Need help with this transistor question...

Discussion in 'Homework Help' started by allhuber, Feb 26, 2012.

  1. allhuber

    Thread Starter New Member

    Nov 5, 2011
    Hi AAC,

    need help with this question:

    In Figure B1, the thermistor, RTH, has a resistance of 75k at 0°C and 25k at 100°C. At 100°C, the transistor reaches its saturation with Vce(sat) = 0.2V and LED is turned on. At 0°C, the transistor is cut-off and LED is turned off. Given that: VLED is 1.8 volts when forward-biased,  = 80 and VBE = 0.7 volt.

    -The minimum voltage of Vb for saturation to occur; (5 marks)
    -The minimum value of R1 for the circuit to operate. Show also that at 0° with the calculated value of R1, the transistor is cut off / conducting.


  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    You need to post what you've worked out so far. For example an estimate of the LED / Q1 collector current when the transistor is in saturation.