Perky123
New Member
Hello guys,
I found this valuable forum just recently and was happy to read so many intelligent and wise oppinions here. I am a student and unfortunately have troubles passing an exam in "Electronics and Measuring Technologies". I really hope that I will find some support here and answers to my probably silly questions (pleaassee )
Here comes the first one - I have the following part of an exam problem (don't lough, that's one of the easiests).
There is a zener Diode in the middle as a voltage regulator which hase a Zener Voltage of 5V. The input voltage is initially set to 11V. R1 = 150 Ohm. The Light Diode (LD) has U=2V.
a) What should be the resistance of R2, so that the LD would have a current of 20mA by U(LD)=2V. Answer: 150 Ohm
b) What current flows through R1 and what through ZD? Answer: 40mA, 20mA
c) The input voltage is now set to 9,5V. What current is flowing through the LD now? Answer: 20mA
d)The input voltage is now set to 6,5V. What current is flowing through the LD now? Answer: 15mA
This exactly is my problem - how come and why is the current of d) 15mA!?? My answer would be 20mA as before!? is it my mistake or is the given answer wrong? I do get the previous answers... but maybe I am not solving it right
Thanks very much guys
P.s. Please let me know if it is ok to post my problematic exam problem in this forum at all, really hoping to find some help 3rd attempt to pass the exam
I found this valuable forum just recently and was happy to read so many intelligent and wise oppinions here. I am a student and unfortunately have troubles passing an exam in "Electronics and Measuring Technologies". I really hope that I will find some support here and answers to my probably silly questions (pleaassee )
Here comes the first one - I have the following part of an exam problem (don't lough, that's one of the easiests).
There is a zener Diode in the middle as a voltage regulator which hase a Zener Voltage of 5V. The input voltage is initially set to 11V. R1 = 150 Ohm. The Light Diode (LD) has U=2V.
a) What should be the resistance of R2, so that the LD would have a current of 20mA by U(LD)=2V. Answer: 150 Ohm
b) What current flows through R1 and what through ZD? Answer: 40mA, 20mA
c) The input voltage is now set to 9,5V. What current is flowing through the LD now? Answer: 20mA
d)The input voltage is now set to 6,5V. What current is flowing through the LD now? Answer: 15mA
This exactly is my problem - how come and why is the current of d) 15mA!?? My answer would be 20mA as before!? is it my mistake or is the given answer wrong? I do get the previous answers... but maybe I am not solving it right
Thanks very much guys
P.s. Please let me know if it is ok to post my problematic exam problem in this forum at all, really hoping to find some help 3rd attempt to pass the exam
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