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4.5. PROTECTIVE CIRCUITS
The 2707 is protected by fast-acting Silicon Controlled Rectifiers
(SCR's) which, when triggered, turn-off the two FETs at the input, thus
disconnecting the drive and DC centring signals. Each triggered SCR also
lights a lamp which gives an indication of the reason for equipment shut-
down.
Overload protection is provided against excessive coil drive current
(CURRENT), shaker table displacement (DISPLACEMENT) and amplifier
temperature increase (TEMPERATURE). Activation of the exciter head/
body protection will trigger the EXCITER circuitry.
To reset the amplifier, the AMPLIFIER GAIN control must be reduced
and turned fully anti-clockwise to the click-position marked "Reset". This
opens the anodes of all protection SCR's. Increasing the gain once more
returns the amplifier to its normal operating condition with the protection
circuitry again enabled.
4.5.1. Current
A front-panel control (CURRENT LIMIT) is used to pre-set the RMS
output current (including the DC component) at which the CURRENT
circuitry trips. The limit can be set anywhere between 2 A and 24 A.
The squared and averaged value of the voltage across the current feed-
back resistor is compared to a reference. If the reference level is exceeded
the CURRENT SCR will be triggered, turning on the red front-panel lamp
labelled CURRENT and disconnecting the system inputs. The time-constant
of the averaging circuitry is 50 sec., matching the minimum thermal time-
constant of the Exciter Heads used with the 8 & K Exciter Body Type
4801. This permits short-term current overload and means the shaker
system can be used in a pulsed mode. The averaging time-constant can
simply be reduced to 2.5 sec. if a "snap-action" current trip is required (see
section 3.4) or if a non-8 & K exciter is to be used.
Interlocking ensures that any attempt to alter the OUTPUT I MPED-
ANCE switch when the amplifier is delivering power to its load will trip the
CURRENT protection circuitry.
The Exciter Head moving coil is not connected to the amplifier until the
three-position POWER switch is at "Load On". When switching on the
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