Keysight DAQ970A User Guide

In the above diagram, the resistance is 60 Ω when all of the actuator channels are open (not connected to
COM). Note that when the actuator channels are open, as shown above, the normally-closed contacts (not
shown above) are connected to COM. Values from 10 Ω to 50 Ω are selected by closing the appropriate
channel(s) on the module.
Snubber Circuits
Whenever a relay contact opens or closes, electrical breakdown or arching can occur between the con-
tacts. This can cause high-frequency noise radiation, voltage and current surges, and physical damage to
the relay contacts.
A breadboard area is provided on the DAQM903A to implement custom circuitry such as simple filters,
snubbers, and voltage dividers. The breadboard area provides the space necessary to insert your own com-
ponents but there are no circuit board traces here. You can build these networks to provide contact pro-
tection when actuating AC power line for reactive loads. Although many types of contact protection
networks can be used, only RC networks and varistors are described in this section.
RC Protection Network
When designing RC protection networks, the protection resistor R
p
is selected as a compromise between
two resistance values. The minimum value of R
p
is determined by the maximum acceptable relay contact
current (I
max
). For the DAQM903A, the maximum allowable relay current (I
max
) is 1 A DC or AC rms. Thus,
the minimum value for R
p
is V/I
o
, where V is the peak value of the supply voltage.
The maximum value for R
p
is usually made equal to the load resistance R
L
. Therefore, the limits on R
p
can
be stated as:
Note that the actual value of the current (I
o
) in a circuit is determined by the equation:
where V is the peak value of the source voltage and R
L
is the resistance of the load. The value for I
o
will be
used to determine the value of the protection capacitor (C
p
).
In determining the value of the protection network capacitor (C
p
), there are several things that you will
need to consider. First, the total circuit capacitance (C
tot
) must be such that the peak voltage across the
open relay contacts does not exceed 300 V
rms
. The equation for determining the minimum allowable cir-
cule capacitance is:
Keysight DAQ970A User's Guide 217
4Measurement Tutorials

Keysight DAQ970A Specifications

General

Scan Rate	Up to 450 channels/sec
Connectivity	USB, LAN, GPIB (optional)
Display	4.3-inch color LCD
Operating Temperature	0 °C to 50 °C
Number of slots	3 slots
Measurement Types	DCV, ACV, DCI, ACI, 2-wire resistance, 4-wire resistance, frequency, temperature, capacitance
Voltmeter	6.5 digit
Resolution	22 bits
Memory	Non-volatile memory