I'm currently trying to figure out how this circuit work :enter image description here

I found this in a microchip documentation dealing with DALI (Digital Addressable Lighting Interface) protocol. The DALI protocol is mostly used to communicate with lighting equipment. I can already understand what is going on for the most part but I'm still having trouble with the function of the diode bridge. I know that diode bridge are used to convert AC to DC voltage. But in this circuit, the input is already a DC voltage, so I'm wondering what is this diode bridge doing exactly?

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    \$\begingroup\$ It would be useful to link the document this was from \$\endgroup\$ – MCG Nov 25 '19 at 13:26
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    \$\begingroup\$ Ensures only positive polarity pulses are received. \$\endgroup\$ – Captainj2001 Nov 25 '19 at 13:31

While the DALI bus has DC power on it, the wiring topology is not polarity-sensitive. That is, either wire can go on either pin of any DALI interface connector. From the Wikipedia page:

"Each device has a bridge rectifier on its input so it is polarity-insensitive."


  • \$\begingroup\$ Idiot-proofing a commercial system makes sense. No one actually read instructions much anymore. At least, not detailed ones. \$\endgroup\$ – jonk Nov 25 '19 at 14:02
  • \$\begingroup\$ It uses the BiPhase encoding scheme to modulate data on changes rather than binary level. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 25 '19 at 14:17

I suspect the bridge is there to protect the circuit and keep it functioning if the DALI bus wires are reversed.

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    \$\begingroup\$ From the Wikipedia article: Each device has a bridge rectifier on its input so it is polarity-insensitive. \$\endgroup\$ – Mattman944 Nov 25 '19 at 13:26
  • \$\begingroup\$ No it is not mainly for protection but for Common Mode Rejection with optical isolation. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Nov 25 '19 at 14:58

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