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Can anyone tell me what is the functionality of the transistor in this schematic ?

enter image description here

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    \$\begingroup\$ Can you figure out what the current through L1 is? \$\endgroup\$ May 4, 2019 at 13:46
  • \$\begingroup\$ What do you mean by L1 ? For 78L15 is 1.5A \$\endgroup\$
    – pantarhei
    May 4, 2019 at 13:52

2 Answers 2

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It lights the LED to show that both outputs are on and above around 8V or so.

Note that the base is at the common terminal - I will call this zero for now.

When the positive output exceeds about 5.6V, ZD1 (a zener diode) breaks over and conducts. If the positive output were at 15V, then there will be about 5mA of current in the emitter. (15V - 5.6V - 0.6V) / 1.8k

The LED cannot light up unless there is a negative voltage output though as there is no bias for the LED.

Once the negative output exceeds 5.6V (for ZD2) plus the LED minimum forward voltage, there is a current path and the LED lights.

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The transistor circuit causes the LED to light only if both power rails are working.

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    \$\begingroup\$ It's not a great quality answer, but if JP1 is connected and 78L15 and 79L15 are power, then the LED will light. \$\endgroup\$ May 4, 2019 at 18:53

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