Zener diode question ask for help - Page 1
Forum section: electronics / beginners

author: limlim7214
author_rank: Newbie
timestamp: October 22, 2017, 07:46:28 pm
content: Zener diode question ask for helpHi,everyone I am a student who come from HongKong,I have a big problem to learn zener diodeI try to solve this question PartQ1 to Q5 with my classmate for few days,but we can't make it,our knowledge have not enough to slove this question, so I request earnestly to find who can solve this question.Question photo please see at attachment,thankyouReferring to the zener regulator shown in Fig. Q1, if VS = 12V, RS = 200?, and RL = 300?,also regulation is required to be kept throughout the question even for a low IZK of 0A, determine:(i)the 3 currents IS, IZ and IL; {only requested variable changes, others are fixed in(ii)to (iv)}(ii) theminimum supply voltagebeyond which regulation is lost;(iii) the value ofmaximum RSbeyond which regulation is lost;(iv) the value ofminimum RLbeyond which regulation is lost; and(v) theminimum power rating of the zener diode for full feasible range of RL values.My Answer of(i)I(supply) = 12v-5.2v/200ohm = 34mAI(Load)= 5.2v / 300ohm = 17.3mAI(Zener)= 16.67mAAbout the(ii) regulation is required to be kept throughout the question even for a low IZK of 0A[which regulation is lost]<-- Is it mean the circult(zener diode) working in normal function?My Answer of (ii) I dont really know what I am claculating(ii)If Iz= 0A , Is = IL = 17.3mA ,Rs= 200ohmVs (min) = 200ohm X 17.2mA = 3.46v [regulation function]But I have a problem here , we cant provide 5.2v to the Z from Vs , how can we make the Z work?So my answer change to 5.2vAm I right?(iii)Rs max = (12v-5.2v)/17.3mA = 393ohm(iv)RSRL = 0ohm(v)I dont know------------------------------------------------------------------------------------------------Please feel free to solve my learning problem Thanks a lot

author: eblc1388
author_rank: Frequent Contributor
timestamp: October 23, 2017, 08:01:19 am
content: You've got the answer right in the (i) & (iii) part of the question.I'll give you some hints regarding the rest.(ii) When the supply voltage is reduced, obviously the current through Rs will go down but the Zener diode will try to maintain the voltage across its terminal at roughly 5.2V, thus the load current IL can be assumed to be unchanged at 17.3mA. Therefore the reduction of current can only happen at Zener diode. When the zener current Iz drops to zero, the zener diode will no longer function. This occurs when the current in the circuit flows through Rs and RL, but not Zener) becomes 17.3mA.(iv) Minimum RL is reached when current through it develops a voltage drop of 5.2V as any more reduction in resistance value will cause the voltage drop to fall below 5.2V and regulation is lost. Your task is to find such value which when place in series with Rs and connects to 12V, will develop across itself a voltage drop of 5.2V.(v) feasible range of RL is from open circuit to minimum value calculated in (iv) above. As current is shared between zener and RL, the power rating in zener is at maximum when RL= open. Under this condition all the current is flowing through the zener diode. You should know how to calculate its power rating.Hope it helps.

author: lordvader88
author_rank: Frequent Contributor
timestamp: October 23, 2017, 08:10:53 am
content: I can't read much ATM but zener's use a lot of power, ~42mW for a 6.3V drop, but hey that's more of a voltage drop than I create as a human sitting here