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    Reverse polarity connection of 16V DALI comms rail

    Hi,
    We have a DALI comms based LED driver. This contains the LED driver and the DALI power supply (this is just a 16V rail with a current clamp on it). However, sometimes, our customers will connect their own 16V DALI rail into a DALI power supply connector…as shown in the schematic. Sometimes, the production staff may accidentally connect the 16V DALI rail in the wrong way round , (as shown in the schem).

    Therefore, we have added diodes D1, D2 and D3 to protect against reverse polarity connection. Part of this is to prevent the NPN BE junctions getting too far reverse biased by a reverse polarity connection.
    We cant just use a reverse polarity diode near the connector, because sometimes there is no external DALI power supply, and sometimes the internal DALI rail needs to go to the outside via the connector.
    Do you think there is a more efficient way to provide the reverse polarity protection here…instead of using three diodes like we have?

    LTspice sim also attached

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    Re: Reverse polarity connection of 16V DALI comms rail

    D1 might have some protective effect but I would doubt reversing the polarity would damage it anyway.
    D2 only protects against reverse polarity from the microcontroller. If it really is producing a negative voltage you have a more serious problem!
    D3 again might offer some protection at the expense of increasing the DALI low level.

    Given the line is current limited and will not work if the polarity is reversed anyway, why not simply fit a reverse biased diode across the supply line. It will do nothing in normal operation and simply 'short out' the line if reverse polarity is applied. You could even use a Zener to give some protection against over voltage (18V for example), it would still shunt to ~0.7V if pushed in forward direction.

    Brian.
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    Re: Reverse polarity connection of 16V DALI comms rail

    Thanks,
    The line is current limited from the left hand source. However, the other side might get a 'pure' voltage source accidentally plugged into it, and the wrong way round, as shown in the schem. (it may (or may not) have a 240R resistor in series with it). (eg , A24V supply or 16V supply may get plugged into the connection on the Right Hand Side, and the wrong way round.)

    D1 is to prevent the BE diode of Q10 getting more than 6V in reverse bias due to the V3 connection in reverse polarity.
    D2 is to prevent the BE diode of Q8 getting more than 6V in reverse bias due to the V3 connection in reverse polarity.
    The simulation included shows the BE of these NPNs going above 6V in reverse bias when D1 and D2 (and D3) are absent.



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    Re: Reverse polarity connection of 16V DALI comms rail

    I see where you are coming from but there has to be some sensible limit to how much protection you provide, especially against idiots!
    Someone could swap the DALI and AC lines over but ask yourself if it's worth up-rating all the components to withstand 400V+, probably not.

    If there is a real danger of someone wiring it wrongly, I would still just add a diode across the supply lines but add a fuse ( 1A ?) at the output. If you got units back with blown fuses, at least you know the failure reason and can blame the customer.

    Brian.
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