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I am working on a DALI system and I came across this schematic for the DALI power supply in a Microchip app note AN1465:

It says the power line can be 12-20 Volts but the current has to be limited to 250 mA or so. Some power supplies limit to 70 mA or 100 mA as well.

dali power supply

I need some help understanding this circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

I made the schematic and ran a DC sweep for various values of R3 (1 ohm, 100 ohm, 1000 ohm) and understood that if I put a heavy load (R3), the voltage at Node1 will go low (mV range). However I need some help understanding what exactly is happening.

Is this schematic better or more efficient than a series resistor to limit the current to whatever I want. For example, I could simply put a 120 ohms resistor to limit the current to 100 mA on a 12 V power supply line.

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Simply use a readily available linear voltage regulator IC. They cost cents and are easy to use, and actually do work reliably across their full range. Current limiting can be achieved either by a linear supply with current limiting built-in, or using something like a low-side shunt resistor (much smaller than 12 Ω) (diode-)attached to the feedback port of an adjustable linear regulator.

Maybe you're already using a supply that has current limiting! Many switch-mode supply ICs have dedicated current sensing and limiting functionality.

Anyways:

However I need some help understanding what exactly is happening.

When you draw a lot of current, the voltage across R1 becomes large enough to put the two diodes in forward operation, i.e. they start to conduct. Current going through the diodes can't go through the base of your transistor, which in turn "turns off".

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  • \$\begingroup\$ Sorry sorry sorry, I forgot to mention the source and I missed an important point. DALI works on 12 V but the current has to be limited to 100-250 mA. Given circuit limits the current not the voltage. \$\endgroup\$ – Whiskeyjack May 13 at 10:47
  • \$\begingroup\$ ah ok, modified a bit \$\endgroup\$ – Marcus Müller May 13 at 10:50
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A series resistor will not achieve what is required - the voltage has to stay above 12V for all currents until you get to the current limit point, and then it must collapse below 4.5V. The supply must be able to supply the input current for all the devices on the line (eg 2mA max for each control gear) without the voltage dipping below 12V, but when low pulses are to be sent by pulling down on the line, the supply must go into current limit within 10us with the voltage falling below 4.5V.

You don't want a linear slope down to 0V as the current increases, for example, your 120 ohm resistor would drop 6V when the current is 50mA, so you only get 6V output, which isn't acceptable for DALI.

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