Another (strange?) question. - Page 1
Forum section: electronics / beginners

author: firewalker
author_rank: Super Contributor
timestamp: September 12, 2014, 01:40:07 pm
content: Draw and explain the voltage VL across the load, of the following circuit, in relation to the Vs. Vs goes from 0 to 10 volts. So we want VL=f(Vs).I am solving question for some hours now and I can;t see it clearly. I think that the person that wrote the question had something other in mind.

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2014, 01:46:04 pm
content: Quote from: firewalker on September 12, 2014, 01:40:07 pmIs this homework? Was the "other person" a lecturer or similar? If not, then what?

author: firewalker
author_rank: Super Contributor
timestamp: September 12, 2014, 01:49:41 pm
content: No. The question is part of big list of questions a candidate has to answer in order to obtain a technicians license. every one can download the question from the organization responsible for the test. I have located many questions with errors or missing infos.Alexander.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 12, 2014, 02:00:26 pm
content: Quote from: firewalker on September 12, 2014, 01:40:07 pmInputting max 10V will not do anything. 20V as in the picture will make things happen.

author: firewalker
author_rank: Super Contributor
timestamp: September 12, 2014, 02:15:19 pm
content: Why 10 volts? Until the voltage across the zener reach 5.6 volts, the zener is like it is missing. The voltage across the diode is:Vs-0,7-(Vs/2)=5.6 => Vs=2(5.6+0,7)=12.6.Everything starts at Vs=12.6 volts. Yes?

author: firewalker
author_rank: Super Contributor
timestamp: September 12, 2014, 02:26:40 pm
content: After the the 12.6 volts the voltage at the base of T1 will keep increasing until 20-0.7=19.3 volts.T2 is on until the voltage at it's base is bigger than Vs-0.7. Or the voltage across R (collector of T1) is biger than Vs-0.7. If that voltage exceeds that limit T2 will go off.Alexander.

author: janekm
author_rank: Unknown
timestamp: September 12, 2014, 05:54:35 pm
content: Quote from: PA0PBZ on September 12, 2014, 02:00:26 pmMaybe that's the point, and the answer is a straight line

author: tggzzz
author_rank: Super Contributor
timestamp: September 12, 2014, 06:30:36 pm
content: Quote from: firewalker on September 12, 2014, 01:40:07 pmIt will depend on the hFEof the transistors, the value of R, and the cold resistance of the bulb. Roughly speaking though, the voltage across the bulb will rise with the supply voltage until, at a certain point it falls to VCEsat.Whether that is what the designer intended is a separate and unanswerable question.

author: onlooker
author_rank: Frequent Contributor
timestamp: September 13, 2014, 02:01:39 pm
content: In other words, with proper components, the circuit should be intended to work as a OVP in that T2 acts as an on/off switch. Thus, hfe is not a real concern.Or more precisely, UL from following Us transitions to UL~= 0V sharply at Us=~12.6V

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2014, 03:25:00 pm
content: Quote from: onlooker on September 13, 2014, 02:01:39 pmIf R=1Mohm and it was a 40W incandescant bulb, yes hFEwould be a concern in that the voltage would not be the same. Rinse and repeat for less extreme values.

author: onlooker
author_rank: Frequent Contributor
timestamp: September 13, 2014, 06:04:30 pm
content: Of course, the problem description is under-defined. But, no word description of any thing really can be perfect and precise.In this case, even if R, RL and hfe were given, how would one know the track width, the power rating of T1 and T2 and more would not be extra variables in VL = F(Vs). Remember these are missing info too. Would one equally use extremes for these to consider a proper answer to the question?I would argue one always needs assumptions about the missing info that fits the purpose in general.In this case, once, one can assume the purpose of the circuit as a OVP, R=1MOhms becomes an unreasonable take that does not fit the purpose, therefore, needs not to be considered for a proper answer.Also, as a OVP, it should function independent of hfe (and R ...) in its proper operation region. In this sense, hfe is not a concern.hfe, R, RL and more are only relevant when one analyzing the limits of the safe/proper operation area. These are unlikely to be part of the required answer.

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2014, 08:00:56 pm
content: Quote from: onlooker on September 13, 2014, 06:04:30 pmYou are reading too much into possible purposes of the circuit - which is only there to test someone in an exam. As such I would give half marks for saying it was an OVP, and full marks for adding in the caveats based on missing information.