# transistor loop to limit the current

I am trying to implement the DALI TX/RX circuit below.

The description says:

The other transistor (T2) is controlled by the current flowing through the resistor (R1). If a DALI slave unit is connected, this current will be the same as that flowing through the power transistor. The value of the resistor is chosen in such a way that when the current exceeds 250 mA the voltage level across the resistor will open the transistor (T2) which in its turn closes the power transistor (T1). In this way the current is maximized to 250 mA.

The designed circuit is here below, the relative part is in the red rectangular box.

Could anyone explain me how does the resistor R1 (in the first picture) value 2.7 ohm opens the transistor when the current exceeds 250mA? On that design, the signal is transmitted as 12V (logic high) and 0V (logic low).

Here below is the circuit lab drawing in case it is needed.

simulate this circuit – Schematic created using CircuitLab

V2 is a pwm that gets 5V and 0V values.

datasheet of T2 and T3 transistor (BC337)

datasheet of T1 transistor (BD135)

• Be careful to check that your design includes an edge rate limit so that you meet the 10us minimum requirement for DALI. Often example circuits from microcontroller manufacturers do not bother with this aspect but it is necessary in the final design. – Martin May 8 '13 at 16:22