Voltage Drop, Voltage Drop, Voltage Drop....

Discussion in 'General Electronics Chat' started by NickNYC, Jun 30, 2011.

  1. NickNYC

    Thread Starter Member

    Jun 30, 2011
    12
    1
    Hello all - I am a newbie on this forum, and I want to express a certain frustration I feel while reading the tutorials and forum commentaries here.

    The term "voltage drop" is used hundreds and hundreds of times throughout this site.

    Yet NOBODY, even once, has defined clearly and exactly what this term means. It seems to me, considering the frequency of usage of the term, that it should be one of the all-time basics, and should not be assumed knowledge on the part of the reader, especially in the tutorials.

    For God's sake....WHAT IS "VOLTAGE DROP"? EXACTLY? Voltage drop is... across such and such component? What does THAT mean?

    Thank you so much!!
     
    JasonL likes this.
  2. someonesdad

    Senior Member

    Jul 7, 2009
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  3. KJ6EAD

    Senior Member

    Apr 30, 2011
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  4. munna007

    Active Member

    Jul 23, 2008
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    its the voltage that is dropped or absorbed by resostors .....
     
  5. edgetrigger

    Member

    Dec 19, 2010
    133
    19
    Only resistors??? what about voltage drop accross diodes, gas discharge tubes, inductors, capacitors, pnpn...........pn junctions etc..
    BTW resistors won't absorb voltage...

    Please brush up your basics with google for your own good.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Welcome to AAC!

    This has been stated, but in Volume 1, Chapter 1, Section 4

    http://www.allaboutcircuits.com/vol_1/chpt_1/4.html

    In learning electronics, reading is required. This site is unusual in that we have a text book at the core of it, so the required reading is local.

    The sum of all voltages dropped in the circuit should add to the power supply voltage. Ohm's Law usually rules, but not always. Quantum physics actually shows it's head in some of this stuff, as in diode drops, LED drops, transistor Base Emitter drops, etc.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,441
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    NickNYC, Welcome to AAC!

    "voltage drop" is an in-circle expression that nobody has to define because we all understand what it means. Here is the lowdown.

    It is like "distance". We do not define it because we know what it means.
    Saying "The distance at New York City" has no meaning,
    whereas "The distance to New York City" has a specific meaning.

    "voltage at ..."
    "voltage across ..."
    "voltage drop ..."
    "voltage drop across ..."

    "almost always" mean the same thing.

    Voltage is a measure of potential difference. It is always the voltage between two points. That is why a voltmeter has two connections. It is the potential at Point A with respect to Point B.

    [​IMG]

    Take a look at the circuit diagram above for illustrative purposes only.
    Saying the "voltage at R1" is ambiguous, while "voltage drop across R1" means (V1 - V2).

    When we say "the voltage at R2" we really mean the "voltage drop across R2" which would be (V3 - VGND). If VGND = 0V then most of us would understand "the voltage at R2" to be equal to V3.

    The "voltage drop" at D1 would be (V2 - V3).

    If someone said the "voltage at D1" or the "voltage at the base of Q1" we would understand that to mean V2.
    Why? Because when you understand circuit topology we know that in this case we are interested in the voltage at this important junction.

    Saying "the voltage drop at the base of Q1" is not meaningful, like saying "the distance at NYC."

    It's good to be part of the in-crowd.
     
    Last edited: Jun 30, 2011
  8. radiohead

    Active Member

    May 28, 2009
    474
    31
    well put, Mr Chips
     
  9. NickNYC

    Thread Starter Member

    Jun 30, 2011
    12
    1
    MrChips!

    Thank you so much for this wonderfully clear and concise explanation of a term that has bugged me now for at least a year. This is the finest explanation of it I have ever seen of this, and leaves no doubt in my mind. I always felt extremely unsure of my understanding every time I saw it.

    I now know that the voltage drop across a component is the measured difference between the voltage on one side of that component and the voltage on the other, as the presence of any component in the current path will change the voltage entering into it. Only an idealized wire would have a voltage drop of 0 across it. No? If that's wrong, please let me know about it.

    I have already referred to your explanation and diagram many times while poring over old tube radio circuit diagrams.

    Thanks so much again.

    BTW, you say in your post ""voltage drop" is an in-circle expression that nobody has to define because we all understand what it means."

    I respectfully beg to differ! If there's a site that claims to teach the basics of electronics, as we have here, an explanation such as yours should be right in there at the top, or at least in the glossary. I'll bet many many people are confused by this term and its usage. Assumption of knowledge of anything by anyone is NEVER a good idea!

    I am very pleased to be a member of this community.
     
    Last edited: Jul 3, 2011
  10. MrChips

    Moderator

    Oct 2, 2009
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    You make a very good point that has plague all educators from time eternal. When you have been in a certain business for a long time you tend to take a lot for granted. There will always be someone that comes along with a different question and perspective that you never would have expected. We always look forward to some fresh faces. That's what graduate students are for.
     
    NickNYC likes this.
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Yes, one of the problems is that after we have been using certain expressions for a while they become part of our "mental furniture", and are taken for granted. That applies with particular force if we have got used to them when young, and we are now...not so young.

    I am sorry to have to point this out though, but you will probably come across other technical terms which will need explanation. For this you may well need to refer to texts, rather than expect to find each idea explained as you come to them in threads on the forum.

    It would not be realistic to expect all these things to be explained in ordinary posts, because this would make for a lot of extra effort. Somehow or other, a balance has to be struck between giving concise descriptions and making the forum accessible to beginners.
     
  12. NickNYC

    Thread Starter Member

    Jun 30, 2011
    12
    1
    This term is especially confusing, at least to me it was, because of the use of the word "drop". Nobody ever talks about it.

    Does it mean "drop" as in dropping an object from a height, or "drop" as in the lowering of the measurement of something, as in a drop in temperature? Nobody ever talks about this or explains it.

    I STILL don't know what usage of the word "drop" is intended.
     
    Last edited: Jul 3, 2011
  13. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I'm afraid it really is a basic idea, describing the voltages developed across elements in a series circuit. Many people would take it for granted: some of us may be older people who have learned about these things as teenagers, and may need to make bit of mental effort to see the issue.

    The idea of a chain of components connected across a supply, with predictable voltages being developed across each component is a very useful one. We need to be able to understand voltage drops if we are going to do much with electronics - starting from real basics like calculating a resistances to run LEDs from a DC supply.
     
    NickNYC likes this.
  14. NickNYC

    Thread Starter Member

    Jun 30, 2011
    12
    1
    What would REALLY be useful first of all is to understand in what sense is the word "drop" being used in this context? What is meant by "drop"? A drop like in a drop in temperature? Or an action, like to drop a ball off a building? Or a noun, like a drop of water?

    Why does NO ONE talk about this??
     
  15. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    If you start with voltage on a symbol for a wire (a straight horizontal line) at the top of the page and add several resistors in series with each other from the top of the page to the bottom of the page, there is a line at the bottom of the page that represents zero volts, or common, or ground. As you connect your imaginary volt meter with one lead to the ground wire and measure the voltage at the tops of the resistors, the voltage reading drops to lower and lower levels as you move down the page. Thus the voltage reading drops (verb) consecutively, and the voltage drops (verb) across each resistor.

    Eventually, laziness creeps into the language and we now have "voltage drop" as a noun.
     
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  16. MrChips

    Moderator

    Oct 2, 2009
    12,441
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    The word "drop" in this context is used as a noun to represent the reduction in a given quantity. It is used quite commonly such as:

    "drop in temperature"
    "drop in price of oil"
    "drop in heart rate"
    "drop in attendance"

    etc.
     
  17. NickNYC

    Thread Starter Member

    Jun 30, 2011
    12
    1
    Thank you Mr Chips! My understanding is assured now...finally!
     
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