Wednesday, 4 April 2012

Light Canvas - LEDs 6: Dim power

The previous post demo-ed all the LEDs roughly in place in the frame, running on the dimmer circuit - but not running bright enough. I've figured out (I think) the LEDs aren't getting enough power. Firstly I wanted to establish what is happening in the circuit:

  • From it's specifications, the 2N222A transistor has a beta value of about 100
  • The current applied to the transistor base (555 chip output current) measured at: 1.16mA
  • Taking the above, 100*1.16mA = 0.116A
  • This calculation is confirmed by the measured of current going through LEDs (via the transistor): 0.1A
  • This 0.1A is a quarter of the current needed to fully light the LEDs - measuring the current when driving the LEDs directly from the power supply gave a reading of 0.39A (pretty close to the calculated value of 0.42A)
  • The transistor's peak base current is 200mA, so the max emitter current (base current * beta value) should be 200ma*100 = 20A - considerably more than the 0.4A needed.
  • It should be possible to get the 0.4A - I just need to figure out how to get more current into the base.
Answers on a postcard...

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