Technik und Service
Seite: 31
Technology and Service
Page: 31
mecablitz 44 MZ-2 / 44 AF-3
ENGLISH
Regulation of the flash-capacitor voltage:
The voltage on the flash capacitor C001 is divided by the voltage divider consisting of R4010 and R4011 and
connected to the measuring input at pin 47 (U MAIN CAP IN) of µC IC401. C404 smoothes the measured
voltage and removes any interference. The µC thus monitors the voltage on the flash capacitor. To charge the
flash capacitor C001, µC IC401 switches pin 59 (MAIN CON CONTROL) to LOW and thus enables the main
converter. When the voltage on C001 rises above 330 V, pin 59 switches to HIGH, cutting off Tr305, Tr301
and Tr304. It is now no longer possible to drive transistor Tr303 of the main converter and the converter stops
running. If the flash-capacitor voltage subsequently drops to about 320 V, pin 59 of µC IC401 switches back
to LOW and the main converter resumes operation.
The Schottky diode D404 at pin 47 of µC IC401 is a protective diode which ensures that the voltage at pin 47
cannot rise above the voltage at the supply-voltage connection pin1 (+VDD) / pin 71 (VDD0) of µC IC401 if
the mecablitz is switched off with the flash capacitor charged.
Converter protection circuit:
The converter protection circuit is intended to prevent the flash-capacitor voltage from rising above 350 V,
something which could happen in the case of a fault in the converter circuits. The flash-capacitor voltage is
connected via R309 to the Zener diode D306. If the flash-capacitor voltage rises to about 350 Volt then, after
the Zener voltage of D306 has been overcome, the Zener current starts to generate a voltage across R312
(and thus at the gate of thyristor Ty301).
When this gate voltage reaches about 0.6 Volt, Ty301 conducts and thus connects the base of transistor Tr302
via R306 to ground. Transistor Tr302 then conducts and connects the battery voltage (minus the voltage drop
across a diode) to the base of the main-converter transistor Tr303. The base and emitter of Tr303 are thus at
practically the same potential, causing Tr303 to cut off and deactivate the converter.
2.3. 17 V supply voltage:
The 17 Volt supply voltage for the IGBT driver stage is derived from the flash-capacitor v 330 V.
Zener diode D310 is connected via R324, R325 and R326 to the flash capacitor. At a Zener current of as little
as about 50µA, a constant voltage of 20 Volt is generated at the gate of the IG-FET Tr312. This transistor acts
as the control element for the regulation circuit and a supply voltage of +17 V is available at C318 for the
IGBT driver stage. In the case of a short-circuit in Tr312, D311 (protective diode) limits the +17 V supply
voltage to a value which cannot cause electric shocks. For this, R324 provides the necessary current limiting
function of Tr312.
2.4. Motor-zoom supply voltage:
The +3.3 V supply voltage for the motor-zoom control is derived from the battery voltage via the polyswitch
R301 and fuse Si301. The parallel-connected transistors in the regulation circuit consisting of Tr308/Tr309
and Zener diode D309 stabilise this voltage at +3.3 V.
