## 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.

Quote:
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.

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut.

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!

Quote:
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]

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..."

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

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

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

If you don't know my whole story, keep your mouth shut.

If you know my whole story, you're an accomplice. Keep your mouth shut.