3
Chapter 2
Theory of Operation
Load Resistor
Bird 889x loads consist of a thin-film-on-ceramic resistor immersed in a dielec-
tric coolant. The resistor, individually selected for its accuracy, is enclosed in a
special housing. When surrounded by the coolant, this produces a uniform,
practically reflectionless line termination over the specified frequencies.
Coolant
The load is cooled by natural fluid and air convection currents. The coolant,
chosen for its dielectric and thermal characteristics, carries heat from the
resistor to the walls of the cooling tank, where radiator fins surrounding the
tank transfer the heat to the air.
When the coolant is heated, thermal expansion causes an increase in the
internal pressure. The vent plug (Figure 3, Detail “B”) relieves this pressure
while protecting the opening from dirt or other contaminants.
Thermal Interlock
When installed, a passive, normally closed over-temperature thermoswitch
opens above the maximum safe load temperature of 236 °C (457 °F), turning
off transmitter power. The interlock will not permit use of the transmitter
until the load has reached a safe temperature.
Blower Assembly
When installed, the blower assembly provides forced airflow with two blowers.
Baffles direct the airflow from the blowers over the radiator fins, doubling the
heat transfer efficiency.
Example -
A 2.5 kW load will safely dissipate 5 kW. A passive,
normally open control thermoswitch closes when the coolant reaches
155 °C (311 °F), turning the fans on.
The baffles interfere with the free flow of normal air currents, causing a 50%
reduction in heat transfer efficiency if the forced airflow is stopped. Thus, a
2.5 kW load will have its maximum power dissipation reduced to 1.25 kW.
Figure 3 Blower Plugs
O-ring seal
O-ring seal
DETAIL “A” -
Shipping Plug
DETAIL “B” -
Vent Plug
深圳市新朗普电子科技有限公司
www.xLp17.com Email:[email protected]
TEL:0755-83986300 FAX:0755-88850515
QQ：55178055 285480356
深圳市新朗普电子科技有限公司
www.xLp17.com Email:[email protected]
TEL:0755-83986300 FAX:0755-88850515
QQ：55178055 285480356
