## Why wrong voltage will spoil a device but not wrong current?

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This sounds like a basic question. I know when I connect wrong voltage into a MCU pin, it may spoil the MCU. But when I pump in very high power supply current, the device does not spoil. I just know it as a practitioner but cannot explain it. Can someone explain the theory? Thank you very much.

Mr Ohm can help you here. Look up Ohm's law and get some understanding of how electricity works.
In a sentence, the voltage is the pump, so you can't actually "pump in a very high power supply current" without increasing the voltage.

Four legs good, two legs bad, three legs stable.

John_A_Brown wrote:
Mr Ohm can help you here. Look up Ohm's law and get some understanding of how electricity works.
In a sentence, the voltage is the pump, so you can't actually "pump in a very high power supply current" without increasing the voltage.

But why not? Pardon me if I sound stupid. I am probably confused.

When using a power supply equipment, I just turn the current knob to increase current while fixing the voltage knob.

I have another example. When charging a smartphone, I can charge using USB. The current supplied will be 500mA. Then, I can also charge from the mains and the current supplied can be as high as 1A. In both instances, the voltage is fixed.

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When using a power supply equipment, I just turn the current knob to increase current while fixing the voltage knob.

No you don't. You turn the current knob to limit the maximum current available. If your circuit takes 500mA at 5V, turning the current knob up to 900mA will not make your circuit take more current. Try it if you don't believe me. If, on the other hand, your circuit draws 500mA at 5V, and your current knob is set at 100mA, the power supply will automatically reduce the voltage to the point where the outgoing current is 100mA. Try that too.

Now go and look up Ohm's law.

Four legs good, two legs bad, three legs stable.

Thanks for taking the time to reply.

Quote:

But when I pump in very high power supply current

Are you saying you are attaching your AVR to a supply that can deliver a lot of amps?

Or are you saying that you actually are measuring a high current?

Here's an example, that might help: Lets say you have a supply that is capable of supplying 30V and 10A. You hook that supply up to something, set the voltage to 30V and switch it on. Unless there is a short the supply will put 30V on the thing connected. But it is not the same with the current. The current will be determined by the thing attached, not the supply. If the thing attached draws 10 microampere then that is what it will draw.

Here's an intellectual exercise - DO NOT ATTEMPT THIS IN REAL LIFE! : The simplest thing you can attach to a supply is a switch. Leave the switch open and turn on the supply. What will the voltage over the switch be? Yes, 30V. How much current will flow? None at all. zero Amps. (The "interesting thing" happens when you close the switch. You now have a short circuit, and the supply will pump all the Amps out it is capable of. If the supply is not protected in any way, then very nasty things will happen. Fire. Sparks. Smoke. SO, DON'T DO THIS IN REAL LIFE.)

Put a 2 MOhm resistor in series with the switch. If the switch is open, what is the volatege over the switch+resistor combination? Yes, 30 V. What is the current through the resistor? Yes, 0 Amps. Close the switch. What is the voltage over the resistor? Yes, 30V. What is the current through the resistor? Enter Ohms law: V = R * I. Solve for I: I = V / R. Get the numberis into that: I = 30 / 2000000 = 0.000015 A = 15 microamps.

You can apply Ohms law to the simple "switch only" case too. Open switch is "infinity Ohms". Closed switch is "zero ohms".

If you see a power supply with a knob for setting current, it is not a setting for current it will always supply. It is a setting for maximum current it will supply before protection circuitry in it jumps in to prevent a disaster.

Summing up: The supply supplies a certain voltage. The thing connected to the supply draws a certain current.

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JohanEkdahl wrote:

Summing up: The supply supplies a certain voltage. The thing connected to the supply draws a certain current.

Thank you very much for your time. Crystal clear now.

Because a 2000 amp 3v power supply can't get more current through a 1M resistor than a 3uA 3v power supply.

Study carefully the concept

I=E/R

Then consider the eagle, the idiot and the rabbit and all will become clear.

Of course, if you've connected your AVR to a 5v power supply and it's drawing 2 amps, you have something connected wrong.

The largest known prime number: 282589933-1

I being the current
E being the electromotive force (voltage)
R being the resistance
Never seen it shown this way before. (They make rabbits big around your way)

Quote:
They make rabbits big around your way

A few years ago I was out with the dog -- she usually chases rabbits. But we saw a jack rabbit that stood up to my chest, must have been at least 15 kg. ... we went the other way. She waited for a different rabbit to chase.

Martin Jay McKee

As with most things in engineering, the answer is an unabashed, "It depends."

It's actually a wallaby not a wabbit....

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

Be wery wery quiet - I'm huntin' wallabies!

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Then consider the eagle, the idiot and the rabbit and all will become clear.

At the risk of being the i part of the equation, sorry, i don't get it. No i did get Ohm's law a quarter of a century ago, but what the hell should this picture or the eagle/idiot/rabbit thing tell me? I do see they start with the letters used in your equation, but then you lost me... Idiot = eagle divided by rabbit??? Where's the joke?

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

Idiot = I or current
Eagle = E Energy ot voltage
Rabbit = R resitance

The Idiot = Eagle over Rabbit or I=E/R

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

Ok, and why does an eagle (flying) over a rabbit equal an idiot? I mean, i know the concept of helper phrases to aid in learning, but this one is so far stretched that it, hhmmm, well, serves its purpose, to be honest :?

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

In Sweden, voltage is denoted by "U", so our Ohms law is e.g. U = R * I . My memory rule for 45 years have been to think of Uri Geller (remember him?).

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]

Same over here in germany (voltage = U). Many years ago i developed my own scheme to remember Ohm's law with the following expression:

``` U
---
R I```

When looking for the equation for an unknown value, you'd simply cover the relevant letter in the expression and automagically come up with the rest showing the equation.

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

Quote:
There comes a point where the spoon-bending has to stop and the independent thinking has to start." [C Lawson]

LOL, Kartman! :D

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]

lightaiyee wrote:
Can someone explain the theory?
Actually, *current* is what will do the damage. Voltage, merely, is the reason why current will flow through a closed circuit --even though current *IS* a flow: Current is the flow of electric charges, either these electric charges are ions, electrons, radiation (without any matter involved), etc.

For example, no matter how much current a 6V source is able to source, it will never source enough current to a 12V incandescent light bulb to light it up in full intensity; on 6V the 12V lamp will only light up halfway (well, not exactly halfway if you do the math and consider the non-linear visual perception of brightness of the human eye, but you can get the concept). On the other hand, the same 6V source of electric current would literally evaporate a 3V incandescent light bulb heating element because the current under these conditions will be well above of the current the 3V light bulb heating element can handle.

William Beaty's fine site might help you get a better grasp of what electricity is. Over there, you will even find out a lot of common misconceptions, just like how do the transistors really work (part 1 and part 2), actually being voltage driven devices (just like the FETs are), against the myth of them being current driven devices: Just look at the Ebers-Moll Equation, where it is clear that the BJT is a voltage controlled device because the current drawn by the BE junction is merely a healthy side effect of a forward biased P-N (or N-P) junction and NOT the primary cause of the transistor conduction. The Vbe electrical potential (or voltage) is the cause of the retraction of the depletion zone which, in turn, allows the Ic current.

I think that you have already been given enough material to study, that will keep you occupied for a long, long time! It is up to you to decide what is enough for you to digest or to take the steps to go even further than that.

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

Giorgios,

while your explanation started out quite pleasantly, i'm sure you have frightened away anyone having problems with Ohm's law by mentioning the Ebers-Moll equation ;-)

This doesn't serve well as a starter into the world of electr(on)ics, it's more for guys with strong stomachs!

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

No doubt you are right in that I might scare away a novice interested in electronics, with an answer like my previous one. Just like pepper cannot be eaten alone in a dish, yet without pepper a dish would have a very washy taste and it would probably be written off someone's favorite choices in the future.

Maybe I have poured some more pepper in that mental dish; yet, this is how I, meaning personally, would like to be answered in that kind of question the OP did, because it would intrigue me to begin searching for more and more, and to find out as much as I could digest.

But, as you said, I really hope I have not scared him (or anyone else) away from such an undoubtedly interesting field as the electronics technology is...

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

Quote:

Actually, *current* is what will do the damage.

I thought it would be "Actually, power will do the damage". Then you'd go on to say that P = U * I, and then you'd insert Ohm's U = R * I into that to get P = I * I * R, so then you are right about it being current that matters (and squared, to boot).

BTW, there is this very nice riddle in the "Art Of Electronics" book where they sum up the power used in New York and then computes how thick a cable would be needed to bring that power to the city at 127 Volts. The cable needs to be several kilometres in diameter, so they ask the reader "How is this solved?"...

And then there is that question of the speed of the electrons, eg in DC flowing through a copper cable.. Guess first, look up answer after that! :wink:

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]

Some MCU maximum limit specifications for an i/o pad will state only a voltage limit.
SIGNAL CHAIN BASICS #66: How to interface a 5V transceiver to a 3V controller (Planet Analog 6/25/2012); go to "Figure 5. Using a single current-limiting resistor causes damage to the input circuitry".
Though latch-up does not occur there is EOS (over-voltage) due to an oscillating bypass of the MCU's i/o protection.

"Dare to be naïve." - Buckminster Fuller

Quote:
I thought it would be "Actually, power will do the damage".
Alright, you are right; and we are both wrong!

As voltage is the cause of current, current is the cause of power dissipation on the resistive parts due to friction (free and moving charge carriers colliding on the immovable parts of the material). The damage will actually be done by the excessive heat that will be produced by whichever reason caused it, which heat will most commonly destroy the mechanical/chemical structure of the overheated material that will melt or break or even change in a molecular, atomic or even in sub-atomic level!

Now, what was the cause of the excessive heat in question? That would be the power dissipated on a component (or a line), which in turn was caused by the electric current through it, which was caused by the voltage applied to it, which was caused by an electric failure or by the aging of that component (or line) or by the user mishandling it, or by whichever other force of nature!

Quote:
Some MCU maximum limit specifications for an i/o pad will state only a voltage limit.
There is no difference in the cause of electric damage, whether we are talking about a melted and broken filament (the heating element of an incandescent light bulb that radiates light due to its high temperature caused by electric current) or about a melted part of a semiconductor that was permanently destroyed by the same exactly reason: By the excessive local heat that was generated by a tiny spark due to an overvoltage condition at an I/O line (5..10V can be more than enough for that!) which melted a microscopic part of the semiconductor material surface, which in turn (while still being in overheated liquid form) was sprayed over the neighbouring junctions and shorted them, causing thereafter that avalanche which finally destroyed that single I/O in question, then the whole die, then the chip, then the PCB, then the whole device, then the vehicle or the house, etc.

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

DC Electrons: "About that far per second," holding hands apart.

I'm remember Mr. deHan's "Pingpong ball pump" illustration.

The largest known prime number: 282589933-1

Quote:
And then there is that question of the speed of the electrons, eg in DC flowing through a copper cable.. Guess first, look up answer after that!
Well, in copper wire it is a few centimetres per hour per Ampere! This explanation confirms that and, taking the subject matter a step further, it gives an answer to what really happens around every electric line carrying electric charges, the 'surface charges' which are tightly connected to electric fields and they should never be confused with the 'skin effect' that is relevant to magnetic fields and appears only in cases of alternating current (of high frequency, mostly). There are two very closely interconnected and orthogonally aligned forces or, better put, fields: The indivisible duality of the electric field and the magnetic field, which duality is called the electromagnetic field.

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Electronics students commonly assume that electrical energy flows inside metal wires. Physics students know differently! Electrical energy normally doesn't flow inside of metals. In fact, the joules being sent out by batteries and generators are located in empty space: they take the form of electromagnetic fields surrounding the wires.
Or, according to this:
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Whenever the electric company sends energy to our homes, does the energy flow inside the wires? Is the voltage and current part of the energy? Nope. The current and the voltage are connected to the flowing energy, but all the energy flows outside of the wires where the electromagnetic fields are. It goes like this: CURRENT/INDUCTOR/M-FIELD, and VOLTAGE/CAPACITOR/E-FIELD. Because even the simplest electric circuit is like a coil and a capacitor, we have no choice but to say that the energy is stored in the fields surrounding the wires, and is not stored inside the wires.

Want some more? Why is Electricity so hard to understand?

Has anyone heard of that sea of charge carriers that surrounds us? Or the aetheric realm, as it was introduced by Plato in Timaeus as the 'fifth element' (the quintessence) and by Aristotle in his apparent 'rumblings' (that occasionally exist in his writings, which actually were pieces of intentionally encrypted information)? This very realm was extensively supported later by Nikola Tesla's resonant coil single wire transmission lines, widely known as they are colloquially being called the Tesla Coils. This happens to be a theory that I am struggling to decipher and comprehend (if I will ever succeed in it), regardless of what the 'educational system' has taught me or any other heavily indebted victims of that system (these poor fellow people who think of themselves as 'educated' thus special individuals, after their graduation from a system that 'leaves no one behind'). What about the so-called Monkeys with a Degree? (that author 'A Hellene' is yours truly)

And, no, I am not trying to write off Science; it hurts me to be seeing those in power to use Science as a cheap tool to pull their agendas through, by deceiving the gullible masses. I am just stating the obvious: That those who distort History on purpose (and there are countless paradigms of this hideous tactic), could have as well distorted facts in other departments, too. For example, who has not heard of that 'mathematical leap' of Maxwell (as exactly it was called by Heaviside), the theoretical 'displacement current' invention? Is not the 'displacement current' theory a scientifically accepted explanation of the 'aetheric realm'? Why are there more than 6,000 patents still remain hidden and out of the reach of people? What else is a false reality out there?

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

Giorgos_K wrote:
... (if I will ever succeed in it) ...
Not if but when.

"Dare to be naïve." - Buckminster Fuller