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# power supply regulation question

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#### dnolan747

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I am a student at Penn Foster Career School and i am having a problem with a question and need a little help...plz.... the question is: the output of a power supply is measured at 12v with no-load connected. the manufacturer's specifications say the power supply has a regulation of 10%. What is the full-load output voltage of the power supply?

A. 13.2v C. 10.8v
B. 10.9v D. 1.2v

I answered C.10.8v and was told that i was wrong i have a retake of this test in a few days and dont want the answer given to me just want to try to understand a little better i am leaning to D. 1.2v because i took the no load voltage and multiplied by 10% and came out with 1.2 and this is what i did the first time but then subtracted 1.2 from 12 and came out with 10.8.. i think i am confused or i am thinking further than i need to

Dean

I would have chosen "C" too. Guess I need to go back to school.

I am a student at Penn Foster Career School and i am having a problem with a question and need a little help...plz.... the question is: the output of a power supply is measured at 12v with no-load connected. the manufacturer's specifications say the power supply has a regulation of 10%. What is the full-load output voltage of the power supply?

A. 13.2v C. 10.8v
B. 10.9v D. 1.2v

I answered C.10.8v and was told that i was wrong i have a retake of this test in a few days and dont want the answer given to me just want to try to understand a little better i am leaning to D. 1.2v because i took the no load voltage and multiplied by 10% and came out with 1.2 and this is what i did the first time but then subtracted 1.2 from 12 and came out with 10.8.. i think i am confused or i am thinking further than i need to

Dean

Dean.

This is not an "intuitive" thing to understand, because the answer seems so obvious, but the answer is 10.9.

Are you aware of the formula?

I would have answered it as you did without knowing the formula.

edit:

actually I got 10.09, which doesn't make sense.

Bear with me.

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OK.

As I said, I had never needed to use this formula, but judging by what I see, something is missing here.

Are you stating this question from memory or are you typing it verbatum?

Are you sure you didn't get it backwards, ie the 12 volts is the value under the load and you are supposed to find the no load value?

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i do know the formula if this is the one you are speaking of which is: %voltage regulation=Vnl minus Vfl divided by Vfl multiplied by 100 or
%voltage regulation=Vnl-VflX100
Vfl
but how do i convert this formula to do what the question is asking me????? I know the no load is 12v and the percentage of regulation is 10% so how do i get the full load voltage value????

the Vfl was supposed to be under the Vnl-Vfl but is was somehow put all the way to the left but i think yall will understand where i was going with that

Are you stating this question from memory or are you typing it verbatum?

i am copying it straight out of my text manual

OK.

Are you stating this question from memory or are you typing it verbatum?

i am copying this straight out of my text manual

I got 10.9.

10 = (Vnl - Vfl)/Vfl*100

After some algebra:

Vfl = Vnl/(10/100 + 1) or

Vfl = 12/1.1

= 10.9

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You see, I Googled the formula and plugged in the numbers provided and each of the answer choices.

With the values assigned, kind of reverse engineering the formula you get 10.09

Oh wait a minute, that might be correct if you are allowed to round off your percent of regulation. Because brain farttt, the 10.09 value is what i get for the percentage after plugging in all the values.

Dean,

Is this test timed? Because if it isn't, you should just memorize the formula and plug in the numbers till you get the closest answer. If it is multiple choice you have 4 numbers to plug in and find what fits.

I encourage you to ask questions about the formula, why the value is what it is and what exactly it represents...but outside of that it is just a matter of doing the algebra.

no the test is not timed in fact it is open book but i guess where i am having the problem is that it is a home course and the biggest problen is i have a problem with algebra... but i am trying to overcome this I thought this home course would be easy but i think i just reallized that an instructor standing at the front of the class is much better.....i am still not seeing where yall are getting the 1.1 in the algebra part but thanks for all of yall's help and i will go back and study the algebra part of that post

Dean

1) divide both sides of equation by 100. - result 10/100 = (Vnl - Vfl)/Vfl
2) Multiply both sides by Vfl -result 10/100*Vfl = Vnl - Vfl
3) Subtract Vfl from both sides -result 10/100*Vfl + Vfl = Vnl
4) Factor Vfl from lest side -result Vfl(10/100 + 1) = Vnl
5) divide both sides by the factor in step 4. -result Vfl = Vnl/(10/100 + 1)
6) solve -result Vfl = 12/1.1

You're rally going to need to be able to do work like this to do electronics.

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The whole question and concept of regulation is stupid if the answer does not intuitively mean 10% of 12v = 1.2v

You're rally going to need to be able to do work like this to do electronics.

yes i know that i will need to be able to do this,,, i am just having a hard time with this i think because this is an at home and online course and i do not have an instructor in a classroom setting and it has been 15 years since i graduated high school and that long since i had an algebra course....but anywat thanks brownout i think that brought a little of it back... my brain is on overload!!!!!!!!!!!!!

Dean,

Is this test timed? Because if it isn't, you should just memorize the formula and plug in the numbers till you get the closest answer. If it is multiple choice you have 4 numbers to plug in and find what fits.

ke5frf i was looking over the thread again and i just caught on to what you were trying to say here and i did plug in all the answers to the formula and that worked better for me i also got 10.09% regulation which rounded is 10% so yes according to this the answer is B. 10.9v but i really thank all of you for helping and i know the algebra will come into play in real life situations

thanks
dean

I think that in real life situations it doesn't make any difference at all.

The question is has the 10.8 V and 10.9 V answers to catch you out. 10.9 V is mathematically correct, because 12 V = (100 + 10)/100 * 10.9 V. In real life, if a power supply had 10% regulation, you would never rely on 10.9 V output rather than 10.8 V, as supply variation, temperature and minor differences in load current would change things more than that.

Also typical unregulated power supplies can have much worse regulation than 10%, and they will also have a large ripple voltage that increases with load. Transformers have a more predictable regulation when you have a resistive load, but they are rarely used with a resistive load.

I think that I would have phrased the question like this:-

The output of a transformer is measured at 12v with no-load connected. The manufacturer's specifications say the transformer has a regulation of 50%. What is the full-load output voltage of the transformer?

A. 24.0 V C. 18.0 V
B. 8.0 V D. 6.0 V

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