HP 3457A Service Manual page 198

K800 column | must be driven low while row | is driven high.
8-66. Contact Circuitry
Two types of relays are used in the 44491A. The first type consists of relays K800-K803 and
K806-K809 which have four normally open contacts. These relays are used as channel relays and al-
ways have all four contacts in the same position. A set position exists for these relays when all four
contacts are closed and a reset position when all contacts are open.
The other type of relay used consists of K804 and K805 which have two contacts normally open and
two contacts normally closed. These relays are used as tree relays and when in the set position have
contacts between pins 10-11 and 4-5 closed and contacts between pins 2-3 and 8-9 open. In the reset
position, contacts are in the opposite configuration with contacts 10-11 and 4-5 open and contacts 2-3
and 4-5 closed.
Channel relays K800-K803 along with tree relay K804 create the eight two wire channels 0-7. When
K804 is set channels 0-3 are selected and when K804 is reset channels 4-7 are selected.
Channel relays K800-K803 along with tree relays K804 and K805 create the 4 four wire channels 10-13.
Both K804 and K805 are always set when making a four wire measurement through channels 10-13,
Relays K808 and K809 are active when channels 8 and 9 are used as actuator channels. When a CLOSE
command is executed to these channels either K808 or K809 is latched in a set position connecting the
high and low input together.
Relays K806 and K807 are active when channels 8 and 9 are used to measure current, However, a relay
protection scheme complicates operation of these relays.
When channel 8 has been selected to measure current with a CHAN or SLIST command the following
sequence of events occur:
® Relay K808 is closed which connects the high and low input together.
© Relay K806 is closed connecting the instrument's AMPS and LO terminals to the high and
low inputs.
© Relay K808 is opened, removing the short from across the input and causing the input cur-
rent to be forced into the AMPS terminal.
K808 is used as a current shunt in this sequence so that at the time relay K806 is closed there is no
potential across the contacts. This protects K806 from arcing, thereby, preserving the contacts.
When the channel is later opened the following sequence of events occur:
© Relay K808 is closed.
© Relay K806 is open.
®@ Relay K808 is open.
Again K808 is used as a current shunt so that when K806 is open there is virtually no potential across
the contacts. Preservation of K806's and K807's contacts is important because low contact resistance is
required to make accurate current measurements.
HP 3457A Multimeter
8-33