DALI transceiver power supply

Question

I have a question related to DALI power supplies. I am implementing a circuit to convert the uC voltage levels (0-5 V) into DALI bus levels (0 - 16 V) in order to implement a master for DALI protocol.I have found this circuit, but I have several doubts .

The section highlighted in red corresponds to the polarising of the MOSFET in order to communicate the power supply with the DALI bus by shorting the grounds. I can also see that the Zener and the 47K resistance are used to provide a stable supply to the MOSFET.

But the circuit formed by the BC817 transistor and two 4.7 ohm resistors seems incorrect to me as it should be similar to the ones you have provided before, which limits the current on the base of the BJT with the aim of limiting the output current. The way this topology is implemented, my impression is that all the current will flow through the 4.7 ohm resistors and the BJT will be useless in this case.

Is it true what I am saying or dou you think this circuit is correct and has another use?

Many thanks.

Answer 1

The circled portion is an over-current protection circuit, and a voltage limiting circuit for the gate of T2. All current flowing through the drain-source of T2 will go through resistors R1 and R2, which have a combined resistance of 2.35Ω. When the voltage across those resistors reaches about 0.6-0.7V (e.g. about 250-300mA), T1 will turn on, pulling the gate voltage of T2 to the negative rail. This will override the "EN" signal from the opto-isolator OK2.

ZD1 limits the positive voltage of the gate when enabled to 12V, which hopefully is below the max VGS rating of the MOSFET. R4 will pull the gate of T2 back to 0V when the "EN" opto-isolator is off. R3 provides current limiting for the zener diode as well as if T1 turns on completely.