Why is the mbed power input defined in terms of Volts?

Thread Starter

eb123

Joined Jul 3, 2017
74
I'm new to the exciting world of electronics. My basic understanding is that it is Current (i.e. flow of electrons) which "powers" components - they have no concept of voltage.

If this is the case, then why do we say that the mbed (or any component) requires x Volts to run? What if there's high resistance causing a reduction in Current before "power" reaches the component? Surely then the Voltage is technically the same, but there is less Current to actually "power" the component.

On another forum I heard someone say that the mbed in particular pulls on average 25ma. Why then would it need anywhere near 5V to run?

I = V/R

R is likely to be extremely low on a small wire. So with 5V, there's way more current produced than actually needed.

Obviously there must be a fundamental misunderstanding of the topic, causing my confusion. If somebody could help "correct" it, I'd greatly appreciate it.

Thanks.
 

MrChips

Joined Oct 2, 2009
30,712
I can see that you are totally confused. Best to start with a clean slate. Forget everything you know about current and voltage.

Where do we begin?
In order to "power" anything electrical or electronic, you need current and voltage.
In order to create current you need voltage. (There are exceptions such as in super-conducting materials but no need to go there for now.)

The calculation for the amount of power required is current multiplied by voltage.

How much voltage is required by any gadget is determined by the manufacturer and the design of the device.
A hearing aid device might need only 1V.
A 1000W audio amplifier might need 100V.

A computer system is based on millions of switching circuits. Each individual switching circuit only needs to generate one of two states, TRUE or FALSE. We can make up any rule of what is defined as TRUE and what is defined as FALSE. For example, we can define 1V = TRUE and 0V = FALSE.

It is possible to create a computer that runs on 1V (just as an example). To power such a digital switch might take zero current. Remember, all we need is to output 1V for TRUE and 0V for FALSE. We can have 1 million such switches all powered by 1V and drawing zero current.

The problem is, every time a switch changes state from TRUE to FALSE, or FALSE to TRUE, we have to move some electrons from one box to another. It is like, every time we open the barn door a few flies go in or out.

In order to make a computer do anything useful we have to open and close these switches a few million times a second. This is what drains current and consumes power. The faster we run the computer the more current it consumes.

The bottom line is, the voltage required to run a computer is determined by the design of the circuits. The amount of current the computer takes is primarily determined by how fast we want to run the program. If we were to STOP the computer, the computer would take next to zero current.
 

nsaspook

Joined Aug 27, 2009
13,086
Not particularly helpful.
It's helpful to drive home a point that's most likely is not your fault but is easily fixed with proper education. The vast majority of people outside of this field (and some in this field) don't understand electricity and the concept of current as a rate (flow of charge over time) instead of a flow of electrical energy that has a completely different set of terms.
 

shteii01

Joined Feb 19, 2010
4,644
Not particularly helpful.
The thing about your original post... I wanted to post something, but I did not want to go all textbook on you...

I was looking at the whole voltage thing you were talking about and I thought about it and I thought some more and I could not come up with anything that would have been helpful. So I left it along.

Then I came back half an hour later and read nsaspook's post and BINGO! You were talking about power and then explaining it with voltage. I kept looking at the voltage and I did not even notice the whole power angle your had. So obviously you don't understand what is electrical potential, what is electrical power. Yes, I am simpleton that missed the whole problem of your statements. But the fact remains. You do not understand the things you were asking about. You think you know what they are, but you don't.
 

#12

Joined Nov 30, 2010
18,224
Just for a pointer, Ohm's Law E=IR and Watts Law P=IE are the first two things to learn. They will get you through a lot in your first weeks and they are still obligatory after decades of design work. You simply must learn them because there is no field of electronics that does not use them on a daily basis.

You will get to the point where you can see any 2 of those numbers and "flip the digits" in your head, at least well enough to know if you're looking at a likely "burst into flames" situation. Don't worry. It will come.;)
For now, use a calculator.:p
 

BR-549

Joined Sep 22, 2013
4,928
"My basic understanding is that it is Current (i.e. flow of electrons) which "powers" components"

Actually power requires pressure plus the flow. No pressure...no work. An electron at 2 volts can do twice the work/energy of an electron at 1 volt pressure. It also works the other way. 2 electrons at 1 volt....can do the same work as 1 electron at 2 volts. It's a combination of pressure plus flow. Don't think of current as water. It's not springy. Water can be pressurized, but not compressed. And gravity controls it. Think of current as pressurized/contained steam. We use heat to pressurize steam........we use voltage to pressurize current.

"they have no concept of voltage."

All components are voltage(electrical pressure) sensitive.

If your really interested in electronics.........you're gona have to read and read a lot. If you don't, you will always be perplexed and confused.

There is an advantage to using the textbooks on this site....there is always a faculty here to answer questions.
 

Thread Starter

eb123

Joined Jul 3, 2017
74
"My basic understanding is that it is Current (i.e. flow of electrons) which "powers" components"

Actually power requires pressure plus the flow. No pressure...no work. An electron at 2 volts can do twice the work/energy of an electron at 1 volt pressure. It also works the other way. 2 electrons at 1 volt....can do the same work as 1 electron at 2 volts. It's a combination of pressure plus flow. Don't think of current as water. It's not springy. Water can be pressurized, but not compressed. And gravity controls it. Think of current as pressurized/contained steam. We use heat to pressurize steam........we use voltage to pressurize current.

"they have no concept of voltage."

All components are voltage(electrical pressure) sensitive.

If your really interested in electronics.........you're gona have to read and read a lot. If you don't, you will always be perplexed and confused.

There is an advantage to using the textbooks on this site....there is always a faculty here to answer questions.
Now we're getting somewhere.
I was under the impression that it was purely the number of electrons flowing through a component (in a given time) which dictated how much work was done. I never realised the voltage had any use to the component (other than being the driving force to keep the electrons moving).

Could you say then that Voltage is a bit like the torque of a motor? Two motors could be rotating at the same speed (current?), but the motor with the higher torque (voltage?) is capable of doing more work (e.g. lifting heavier loads). Is that a useful analogy?

Funnily enough, when I read elementary articles on electricity on sites such as Sparkfun or http://electronicstheory.com/COURSES/ELECTRONICS, I leave feeling I have a solid understanding of the subject. It's only when I try to advance further that I encounter concepts that seem to contradict what I previously thought to be true - very confusing indeed.

Could you guys recommend some resources to get me on the right track?

Thanks.
 

MrChips

Joined Oct 2, 2009
30,712
Sometimes analogies help. Sometimes analogies lead to contradictions.
At this point it is best to drop the analogies and work with the real thing.

Current is charge flow.
Units of charge is coulomb.
Units of current is coulomb per second = ampere.

To make charge flow in an electrical circuit you need a potential difference between two points.
Units of potential is volt.

Power is current x voltage.
Units of power is watt = ampere x volt.
 

BR-549

Joined Sep 22, 2013
4,928
eb123.......how old are you? Where do you live? What is your educational background?

What is your working background? Why do you want to learn electronics? Is it for personal hobby knowledge or do you want to make a living at it?

To clearly understand the relationship between voltage and current.......you will need to understand trigonometry and algebra. Do you have that background? If you don't........you will have to study math.....along with electronics.

Electronics is the field where you put all the boring subjects, such as physics, chemistry and math, that you thought that you would never use.........to use.
 

Thread Starter

eb123

Joined Jul 3, 2017
74
eb123.......how old are you? Where do you live? What is your educational background?

What is your working background? Why do you want to learn electronics? Is it for personal hobby knowledge or do you want to make a living at it?

To clearly understand the relationship between voltage and current.......you will need to understand trigonometry and algebra. Do you have that background? If you don't........you will have to study math.....along with electronics.

Electronics is the field where you put all the boring subjects, such as physics, chemistry and math, that you thought that you would never use.........to use.
I'm studying Computer Science at a UK university.
Haven't used trig for a while but my algebra is pretty decent.

After graduating I'd quite like to move into embedded systems.
 
Last edited:

MrChips

Joined Oct 2, 2009
30,712
I'm studying Computer Science at a UK university.
Haven't used trig for a while but my algebra is pretty decent.

After graduating I'd quite like to move into embedded systems.
I would have to assume that you did algebra, geometry, trigonometry, calculus, and physics at 'O' Level and 'A' Level unless the UK science curriculum has changed dramatically since the time I took those subjects.
 

Thread Starter

eb123

Joined Jul 3, 2017
74
Y
I would have to assume that you did algebra, geometry, trigonometry, calculus, and physics at 'O' Level and 'A' Level unless the UK science curriculum has changed dramatically since the time I took those subjects.
Yep, I did all of those.
 

Thread Starter

eb123

Joined Jul 3, 2017
74
I didn't do 'A' Level Physics, but yes we covered both those topics superficially in GCSEs. From what I remember, it was all pretty straightforward stuff, but there seems to be so much more underneath which can be quite tricky to understand.
 

BR-549

Joined Sep 22, 2013
4,928
eb123......very very good. Then you are used to using math as a justification and reason for cause. I personally hate this philosophy and understanding.....but it works and it is the way they teach today.

Teaching that anything is caused by math is a big cheat to you and everybody else. But such is the current state of education.

And being that you are learning a profession for a living.......and a competitive one......you will need to be satisfied with their way of teaching.

The textbook at the top of this page.....teaches "conventional current" and mathematical electronics. This is what you need to converse with your peers.

But it still takes much study. I wish you good study, because luck won't work.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
Could you say then that Voltage is a bit like the torque of a motor? Two motors could be rotating at the same speed (current?), but the motor with the higher torque (voltage?) is capable of doing more work (e.g. lifting heavier loads). Is that a useful analogy?

.
Motor Torque (load) is relative to current not voltage in and of itself.
Motor speed is relative to other factors depending on the design characteristics of the motor.
Max.
 
I hate to to this, but I want to throw out this http://www.mouser.com/catalog/supplier/library/pdf/FairchildLogic.pdf document.

One of the simplest circuits are logic elements. Early on, 5V was a nice compromise with silicon transistors. They had an inherent 0.6 V or so drop across the junctions at room temperature. Germanium transistors with a 0.2 V drop existed at the time, but they had problems.

As technology advanced, we do have a 0.9 V or so logic family.

The lower the voltage, the lower the power a circuit consumes. With CMOS the highs and lows are very close to the power supply rails and the input pin current requirements are minuscule.
 

Thread Starter

eb123

Joined Jul 3, 2017
74
Motor Torque (load) is relative to current not voltage in and of itself.
Motor speed is relative to other factors depending on the design characteristics of the motor.
Max.
Sure, bus an anology for force, could say that torque is a bit like voltage?
 
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