User Manual

In this section:
Cables ...................................................................................... 4-1
External factors ........................................................................ 4-1
Reactive loads .......................................................................... 4-2
Make a modification ................................................................. 4-3
Cables
Many electronic test systems use relay switching to connect multiple devices to sources and
measurement instruments. In some cases, multiple sources and measuring instruments are
connected to a single device. Switching allows automating the testing of multiple devices (for more
information about switching, go to the Keithley Instruments website
(http://www.tek.com/keithley) and
search for Switching Handbook).
When switching signals greater than 30 V RMS or 42.4 V peak take care to prevent contact
with live circuits which could cause electrical shock resulting in injury or death.
Shielded cables should be used with the 3760 high current card when switching above 50 V or high
current. The shield should be connected to circuit guard. This helps prevent excessive radiation from
the cables from interfering with any equipment. The cable rating must not exceed the maximum
allowable signal levels as defined in the specifications.
External factors
The 3760 is specified for a resistive load, however, external circuit capacitances and inductances can
cause excessive currents and voltages across the relay contacts. The currents and voltages must be
limited to within the maximum allowable signal levels.
• Capacitance - Charged capacitances in the circuit can cause excessive currents (surge currents)
to flow through the relay contacts if the source is shorted out or the output is switched to
capacitive loads. This excessive current can weld the relay contacts together. Therefore, use
series resistors, where needed, to limit the maximum current that can flow to within contact
ratings. The maximum signal levels are stated in the specifications.
• Inductance - The inductance of cables can produce high voltage and arcing across relay contacts
when switching. The peak transients must be limited to within the maximum signal levels as
defined in the specifications.
Section 4
Operation considerations