on. The ohms sense high and low inputs are also protected with clamp diodes, current limit-
ing resistors and transient protection capacitors. In addition, L201, R205 and C203 form a filter to
reduce the peak transient voltage at the collectors of Q214 and Q216 to less than 800V.
Ohms Over Voltage Protection
Transistors Q213 - Q216 form a protection network with a negative breakdown of 2BVce=700V. The
resistors R213 and R214 provide the base bias for these transistors through the junction leakage current
of Q212 in normal operation. For large negative overloads, the gate of Q212 will forward bias and
provide a current path to ground. Positive breakdown of greater than +1KV is obtained through the
reverse breakdown of CR201.
Amps Section Protection
Transient input protection of the AMPS section is provided by L401 along with clamp diodes CR401 -
CR404. For any voltages (IRshunt) greater than approximately | volt, the input clamp diodes begin to
turn on and shunt current away from the current shunt resistor thus preventing an over power condi-
tion from being sustained.
Current Protection
Fuse F101 in series with the current shunt (R401) protects the shunt from excessive inputs currents
(>L5A).
8-21. Input Switching
The input switching circuitry consists of relays and MOSFET switches. The MOSFET switches are
located within input hybrid UIOI and their purpose is to provide signal paths to the input amplifier and
HP 3457A Multimeter
8-6