ITCI Brasov CoBra, Hardware Description

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IC2 , which has the output pin 6 in a high state (0V) at startup (power-
on), flips to a low state (–5V). The Zener diode Z6 opens up holding the
potential of pin 2 of IC3 relative to ground at approx. –1.2V. The output voltage
will be equal to the sum of the voltage on S2 , R9 and the voltage on R11 (1.2V
— datasheet value), that is approx. 0V. Rearming the +5V voltage can be done
by turning the main switch off and then on. Due to the tight admissible
tolerances for the +5V voltage (±0.25V), in order to avoid accidental increases
of the +5V voltage, a crowbar protection was also designed, built with
TH1 , Z7 ,
R17 , which upon exceeding a voltage of approx. 6V triggers the overcurrent
protection by opening TH1 .
The protection on the +12V source works as follows: when a shortcircuit
on +12V occurs, on the base of transistor T1 a positive potential less than 1V
relative to ground appears, which divided by R3 , R4 is applied to pin 7 ( ALO ) of
IC1 , blocking the oscillator. Thus the –5V voltage dissapears, Z2 is blocked,
transistors T2 , T3 are opened and through T2 saturated the oscillator built with
IC1 is held in a blocked state. The light bulb B was included in the +12V
stabilizer schematic due to the nonlinear character of its resistance, this way
achieving a limitation of the shortcircuit current by limiting the base current for
transistor
T1 . Through T3 saturated, the +5V voltage will be close to 0V too, so
in conclusion a shortcircuit on the +12V output will cut all three voltages off.
rearming is done by turning the main switch off and then on.
The protection on the –5V source works as follows: a shortcircuit on the
–5V output causes Z2 to block, T2 , T3 get saturated and through T2 , R3 , R4 the
potential on pin 7 of IC1 is close to 0V, blocking the oscillator built with IC1 . It
is obvious that this state is maintained until the source is rearmed. All the three
stabilizers are blocked until turning the main switch off and then on.
Powering up:
After verifying the connections, one pin of diode Z7 is disconnected to
avoid triggering the crowbar protection, S1 is positioned in the middle and S2
set to its minimal resistance, and then the circuit is plugged in. The presence of
the +5V voltage is verified. If it is not present, the oscillator built with
IC1
should be checked. After this, the presence of the +12V voltage is verified after
which the +5V voltage is adjusted without load. The oscilloscope is connected
between the cathode of
D5 and ground. The system should be oscillating.
After this, a load of approx. 1.3 Ohms and at least 12W is connected to
+5V, a load of 39 Ohms/5W connected to +12V and a load of 100 Ohms/0.5W
connected to –5V.
All voltages are again checked, adjusting the +5V voltage with S2. The
voltage on D5 is visualized on the oscilloscope checking that the frequence is
around 33 kHz (+2 kHz, – 4 kHz) and the amplitude around 18V.
A fine adjusting of the frequence can also be done by modifying the
resistance of R10 .
In order to get to the maximum efficiency, the fuse is pulled out of the
socket on the PCB, inserting instead an AC ammeter ( MAVO-35 set to the 5A~
position). The main switch is turned on and S1 is carefully adjusted until a low
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