(1) The protection amplifier (fig 2-7) is a single operation device
designed to remain in a stable state until a voltage is applied to it that is in
excess of the operating point.
Once overloaded, the amplifier operates the
protection relays and holds them operated until power is removed from the
protection amplifier. The three transistors in the protection amplifier are Q303,
Q304, and Q305.
Under normal (non-overloaded) conditions there is sufficient
current flow through Q305 to hold relays K701 and K801 in their operated positions.
Transistors Q303 and Q304 are cut off, and remain so until the voltage across R313
raises to such a value as to start conduction.
Once conduction is started, each
transistor assists the other into faster saturation which lowers the voltage at the
emitter of Q303.
This emitter is connected to the base of Q305 and this voltage
being lowered causes Q305 to cut off, releasing the protection relays by stopping
the current flow to them.
R314 is used to calibrate the operating point of the
protection amplifier. The diode bridge composed of CR310 through CR313 is used to
protect the transistors of the protection amplifier from overloads and voltage
surges which might damage them.
(2) Two diodes have been included in the protection circuit to "speed up"
the operation of the protection amplifier when moderate overloads occur. CR307 and
CR314 present a low resistance path around R311 whenever the voltage drop across
that register exceeds the conduction voltage of the diodes. Without these diodes,
the time constant of R311 and C309 is sufficiently long to slow the operation of
the protection amplifier enough to damage the thermocouple or other components
under conditions of moderate overload, if the "speed up" diodes were not provided.
(3) Resetting the protection amplifier is accomplished by cutting off the
voltage to it.
It is always advisable to determine the cause of the overload
before attempting to reset the circuit.
The voltage may be cut off to the
protection amplifier by two methods.
The MODE switch may be placed in the OFF
position or the POWER switch may be operated to any position other than ON. Either
of these actions will cause an interruption in the battery power to the protection
amplifier. The preferred method is resetting the protection circuit with the MODE
switch.
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