139 / 2663 CIRCUIT DESCRIPTION
Art. No.: 80116-201
09 / 2004
Motherboard
The following functions are also found on the motherboard:
• Starting current limitation
• Supply voltage changeover and power rectification
• Power transformer for low voltage supply
• Rectifier for low voltage
• Chipselect generation and level adaptation of TTL level to 15 volt level for CMOS components.
• Generation of the external control bus and of signal lines from the data from PIO outputs
• HF leakage current measurement
• HF leakage current limitation
• Changeover between floating and capacitance-grounded output
• Input circuit for the footswitch
• Various level adaptations.
The motherboard detects the other PCBs via connectors and contacts them.
Starting current limitation
The supply voltage moves from the power supply to the motherboard via the connector J 11.
Immediately after switching on the ICC, the supply unit capacitors are charged which causes a heavy
power surge by which, without protective measures, the fuses for the in-house electrical system would be
triggered.
Therefore a starting current limitation has been realized on the motherboard which limits the current peak
to noncritical values after switching on.
The corresponding power current first flows into the ICC via the resistor R5 and the fuse F1. The relay
contacts REL 1 are still briefly opened. Power current is therefore temporarily limited to noncritical values
due to resistor R5.
As soon as the 24 volt supply voltage is available, relay REL 1 can pick up, bridges resistor R5 and fuse F1
with its relay contacts, and relieves both of them. This short span of time is sufficient to effectively limit the
starting power surge.
LED D2 lights red when the relay REL 1 is actuated and thus serves as a control.
Supply voltage switchover and power rectification
The unit can be changed over either to the 230 volt or 115 volt supply voltage range. The remaining voltage
deviations within the specification are offset by the controlled power supply units.
The supply voltage can be switched over by shifting the connectors J 16 to J 22.
In the 230 volt range, the bridge-connected rectifier BR1 functions as intended as a true bridge circuit. The
wire bridges must be connected as follows:
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