3.2
Overload Circuits
3.3 Power
Supply Circuit
Electronic Wattmeter
Chapter 3
A considerable proportion
of
the EW604 circuitry is concerned
with coping with
overload. The overload condition is catered for
in
two ways
on
both the current and the voltage inputs, firstly
by
indicating when
an
input exceeds the linear input range
and
secondly
by
protection against damage for larger inputs.
In
the main circuit of
fig
4.3 the current input, TR9
and
TR1
0
detect overload conditions at the output of AC and AA switching
on
TR11
thereby illuminating the overload indicator. The current
circuit is protected against excessive inputs by the 1
OA
slow-
blow
fuse. Diodes
01
and
02
protect the input of
AC
from
excessive
voltages until the fuse ruptures.
With the
voltage input,
TR6
and TR7 detect voltage excursions
outside the
linear range and illuminate the overload indicator
via TR8. Excessive input
voltages cause the emitter base
junction of
TR6
to conduct, the current being limited
by
R2,
R3,
R4
and
R5
and the switched voltage range resistor, these
components being of sufficient wattage rating to withstand the
normal mains supply voltage indefinitely.
The power supply provides from a
single rectified
and
smoothed
output:
-7.5V
zener regulated, +0.7V
and
+24V unregulated.
22
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