Technik und Service
Seite: 30
Technology and Service
Page: 30
mecablitz 44 MZ-2 / 44 AF-3
ENGLISH
In order to ensure that the mecablitz can be switched on again immediately with the main switch, the µC
IC401 needs a RESET signal (IC402). However, this can be done only if the +5 V supply voltage for the µC
has dropped below 4.2 V. It is thus necessary to discharge the charging capacitors of the converter circuit
rapidly when the mecablitz is switched off with the main switch.
When the flash unit is on, LOW potential is present at pin 49 (+5V ON/OFF) of the µC IC401. and Tr313
conducts. HIGH potential is present at pin 6 (inverting input) of IC301B. This means that the output of IC301B
(pin 7) is LOW. In standby mode, there is HIGH potential at pin 49 HIGH and Tr313 cuts off. If the mecablitz
is now switched off with the main switch, the +6 V supply voltage drops. A charge-reversal current therefore
flows through C320, and this current momentarily makes pin 6 of IC301B negative with respect to ground
and thus negative with respect to pin 5 (non-inverting input). The output of IC301B (pin 7) switches to HIGH
and the charging capacitors of the converter circuit are discharged rapidly via this output and resistor R328. It
is now possible to generate the RESET signal needed for IC401 when the flash unit is switched on again with
the main switch.
2.2. Main converter:
When the mecablitz is switched on, the auxiliary converter (IC302) and the following stabiliser (IC303) first
generate the +5 V supply voltage (IC303, pin 5). The comparator circuit with IC301A checks the battery
voltage. When the battery voltage V
Batt
rises above about 1.8 V, IC301A switches its output to HIGH and drives
the base of Tr305 via R311. At this time, the emitter of Tr305 (which is connected to µC IC401, pin 59 MAIN
CON CONTROL) is LOW. Tr305 conducts and connects the base of Tr301 to ground via R303. Tr301
therefore conducts and drives the base of Tr304 via D302 and R304. Tr304 conducts and the base of the
main converter transistor Tr303 receives a drive signal via R308 and therefore conducts.
In the primary circuit of the converter, an increasing current flows from the batteries via the emitter-collector-
junction of Tr303 and the primary winding 6-3 of the converter transformer T301 to ground. This induces a
voltage in the secondary winding 5-2 of the converter transformer T301 and this charges the flash capacitor
via R308 and the rectifier diodes D304 and D305. The voltage drop across R308 which results from this
charging current is polarised such that it helps to keep the two transistors Tr304 and Tr303 in the conducting
state.
When the core of the converter transformer T301 becomes saturated, the current in the primary circuit is
limited only by the resistance of this circuit. Since the current in the primary circuit remains constant, no further
voltage is induced in the secondary winding. The polarity of the voltage on the secondary winding now
reverses and the secondary winding 2-5 of T301 now forms an oscillating circuit with C305 and this oscillates
by half a sinusoidal cycle (negative half-wave) with the energy stored in the core of T301. The resulting voltage
rise across R308 cuts off Tr304 and this causes the converter transistor Tr303 to cut off.
After a quarter of the sinusoidal cycle, the voltage across R308 drops again. At about the peak of the half-
wave, the voltage across R308 permits Tr304 and Tr303 to conduct again. The next cycle of the main
converter now starts with the current rise through the primary winding 6-3 of the converter transformer T301.
