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I want to install a different color LED for the Xbox Series S "on" light. The LED can obviously be either on or off, as well as dimmed by a setting in the Xbox menu. Rather than just replace only the LED (LED1000) like others have done I want to make sure the resistor for the replacement LED is appropriate as well.

I got hold of the schematics and found that it has two NPN transistors, Q1000 which I get but it also has a second transistor (Q2000.) I can't really understand why the second one is there. Unfortunately I do not know the actual transistor make or model so I do not have the datasheet.

I have two questions:

  1. What is transistor Q2000 actually doing?
  2. What is resistor R2000 doing? Is it limiting current for both the LED and the Q2000 base pin?

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1 Answer 1

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When the current through R2000 becomes large enough so that the voltage drop over R2000 reaches the base-to-emitter voltage of Q2000 (about 0.6 V), then Q2000 activates and allows current to flow into its collector. In other words, when the LED current reaches 60 mA, Q2000 steals base current from Q1000 so that Q1000 cannot pass more current.

This is a constant current sink. It regulates the current, independently from the LED's forward voltage or the V_5P0 supply voltage, but Q2000's VBE might be affected by the temperature.

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  • \$\begingroup\$ indeed, written to the right of R2000 is that exact calculation, except it calls the parts Q2 and R2 \$\endgroup\$ Commented May 31, 2023 at 3:53
  • \$\begingroup\$ Yes CL. Thank you for your easily understandable reply. \$\endgroup\$
    – finalman
    Commented May 31, 2023 at 4:51

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