Philips PM6302 Instruction Manual page 33

33
The overloaded amplifier (307/308) makes from the 20 V-supply voltage signals with steep edges. The signal
for searching stage 305 is supplied by stage 309. It is controlled by means of a current, which is taken from
rectifier arrangement 407/408. This rectifiers are supplied with a symmetrical 20 V voltage from the mains.
part.
When push-button R is not depressed, the operating points of the overioaded amplifier and the control stage
(309) are shifted in such a way that control from the mains part is blocked, transistors 301 and 305 are opened
and 302 remains blocked.
3.1.2.2. Operational amplifier 306 is switched as a voltage-current converter. The bridge rectifier is d.c.-free
3.1,3.
incorporated in the feedback arm in series with resistor 662 (Fig. 25). The voltage drop across this resistor
only depends on the current which is taken from the amplifier output through the bridge rectifier. In such a
way a linear relation between the rectified voltage and the control voltage is obtained even for small amplitudes
of the control voltage, near the zero passage.
In order to adapt the sensitivity of the zero indicator to the application purpose (Fig. 15}, a circuit with diode
405 and resistor 617 is parallel-connected in flow direction to measuring instrument 821. In case of an
increasing diagonal current of the rectifier bridge, the current through diode 406 increases also and consequently
the scale-deviation change of the measuring instrument decreases. In this way the characteristic of the zero
indicator becomes logarithmic. This curve is not appropriate in the searching mode; therefore, in that case
diode 4085 is short-circuited by switch 801/D and the indication becomes linear.
For searching mode, see 3.1.4,
Supply voltage generator
A switchable supply part is used to supply the bridges.
31.3.1. The d.c. supply voltage is produced in bridge rectifier 415, which is taken from a well-screened winding of
the mains transformer. This winding is floating and symmetrical to earth. The filtering serves to reduce the
ripple voltage to a minimum.
The internal resistance of the source amounts to approximately 11 Ohm,
2.1.3.2. The supply oscillator contains a Wien-fourpole in the positive feedback loop and an amptitude-dependent
3.1.4.
resistance (409-410 and 640, 641) in the negative feedback ioop .
Operational amplifier 313 is protected against overload by means of anti-parallel connected diodes 413/414
at the input and by clamp diodes 411/412 at the output. Output transformer 71 transforms and separates
the output voltage, As a result of the very high voltage feedback {open amplification reduced from
approximately 104 to 3) the output impedance is extremely low.
The oscillator frequency of 970 + 19 Hz is electronically switchable to 97 + 2 Hz. In mode 97 Hz capacitors
521 and 525 are short-circuited by paralle|-mounted transistors 314 and 312. These capacitors are part of the
double, series connected, capacitors of the Wien-fourpole viz 518, 521/524, 525. If only capacitors 518 and
524 are active, the frequency amounts to 97 Hz. The control of the transistor switch is shown in Fig. 16.
Switch combination D {801/F) and SEARCH (801/D) guarantee this frequency changeover for C mode and
push-button D (801/F) depressed.
In mode SEARCH the frequency remains 970 Hz.
In case of external supply, the shunt impedance of the Wien-fourpole is short-circuited by the switch operated
from socket 856, The external supply voltage is supptied to the inverting input of the operational. amplifier
via resistor 647.
Arrangement for mode SEARCH
Push-button SEARCH (801/D) switches over the bridge arrangements into voltage dividers consisting of
two components, conform to Figs. 8, 9 and 12. At the same time the characteristic of the zero indicator is
linearized (801/D 5-6), the blocking of the interna! frequency changeover for L measurement is completed
(801/D 15-16) and the blocking of the sensitivity increase is prepared for the three first R measuring ranges
(801/D 12-13).
This last blocking becomes active when push-button R (801/A 12-43) is depressed.
In this searching circuit, the impedance of the sample is compared with the measuring-range resistance and the
measuring-range switch is operated until the indicator points to the ww field on the measuring instrument.
There are limit cases, for which the coarse range indicated does not apply. The measurement then must be
performed in the adjacent range.
SSn Shao S