Keithley 7710, User Manual

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13
Heat sink temperature measurements
Measuring the temperature of a heat sink is a typical test for a system that has temperature mea-
surement capability. However, the 7710 card cannot be used if the heat sink is being floated at a
dangerous voltage level (>60V). An example of such a test is shown in
Figure 3 .
In Figure 3 , the heat sink is floating at 120V, which is the line voltage being input to a +5V regula-
tor. The intention is to use channel 1 to measure the temperature of the heat sink, and use chan-
nel 2 to measure the +5V output of the regulator. For optimum heat transfer, the thermocouple
(TC) is placed in direct contact with the heat sink. This inadvertently connects the floating 120V
potential to the 7710 card. The result is 115V between channel 1 and channel 2 HI, and 120V
between channel 1 and chassis. These levels exceed the 60V limit of the card creating a shock
hazard and possibly causing damage to the card.
WARNING The test in Figure 3 demonstrates how a dangerous voltage can unintention-
ally be applied to the 7710 card. Keep in mind that in any test where floating
voltages >60V are present, you must be careful not to apply the floating volt-
age to the card.
Figure 3
Example of an unsafe measurement practice
Card handling precautions
The solid state relays used on the 7710 card are static sensitive devices. Therefore, they can be
damaged by electrostatic discharge (ESD).
CAUTION To prevent damage from ESD, only handle the module by the card edges. DO
NOT touch the backplane connector terminals. When working with the
quick-disconnect terminal blocks, DO NOT touch any circuit board traces or
other components. If working in a high-static environment, use a grounded
wrist strap when wiring the module.
Touching a circuit board trace may also contaminate it with body oils that may degrade the isolation
resistance between circuit paths, adversely affecting measurements. It is good practice to handle a
circuit board only by its edges.
120V
+5V
Regulator
Heat Sink
Ch 1
Ch 2
5V
+5V
OUT
IN
Heat sink floating
at 120V.
Thermocouple
115V
120V
7710 Module
Test Circuit
DO NOT use the 7710 card to perform this type of test. It exceeds
the 60V limit creating a shock hazard and could cause damage to the
card. Excessive voltages:
The voltage differential between Ch 1 and Ch 2 HI is 115V.
The voltage differential between Ch 1 and Ch 2 LO (chassis) is 120V. WARNING
Ch 1 configured to measure TC temperature.
Ch 2 configured to measure 5V regulator output.
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