bus-fuse 12C-Circuit

All I²C cards published so far in this magazine (see References/bus-fuse) are normally powered by the 5 V supply in the PC via the 6-way DIN socket on the 12C PC insertion card. A drawback of this arrangement is that the 5 V supply in the PC is at risk of being short-circuited by a fault on any of the I²C cards connected. Since most PC power supplies are capable of supplying very high currents at the 5-V output (20+ A is typical!), short circuits can cause a lot of smoke and annoyance. The circuit shown here protects your costly N, hardware by inserting a fast I A glass fuse in the 5 V supply line between the I²C interface card in the PC and the first I²C card in the chain. Alternatively, for even better security, a fuse card may be inserted into every cable that connects two I²C boards.

Two LEDs are fitted on the fuse card to signal the presence of the 5 V supply voltage ahead of and behind the fuse. The LEDs cause an additional current consumption of about 20 mA. The printed circuit board shown allows five fuse cards to be built.

bus-fuse 12C-Circuit Parts list,

Resistors:

• R₁. R₂ = 330 Q

semiconductors:

• D₁, D₂ =LED, 3 mm, red

Miscellaneous:

• K₁. K₂ = 6-way mini-DIN socket for PCB mounting
• Fl = fuse, 1 A (fast), with holder for PCB mounting
  PCB Ref. 934016

References:

•  I²C interface for PCs. Elektor Electronics February 1992.
•  ADC/DAC and I/O for I²C bus. Elektor Electronics March 1992
• I²C LED display. Elektor Electronics June 1992.
• I² Opto/relay card. Elektor Electronics February 1
• 993.