Fluke PM3082 Service Manual page 363

5.6-2
To reduce the switching losses in
V1019, a dV/dt limiter, often called
"snubber", is used. C1021 decreases
the dV/dt of the collector voltage of
V1019 during switching off, as the
current to the transistor can pass
during a certain time through C1021.
This slowing down of the collector
voltage will reduce the switching
losses during switching off. During
the ON cycle, the energy in C1021 is
transferred to L1006 and the
capacitor is discharged. During the
OFF cycle, the energy in L1006 is
transferred to C1018 and during the
next ON cycle, the energy in C1018 is
delivered to the transformer. In that
way, no energy is wasted. As a
consequence of this system, the
voltage at the transformer is slightly
increased during the first part of the
ON cycle, but this has no
disadvantages.
Voltage regulation takes place by varying the control voltage from R1046 to the gate of V1014. A
more positive voltage will cause a smaller peak current through the transformer and this results in
smaller output voltages. The converter frequency can be 20 to 50 kHz. This depends on the mains
voltage and the load of the power supply. The lower the mains voltage, the lower the frequency. A
lower load means a higher frequency. R1018 compensates for gate-cathode voltage variations of
V1014 due to the temperature.
Line trigger circuit
For triggering purposes, a sinusoidal signal at the mains frequency is available. Of course there will
be no LINE signal if the mains voltage is DC.
A small signal is picked up with capacitors C1002, C1003, C1004 and C1006 and amplified in N1046.
This results in output signal at pin 1. This circuit provides a sine-wave with low distortion and with an
amplitude of 3 to 8 V, depending on the mains voltage.
Diagram 2
Diagram 2 comprises the following circuit parts:
- trace rotation control
- fan control
- illumination control
- EHT converter
Trace rotation control
To supply the trace rotation coil, of which the resistance is about 200 Q, a voltage of -10 V to +10 V
is created in amplifier V1146-1147. Control takes place via a part of N1101 by means of the signal
DAC3 which can be 1 to 10 V, together with the signal DAC0 with a level between 1 and 3V. The
signals DAC3 and DAC0 originate from the microprocessor unit A3.
Fan control
The speed the cooling fan depends on the temperature in the oscilloscope. This temperature is
measured at the microprocessor unit A3 by a NTC resistor. The microprocessor generates the signal
DAC1 with a level of about 1.7 to 4 V. The fan is supplied by amplifier V1148 which is controlled by
this signal. The output from the amplifier is a DC voltage of -10 to +10 V.
Figure 5.6 Timing diagram converter circuit
UNIT DESCRIPTIONS