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.
Determine:

-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.

THANKS ALOT!!!

 

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.