Basic question I never understood

Discussion in 'General Electronics Chat' started by Nathaniel Zhu, Nov 16, 2015.

  1. Nathaniel Zhu

    Thread Starter New Member

    Oct 16, 2015
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    Here's two thing that I never understood.

    In electronics, the focus is hugely on voltage. If for example, 5V is too much for a microcontroller, we convert it down to 3.3V.
    BUT, that part confuses me. Voltages are just potential. They aren't anything physical or real. Current is what is real.
    If you had 5V at 2A (which say would fry a device), and backing it down to 3.3V volts using lm317....what good would that do? The current is still going to fry the circuit (even though I know it doesn't but I don't get why). What happens to the current?

    I just don't get how Voltage is relevant to whether circuits become fried or not. Why isn't it current?

    Also, what is the difference between 5V, 500ma and 1V, 500ma. In terms of current, what happens to it? does it mean that the 500ma is processed differently whether you have 5V or 1V? Does the 5V current get less hindered than the 1V current?


    Also, why is AC output only given as Voltage? Like 110V/220V. In DC we get something like 5V/500ma. But with AC, I can only find 100V/200V. No mention or even a question about where the current component went.
     
  2. Dodgydave

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    Jun 22, 2012
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    Voltage is the " pressure" or the "pushing force" of electrons, current is the "quantity or amount" of that are moving around , more electrons per second is more current,

    the two are dependant on the load characteristics, like resistance, reactance, impedance, size of cable,etc, the lower the resistance the higher the current, and vice versa, .

    One way to make it simpler is consider a water tank and pipes connected to the tank.
    Voltage would be the tank of water, and current would be the flow of water in the pipe, so a bigger tank means more water hence more voltage, and a bigger pipe means more water flow hence bigger current flowing,
     
    Last edited: Nov 16, 2015
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  3. MikeML

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    Devices are designed to operate on specific voltages. The current they draw from their power supply mostly obeys Ohms Law, and is proportional to applied voltage. If you double the voltage, the current (at least) doubles, and the power dissipated in the device goes up by a factor of more than four!

    It is usually the power dissipation that destroys the device...

    Think of it like this: For most electronics devices, the independent variable is applied voltage; the dependent variables are current and power...
    One exception is LEDs; there you set current, and voltage is what voltage needs to be...
     
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  4. joeyd999

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    No. A bigger tank means more capacity. A taller tank means more voltage.

    In the specific case he mentioned, 3.3V vs. 5V, dielectric breakdown is the failure mode. Lower voltage FETs have thinner gate insulation. High voltage causes the failure (punch through), not current or power.
     
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  5. wayneh

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    That is sort of correct - if there is truly no path for current flow, voltage becomes almost an abstract idea. (Like the discussions we've had here about two planets, far apart, with different voltages.)

    But there are paths for current flow, those paths can be destroyed by excess "pressure". Pipes burst under high pressure. Sensitive electronics fail under high voltage.
     
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  6. Nathaniel Zhu

    Thread Starter New Member

    Oct 16, 2015
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    I've heard many of these explanations countless times (textbooks, more textbooks, and more, and then videos) and I've thought about them for so long. These concepts are so general- too general.
    My problem is trying to apply these general concepts to specific cases which I'm having trouble.

    Joeyd999- So by punch through and dieletric breakdown, do you mean something like a circuit being fried by a Spark? So It's like the actual current isn't high enough to fry the circuit, but the initial jolt is?

    MikeML- If voltage and current are proportional then why use voltage? Who decided on voltage? Also, there are transformers with 5v,500ma, 5V 1A, and 5V 2.1A. if I paid attention to only voltage, it would blow up my circuit?
    When a schematic say, "input 5V" and doesn't specify the current, I don't get the meaning of that. Does it mean that 5v, 20A also work?

    Wayneh- Oh i think it's starting to make more sense. Voltage is like the pressure I put on a glass wall? If I put enough pressure on it, it will break and destroy the glass wall. Then, by path, what specifically defines the path? Is it the wires? Or the channels in a transistor? Or the filament in a light bulb?

    So can I analogize current to Mass? because if I consider the potential of mass, it's a lot easier to get than potential of electrons.
    Like 300ma for example is like Mass. And 5V, 300ma describes the potential of the mass. And 100V, 300ma means that the same quantity of mass REALLY REALLY wants to move badly?

    THanks for your help guys!
     
    Last edited: Nov 16, 2015
  7. Alec_t

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    Believe me, if you ever inadvertently get your finger on a high voltage point in a circuit you'll find out that voltage is VERY real :).
     
  8. joeyd999

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    I wouldn't call it a 'spark', although a spark is an example of the dielectric breakdown of a gas under stress from a high voltage.

    The gate of a MOSFET is a very thin layer of insulating silicon dioxide. Exceeding the dielectric strength of the layer causes a breakdown of the SiO2, allowing electrons to flow where they shouldn't -- thereby destroying the gate and, thus, the transistor. The thinner the gate oxide, the lower the dielectric strength.

    Punch through (I should have said earlier) is a different phenomena. This is when the voltage between drain and source is high enough to drive current through the channel, even when the transistor is in the off state. The power dissipation (a product of the Vds & Id causing highly localized heating) in this case will quickly cause the part to self-destruct. The narrower the channel, the lower the max Vds.
     
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  9. crutschow

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    As noted, voltage is the pressure behind the electrons and current is the rate of flow of those electrons.
    In most electrical devices the rate of flow of the current depends upon the voltage and the internal impedance of the device.
    This means that the voltage and current in a circuit are not independent of each other.

    The basic equation describing the relation between voltage and current is Ohm's law which states that
    Current equals Voltage divided by Impedance.

    The current rating of a device is normally the maximum current it can deliver (if a power source) or uses (if it uses power).
    The actual current depends upon the voltage and impedance in the circuit.
    Thus if you have a 10V source with a 2 ohm load, the current will be 5A (assuming that the source is capable of delivering that amount of current).

    Also note that power in a DC circuit equals voltage times current.
    Thus an increase in either voltage or current increases the power used.
     
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  10. joeyd999

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    The 'path' is the vector along which the electric field strength (voltage) is concentrated, thereby providing the force which causes electron motion, which, in turn, is current. Any conductor, or partial conductor, can provide such a path. A path can also occur in a vacuum (i.e. vacuum tubes).
     
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  11. atferrari

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    Hola Nathaniel,

    Keeping analogies aside (I try to avoid them, if at all possible), I ask you: have you ever tried (played with) a battery, resistors (starting with several Mega ohms down to few ohms), plus a digital multimeter measuring current and voltage? Just in case, be aware that a battery can provides that elusive potential. Play with different values and see that Ohm found the right relationship between voltage, current and resistance.

    Tell you what; once you play with all that for a couple of days in different combinations, taking care not to blow the fuse of the multimeter, you could reread your post and maybe be able to answer your own post.

    As a (stupid) kid, I learnt somehow that a 9V battery meant something potentially painful by touching the terminals with my tongue. If you insist doing that, you could find out how physical and real it is. I managed to "see" what I recall as a short blue flash...! And the salty aftertaste as well...

    I suspect that in one way or another, everybody here had maybe a similar feeling, doubt or intrigue but, by now, they cannot recall how the concepts became second nature to them.

    Sorry, but prior looking for analogies I would try to get it straight in its own terms. Buena suerte.
     
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  12. timbo614

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    Nov 16, 2015
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    "Also, why is AC output only given as Voltage? Like 110V/220V."

    It is not stated in houses building etc but there will be a maximum number of Amps you can use. Go and look near your house fusebox, near the meter you will see a fuse - mine is 100A. So My house would have a "rating plate" of 240v,. 50hz, 100Amp max. Most thing do have a rating but normally Volts and Watts to get the amps... well that is this site in many places.

    A note: this is my first post. In it I would like to thank the authors of the site. I am ageing software guy and I recently started hobbying in electronics (I always treat electronics as a "black boxes" when writing code). This site has been and stil is an absolute boon, extremely clear and helpful.

    With a bit longer reading the light bulbs will start coming on. It took quite a while and I returned to very first chapters quite often!

    I have done (and am still) doing what atferrari suggests, with just a breadboard, small 1.5v to 6 volt batteries, resitors, LEDs and a few other components you can learn a lot.
     
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  13. Nathaniel Zhu

    Thread Starter New Member

    Oct 16, 2015
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    As a kid, I stuck my finger in a 110V power socket. I know. And sometimes, when I'm plugging in my laptop power adapter, I'd slip and touch the prongs just as it connects with the AC. BZZZZZZZZZZ!

    The dumb thing about me is that I've been messing with electronics for a while. Aced my physics exams and watched and done a lot of practice problems in electronics. I can honestly say that after doing all that, I still don't have the "intuition".

    All the stated explanation (except joey's more detailed explainations which makes more sense), I've heard over and over again and thought about over and over again until I got frustrated. Then fast forward 4 years and I still don't get it.

    It feels like being able to drive a car but not knowing why or how you can drive.
    -----------------------------------------------------------------------------------------------------------------------

    Here's the bottom line questions

    1. If I input 5V, 500ma vs. 5V,2A, is there a difference in how it affects a 5V device?
    -Observation: A cell phone that is 5V, 2A charges faster than 5V, 500ma.
    - Hypothesis: If I charge a cell phone with 5V, 4A it won't fry?
    -Hypothesis: If I charge a cell phone with 5V, 2A, it won't fry but if I charge it with 10V, 1A, it will fry?
    -Why would 10V, 1A fry and 5V, 2A won't when their power dissipation is the same?
    -Is Voltage the ONLY thing that determines whether something fries or not?

    2. Current implies the movement of electrons right? Current = Q/t.
    - Then 500ma vs. 1A means that twice the current flows for 1A than 500ma within the same time.
    Current seems pretty concrete and absolute to me. Whether it is 5V or 100V. If only a unchanging defined number of electrons pass through, why does voltage matter? I'm hearing that it destroys the silicon dioxide layer in the case of transistors or causes punchthrough.

    3. Now why is there such a big difference between 3.3V and 5V if the current is similar? (I'm more specifically considerng the ESP8266-12 wifi module I just fried). So, then can I assume that voltage affects the behavior of the current? In the case of a transistor according to joey, it can determine whether it destroys the silicon dioxide layer. But if the same current passes through, why does one destroy it and the other doesn't.

    Here's my analogy, you have a gate. You have the gate made of concrete. You send in 100 soldiers through the gate all carried by the force of lets say horses (higher voltage). And you have 100 soldiers all pushing their way through without horses (lower voltage). They push through the city. In the end, you still have the same 100 soldier moving through - here's the MAIN THING- The same 100 soldiers moving through PER TIME (Q/t). Whether your voltage is high or low, the same number of people get through. Why does voltage matter at all then? (I know it does but still). PHYSICALLY, how would these soldiers behave differently (different voltages) if they all moved in and out exactly the same (current)?

    4. Observation: Tig welder is high current and low voltage. Van ger graff generators is high voltage, low current.
    - So, voltage essentially determines how easily current can be stopped? So, 5V 1A is more easily stopped than 10V, 1A.
    - Does that mean that the 5V, 500ma vs 3.3V 500ma input results in a different output? Since 3.3V can't push as hard.
     
    Last edited: Nov 16, 2015
  14. crutschow

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    Why is it so difficult to understand that voltage is the force behind the electrons, similar to water pressure is the force behind water flowing in a pipe?
    The more voltage (pressure) the more electrons (water) flows for a given resistance (valve restriction).

    A difference in voltage means there's a difference in power when the current goes through a given resistance (power equals current times voltage).
     
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  15. PeterCoxSmith

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    Feb 23, 2015
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    It is all in the datasheet. For example the PIC32MX795F12L datasheet has a section called Absolute Maximum Ratings and it states:
    upload_2015-11-16_19-38-47.png
    This means that if you apply >4.0V to VDD you will break the chip. So if you apply 4.3V, then apply 3.3V you find the chip is no longer working. This is much like applying too many newtons force to a piece of glass and you break it. Volts are joules/coulomb and joules are work done by 1 newton moving through 1metre.

     
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  16. wayneh

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    Generally speaking, the current drawn at a regulated voltage is determined by the load. The current rating on the charger is the maximum safe current for that charger, what it can safely continuously deliver at the stated voltage. So you could have a 5V charger rated to 300A and it would be no different for the phone. If you raise the voltage, the load is not designed for that and the weakest link will reveal itself.

    Not everything depends on the amount of charge that has moved. That is indeed important for a battery or a capacitor, as long as the voltage does not exceed some maximum. But excessive voltage in a circuit can damage components that were not designed for those voltages. Just as the plumbing pipes in your house might burst if the pressure in them was doubled or tripled. At one moment there is no path for current, but because the high pressure causes a fault or failure, now a path appears and current might flow where it was not intended.

    I'll leave explaining this phenomenon to someone that understands it better than I do.

    That's not a relevant analogy to most electronics. Raising the voltage will usually increase the current, and risks destroying the equipment that carries the current. Picture your concrete gate being crushed and destroyed by an overwhelming stampede of forces.

    Well, sort of. A higher voltage requires more resistance to hold back the current. I wouldn't say it's "easier" or "harder", except that you need a numerically larger value resistor. Like a smaller orifice in a valve to throttle water flow in a pipe.
     
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  17. #12

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  18. Nathaniel Zhu

    Thread Starter New Member

    Oct 16, 2015
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    Thanks guys. I think I finally got it!
     
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