APPENDIX C
HIGH VOLTAGE SWITCHING WITH THE 53A-334
Because a scanning system has multiple sources that can be switched to a common bus, with
each source having system-dependent amounts of capacitance, several considerations besides
the 200V rating of the relay contact affect the level of voltage that can actually be switched.
It is highly recommended the C3/C4 section of the Speed Select switch be set to C4 for voltages
over 100V dc.
The first, and most serious, problem is caused by high voltage switching into a capacitive bus.
The severity of the problem depends on current and heat limitations of the relay. The contacts
on the 53A-334 are rated at lOW and 0.5 amp. Even though the
currents
appear to be very low
in a high impedance measurement application, the bus capacitance causes extremely large
inrush currents which may be enough to seriously damage or degrade the reliability of the
relay contacts.
It is not easy to determine analytically the amount of voltage and capacitance that will cause
enough inrush current t o degrade t h e contacts. T e k / C D S h a s f o u n d t h e following
approximation to work, though:
The voltage times the capacitance should not exceed JO' volt-farads for a 10W
contact.
(40V into a 250 pF bus, for example.) I f this value is exceeded, the 53A-336 (which has a
contact rating of 150W) may be used, or resistors may be placed between the scanner card
common and the bus. On the 53A-334, if the backplane analog bus is used, the resistor may be
installed in place of a switch on the board. I f the front connector is used for a scanning bus,
the resistor may be installed after cutting a current trace (consult the factory). A resistor of
20 to 200 Ohms is typically used, depending on how much series resistance can be tolerated in
the application. Consult the factory for installation or resistor selection assistance.
The second problem is that the previous switched voltage may still be present on the common
bus (due to the bus capacitance). A situation could occur where the previous voltage is -200V
(DC or instantaneous AC) and the new voltage is +200V, placing 400V across the relay contact
and exceeding the 200V rating. This problem may be relieved by placing discharge resistors
on the bus. A typical discharge resistance might be 1 megohm and can typically be attached
within the hood of the mating connector between the bus line and analog ground.
The selection o f this resistor needs to take into consideration the required measurement
impedance of the application and whether the time constant of the resistance with the bus
capacitance is sufficient to discharge the voltage to the required level within the switching
time of the scanning card. The bus capacitance depends on the amount of cabling. The typical
capacitance of a Tek/CDS 53A-7I5 Cable, for example, is 15 pF per foot. T h e minimum time
between release of one voltage and contact closure for the next voltage on a 53A-334 with the
C3/C4 section o f the Speed Select switch set to C4 is 3.0 msec. (See below for an example
analysis.)
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