Bird Technologies 8650B Series - Manual (Page 17)

5
Chapter 2 Theory of Operation
The Econoload Load Resistor is unique in having its coolant flow over the
outer surface of the substrate. This eliminates the need for an intermediate
heat transfer system thus reducing the load size to a minimum.
Heat Transfer
The 50 ohm resistor consists of a substrate coated with resistive film. The heat
generated by absorption of RF power is transferred from the film to the cool-
ant flowing over it and through a restricted chamber surrounding the resistor
body. This coolant passes over the entire length of the resistor and is cooled in
the forced air heat exchanger.
Interlock
Dissipation of the heat generated by RF power is critically dependent on a
minimum coolant flow of eight gallons per minute at all times, regardless of
coolant temperature. If coolant flow drops below a preset value, the pressure
switch opens and immediately causes the transmitter to shutdown. The pres-
sure switch is a “normally open” type (i.e., its electrical contacts open when
deactivated), and are closed during normal operation.
After coolant flow is restored, a time delay relay keeps the interlock open for an
additional 2 seconds. This ensures proper operation of the cooling system before
RF power is applied to the load thus preventing resistor damage or burnout.
The interlock circuit also contains a temperature switch to monitor the cool-
ant temperature. This switch is normally closed. It will only open if the system
becomes too hot to adequately cool the load. Once the system cools, the switch
closes. Though, as with the pressure switch, the time delay relay impedes the
closure of the interlock contacts for two seconds.
Flow Interlock Control Circuit
The interlock control circuit provides instantaneous fail-safe protection of the
transmitter and load in the event of even momentary interruption of the cool-
ing water supply. This protection is necessary because dissipation of the heat
generated by the RF power is critically dependent upon a required minimum
water supply at all times regardless of system water temperature.
The water pressure switch, installed on the RF load’s water input is a “nor-
mally open” type; (i.e., its electrical contacts open when deactivated). Closed
during equipment operation, the switch is adjusted to open whenever water
pressure drops below the safe flow level for the specific unit. When this occurs,
the time delay relay is instantly deactivated, thereby opening the interlock
circuit and causing immediate transmitter shutdown.
Also, the centrifugal impeller of the water pump is carefully selected for the
necessary flow rate. The proper operation of this equipment depends on these
conditions being maintained.
CAUTION
Do not attempt to alter the pressure switch setting or disturb the coolant pump.