Understanding DALI power supply

Question

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.

I need some help understanding this circuit.

^{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.

Answer 1

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".

Answer 2

For the benefit of anyone coming here looking for DALI power answers, and considering the LM317 current limiter circuit I linked to above (reproduced here), which is the one I have used with success:

Here are two scope traces from this circuit showing first the DALI transmit and the DALI response from a particular set of Drivers at the end of 150m of 1.25mm cable.

The first is with a TO252 LM317 that was cheap (from "Unisonic") - on the left is the end of the transmit, on the right is the response from a unit 150m away.

And here is exactly the same setup, but after reworking the boards to use a branded LM317 (OnSemi LM317MADTRKG). This shows the whole transmit.

One of these replies is valid, and one is not! So do yourself a favour and learn from my mistakes. This circuit is not your normal current limiter - there are no smoothing caps. A fast response and recovery is critical, as is testing - this was only reproducible with a lot of wire and a particular brand of drivers.

Answer 3

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.

Answer 4

Careful with this circuit - it limits the current to about 70mA, which is not enough to drive a full bus of 64 units. I'd suggest maybe 150mA for that (max draw for each unit is 2mA, plus you have the same circuitry on the master unit - 65 x 2mA = 130mA minimum). Reducing the resistor to 5R1 should do it.

Otherwise the AN1465 note is fairly good, I've built a lot of units based on that circuit.

EDIT: I'm actually working up some of these now, just tried swapping in a 5R1 - the solder melted and the PZT222A fell right off the board. Which did limit the current, but it seems a bit drastic.

The circuit I normally use is the one referenced in this post: Current source vs current limiter - an LM317 with a series resistor. I use a 5R1, limits power to about 240mA in theory.