# How can a DALI power supply handle shorting

I'm trying to understand how a DALI power supply can handle being shorted, when a controller is transmitting a signal. The AN1465 application node helped a lot with understanding a usage, but I'm still not quite sure about how the power supply handles the shorting.

Is it using a form of short circuit protection? I know that the supply is limited to 250 mA, but what about the shorts?

I'm still not quite sure about how the power supply handles the shorting.

From the document you linked is this short circuit current limiter: -

That circuit will limit the maximum external current to about 60 mA when shorted out although it won't be capable of driving a full bus worth of units.

• But how? Wouldn't the transistor alwasy be on? And what purpose does the diodes have? I know the voltage drop across is gonna be around 1.4V, but why is it needed? Jan 19, 2022 at 15:38
• The transistor is always on but, as the current limit is reached it begins to turn off. It's a standard circuit used in many many places. If the voltage across the 12 ohm resistor tries to get too high (greater than about 0.7 volts) it starts to reduce the base current and this turns the transistor off. Jan 19, 2022 at 15:41
• Ahhh, so just to completely dumb proof it. If the current through the R2 gets too high, then the relation between the I_b and I_e of the transistor, results in the transistor turning off? But wouldn't current still be able to flow through the diodes? Jan 19, 2022 at 15:51
• The current that can flow through the diodes is limited by the 1 kohm resistor to maybe 10 mA on a raw 12 volt supply. So maybe up to 70 mA is possible on a dead short. Jan 19, 2022 at 15:56
• Okay. Thank you for the great explanation Jan 19, 2022 at 15:58

If it has 250mA current limit, it will limit current to 250mA in all cases, even in short circuits.

Which happens a lot, because when any device wants to communicate a logic low level on the bus, it will short circuit the wires.