Preface; Power Supply; Auxiliary Converter - Metz mecablitz 44 MZ-2 Servicehandbuch

Technik und Service Seite: 29
Technology and Service Page: 29
mecablitz 44 MZ-2 / 44 AF-3
ENGLISH

1. Preface:

The mecablitz 44 AF-3 flash unit was developed especially for use with the autofocus system cameras
manufactured by Canon and Nikon. This flash unit has various special flash functions in addition to TTL flash
control, including motor-zoom control and a maximum flash range indication in the LC display. The various
versions of the mecablitz 44 AF-3 differ basically in that they have different printed circuit boards for the
controls and camera-specific foot modules.

2. Power Supply:

The power for operation of the mecablitz is provided by four size-AA cells (dry or rechargeable batteries)
loaded in the battery compartment. The polyswitch R301 also acts as the main fuse (maximum continuous
current about 7 A). The primary circuit of the main converter receives its input voltage directly from this fused
switch. The other low-voltage circuits in the mecablitz are additionally protected by fuse Si301.
In order to ensure that no dangerous voltages are present at the battery terminals in the case of a fault, the
Zener diode D301 limits the voltage on the supply-voltage line (battery terminal) to a maximum of 33 V.

2.1. Auxiliary converter:

Voltage stabilisation:
When the mecablitz is switched on, the auxiliary converter (the circuit around IC302) starts to run first. This
circuit operates as a blocking oscillator with L301 as the coil, Tr306 as the switching transistor and the
Schottky diode D308 as the rectifier. The circuit compensates for the variations of the battery voltage V +6 V
Batt
which occur, for example, after a flash is triggered. The output voltage of the circuit at the cathode of D308 is
divided by the voltage divider consisting of R317and R318 and is available at pin 1 of the converter IC302 as
the ACTUAL value (approx. 1 V) for comparison with a reference value generated within the IC (DESIRED
value). Comparison of the DESIRED and ACTUAL values within the IC controls the frequency and the pulse-
pause ratio of the signal which drives the gate of Tr306 such that an almost constant output voltage of +5.5 V
is generated on the charging capacitor C311 of the auxiliary converter. The frequency and pulse-pause ratio
are also adjusted automatically by IC302, depending on the magnitude of the battery voltage V .
Batt
The following voltage stabiliser with IC303 maintains a constant supply voltage of +5 V (IC303, pin 5) for the
connected ICs and circuit sections. This second stabiliser is necessary to ensure that the ICs (such as µC IC401)
do not receive an excessively high supply voltage (+6 V) when the flash unit is operated with fresh batteries.
Discharge circuit:
The mecablitz has a timer function which automatically switches it to a power-saving standby mode three
minutes after the last action is executed on the camera or the flash unit ("Auto-Off"). If the mecablitz is switched
off with the main switch while in this standby mode, there would be problems if it is immediately switched on
again, since the flash unit would remain in the standby mode.