Erbe ICC 200, Руководство по обслуживанию

Страница: 150 / 266

150
/ 266
3 CIRCUIT DESCRIPTION
sparking which occurs during the cutting and coagulation process at the active electrode. Both control
principles have concrete applications.
The control of spark intensity can be activated via the “HIGH CUT” key.
On the Senso-board, the DC voltage U_FUNKE occurring during a spark is gathered via an isolation
amplifier (description there) and directed to the control-board.
The required set value of the spark intensity is transmitted via the control bus to the digital analog converter
IC8. Its analog output signal is combined and compared at the comparator IC4 with the actual value of the
spark U_FUNKE. If the actual value is greater than the set value, output 12 tilts to frame potential and
illuminates the red LED D1. In addition, the output signal moves to the input Pin 2 of the NOR circuit IC5.
The gate IC7 with the monoflop IC10 switches off the QK output stage for a minimum period of time. In
this way, the supply voltage for the HF generator is reduced and therefore also the intensity of the spark.
Actuation of the QK output stage
The MOS field effect transistors of the quasi-complementary output stage require brief start or stop pulses
for their actuation during the zero crossing of the sinusoidal output current.
As is known from the section “Synchronization of the QK output stage”, there is a pulse at the output of the
priority encoder IC11 which is already synchronous to the zero crossings of the output current. This pulse
must now be prepared in such a way that a start or stop pulse is produced for each of the two output stage
transistors at the right point in time respectively.
If one assumes that neither of the two QK transistors are switched conductively after a pause, Transistor A
first requires a start pulse. The transistor then switches on and produces a sinusoidal half-wave within the
oscillating circuit of the QK output. At the end of this half-wave, this Transistor A must be switched back
off using a brief pulse. Then Transistor B should take over the second half-wave and be conductive. Therefore
Transistor B requires its short start pulse after Transistor A has switched off. It remains conductive until the
zero crossing of the second half-wave and must be shut off again at its end using a pulse.
In this way, a complete sine-wave oscillation has resulted and the process repeats itself constantly until the
QK output stage is switched off.
The output of the circuit IC11 produces a pulse with every zero crossing on the sinus current. However, the
actuation requires, as described, two pulses during a half-wave, specifically a start and a stop pulse. It is
therefore necessary to double the frequency of the available pulse. This occurs in the EXOR gate IC12
together with the RC low-pass R67, C15. Using this low pass, the signal at input 1 of gate IC12 is slightly
delayed compared to input 2, which leads to an output pulse with the length of the delay time for every
change in the input signal. The input frequency is thus doubled. The resulting pulses are extremely short.
By means of small, though non-negligible running times in the circuit, the signal received in the meantime
is no longer completely synchronous to the current zero crossings of the output signal. Using the monoflop
IC13, the correct phase relationship of the actuation pulses to the output current can be re-established by
specifically extending the pulses. The necessary delay time can be adjusted using the trim potentiometer
TP13.
By halving the previously double frequency in D-flipflop IC21, Part 2, the original operating frequency of
the QK output stage is recreated with a pulse duty factor of 1:1. Output pin 13 has a HIGH level, while
Transistor A is addressed. The inverted output 12 has a HIGH level, while Transistor B of the QK output
Control board
Slot J3