Theory of Operation 5
Keysight 34970A/34972A Service Guide 149
Relay Drivers
Two types of single-coil relays are used on the switch modules: latching and
non-latching. Typical driver configurations are shown below.
The non-latching relay contacts are in the set position (closed) when current flows
through the coil. When the current is removed, the relay resets (opens). The
positive side of the relay coil is connected to +5NL. The negative side of the relay
coil is connected to ZGND through a single NPN transistor. The transistor and
+5NL must be on for the relay to stay in the set position.
The polarity of the current flow through the latching relay coil determines the set
(closed) or reset (open) position of the relay contacts. Latching relays are driven by
row and column latches or complimentary transistor pairs.
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