# What exactly is an Amp?

Discussion in 'General Electronics Chat' started by dante_clericuzzio, Jun 14, 2016.

1. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
134
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I understand that from Ohm's Law Current I = Voltage (V)/ Resistance (R)....and it says that the current (amp) provided by the ratio of voltage over resistance. And how much amp is being output is depending the load...

Now the amp output depending on the load ...this one i quite don't understand --- for instance just take a common example of a smart phone. If the amp is depending on how much needed by the smart phone battery why then the charger have various amperage rating. Some are 1 amp while others maybe 1.5 amp or 2 amp....

why the charger need to specify the amp when the phone only will take the required amp it needs? anyone would you mind to advise my shallow knowledge of this..

Why not use 5 to 6 volt battery direct connect to the phone...without any circuit will the phone take just enough amp it needs and will not exceed or under charge

Last edited: Jun 14, 2016
2. ### Marley Member

Apr 4, 2016
222
54
Current (measured in Amps) is the flow of electricity. Actually proportional to the number of electrons per second.
Ohms law tells you that it depends on the applied voltage and the resistance of the load.

For a source of power, like a phone charger, there is always a maximum current it is designed to handle. Allow the current to go over this and the power supply will be damaged. The actual current drawn will always depend on the load , the phone in this case.

For a phone, the charging input is designed to work with 5V. A greater voltage may damage the electronics inside the phone.

A phone is an electronic device and the load at the charging input may not obey Ohms law directly - the current may not be directly proportional to voltage. This is because it has voltage sensing devices inside that can change its load resistance depending on the voltage applied.

But to answer your question see Ampere.

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3. ### WBahn Moderator

Mar 31, 2012
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Perhaps a more commonplace example might make some of your confusion go away.

Let's say that you have some gas appliances, say a hot water heater, a stove, and a ceramic kiln out in the garage. You also have a gas line of a certain size coming into your home. The bigger the diameter of the line the more gas it can flow per minute (while staying above some minimum pressure). Let's make up some numbers (without ANY attempt to be reasonable) and say that the hot water heater needs 10 CFM to operate, the stove needs 100 CFM, and the kiln needs 1000 CFM. Now let's say that the gas service uses a pipe capable of delivering 500 CFM.

So how much gas will be flowing if you just have the water heater connected? Only 10 CFM because that is all the water heater needs and it won't draw more than that. Similarly, if only the stove is connected then 100 CFM will flow. In both of these cases the supply is capable of delivering more than the load is asking for, so the actual flow is determined by the load. But what if only the kiln is connected? Now the limitation of the supply comes into play and only 500 CFM will flow -- the kiln wants more than the supply is capable of delivering, so the supply determines how much will actually flow.

Do you see how that maps directly over to the case of a voltage power supply powering a load?

But chargers are often configured primarily as a current supply (not a voltage supply) and they will increase or decrease their voltage output (within limits) in order to maintain a target current output. This is quite different than the case of a voltage supply in which the current will be whatever it is, up to whatever capability the supply has.

Batteries have preferring charging profiles and different batteries are more tolerant than others to deviations from the nominal profile. For batteries that are sensitive to the actual profile (i.e., can be damaged if the actual profile differs too much from the ideal) the charger has an electronic controller built in that monitors several parameters and adjusts the voltage output in order to achieve the desired current output. These systems are far too complex to even attempt to explain or understand reasonable based only on Ohm's Law. The bottom line on that score is that Ohm's Law only applies to systems that are "linear" and smart phone chargers are HIGHLY nonlinear.

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4. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
134
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Does it mean it is possible to directly charge a phone with 5 volt battery without any circuit? So the amperage is automatically adjust itself according to what the phone need?

5. ### Marley Member

Apr 4, 2016
222
54
For a modern phone, with a standard USB connector: Yes. Providing the supply or battery is close to 5V (close means within 0.2V - this is IMPORTANT).

The current will automatically adjust itself to what the phone needs. This is true of most electrical devices (not all).

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6. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
134
3
This make it very interesting why do we need chargers and why the manufacturer dont just create 5 volt battery to charge phones

7. ### MrChips Moderator

Oct 2, 2009
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Why do we need chargers? That is because line voltage is 110/120VAC @ 60Hz or 230/240VAC @ 50Hz depending on the country you live in and your smartphone needs 5VDC.

Why not just create 5V battery? That is already being done. They are sold worldwide as portable power sources for charging smartphones and other USB devices. However, you still need a charger to charge the 5V battery, wouldn't you?.

btw, inside the USB device (or smartphone) is an intelligent battery management system (you can call it a charger). Your USB device does not necessarily need exactly 5V. The intelligent charger will take the 5VDC and convert it to whatever it needs, sometimes 3.6V. It will consume whatever current it needs limited to the maximum current that the external charger can supply.

You cannot use Ohm's Law to calculate the current because Ohm's Law only applies to linear systems such as a fixed resistance. A smartphone is a non-linear system, i.e. it is not a fixed resistance (repeating what WBahn has already stated).

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8. ### BR-549 Distinguished Member

Sep 22, 2013
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Current is a rate. Voltage and resistance are not expressed as a rate. Current is, it has a time factor. Batteries have a low charging rate, compared to the discharge rate. In other words, one can discharge (let current out) at a high rate, but can only charge (put current back in) at a relative slow rate.

In theory a battery has resistance only when charging, not discharging. This one way resistance is responsible for the difference in charge and discharge rates.

9. ### crutschow Expert

Mar 14, 2008
16,209
4,333
Not so.
What do you mean "in theory"? A theoretical perfect battery would have no resistance for either charge or discharge.
Real batteries have resistance in both the charge and discharge mode, otherwise they could deliver infinite current, which obviously they can't.
The resistances may be somewhat different though, depending upon the battery state of charge.

10. ### WBahn Moderator

Mar 31, 2012
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Let me guess, is it because the charged electrons is rotate in the opposite direction of discharged electrons?

Sheesh.

Since a battery doesn't have any resistance when discharging, I guess I can use four 3 V watch batteries to start my truck, huh?

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11. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
134
3
I need some more understand

In a step up voltage circuit using transformer...people say that the current is being used by the transformer to step the voltage..for example 3 volt step up to 500 volt

Does it meant all the current being used? and only left the voltage and if so why is it possible voltage exist without current in the step up circuit and on the other hand is it possible for the current to exist without voltage? kindly advise

12. ### Marley Member

Apr 4, 2016
222
54
The important thing to remember here is that voltage and current are different things. Often confused by people who know nothing about electricity.

A transformer steps up (or down) the voltage.

Once again, the current depends on the value of the load resistance only.

Another thing to note is that the power into a transformer is the same as the power out (actually there is a a small loss - but we will ignore that here).

Now: Power = Current x Voltage. (P=IV)

So consider a transformer that has 10V going into it (primary) and 100V coming out (secondary). Voltage step-up ratio = 10.
If this transformer has a 100 ohm load resistance, the secondary current will be 1A. The power in the load is 100W
Because the input power is the same as the output power, the input current has to be 10A (10A x 10V = 100W).
So, this transformer has stepped up the voltage by 10 times. The current is stepped down 10 times - to keep the power equal.

Hope that helps!

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13. ### BR-549 Distinguished Member

Sep 22, 2013
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crutschow, a battery is the closest thing we have to a infinite current and power source. At least that used to be taught. And it was a while ago.

Wbahn, we use 12 volts now for trucks. We used to use 6 volts. Couldn't we use 3 volts with proper capacity?

The charge and current discharging out of a battery is rotating from - to +. It is physically rotating. To recharge we must rotate the current from + to -.

You're starting to get it.

14. ### wayneh Expert

Sep 9, 2010
13,435
4,272
He got it right - 4 x 3V in series to give 12V. And no, it will not start his truck because the internal resistance will limit the current to well under 1A. Even a car battery suffers from internal resistance problems on cold days. That's why someone is selling a battery/capacitor combo device, to overcome internal resistance for a second or so.

15. ### wayneh Expert

Sep 9, 2010
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The designer of the device has to choose a charger for it. If the device requires a 1A charge rate, the designer might choose a charger rated to 1A (the minimum acceptable), 1.5A or 2A. All would work. The key is simply that the charger meets the minimum requirements for current (and possibly other factors).

And if it helps:
"Amps" is the instantaneous rate of water spraying out your hose. Coulombs of charge per second
Coulomb, amp-hour or mAh are all measures of water accumulated in a bucket over time. Battery capacity or capacitor charge
Voltage is the pressure of your water supply
Resistance is the loss of pressure due to flow in your hose. With no flow the pressure or voltage at your nozzle is the same as at the source. Once current flows, a ∆P appears across your hose or a ∆V across your circuit.

If it doesn't help, ignore it.

16. ### BR-549 Distinguished Member

Sep 22, 2013
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A 12 volt battery has 6 cells (batteries). It starts the truck. Capacity.

But this doesn't help the TS.

17. ### wayneh Expert

Sep 9, 2010
13,435
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WBahn and I were both responding to (and refuting by example) that statement.

18. ### BR-549 Distinguished Member

Sep 22, 2013
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You may refute an ideal component if you want. It doesn't bother me. MY ideal battery doesn't have discharge resistance.

My ideal components are of a higher quality than yours.

That's all for me here.

19. ### dante_clericuzzio Thread Starter Member

Mar 28, 2016
134
3
Does it meant that the load is the one holding the power instead of the supply voltage? For example a battery does not have power just voltage until it is connected to the load the load will consume what is required and produce the power it needs.

Let say a charger capable of delivering 5 Amp and the phone is only required 1 Amp...will the phone burnt or only take 1 Amp and charge normally?

Is it possible to know amperage of a battery how much limit it can supply? Let say 9 volt battery how many amp can it supply at max or min at zero load or no load at all.

How do we know and how much amp the 240 AC current that we use at home...why is it seems capable of powering almost any electrical appliance from the smallest to the highest power let say from 1 watt up to maybe 10,000 watts or maybe 100,000 watts..how does the electrical company determine it can do that?

Last edited: Jun 14, 2016
20. ### Techno Tronix Member

Jan 10, 2015
140
10
1 ampere = 6,241,510,000,000,000,000 electrons per second.
An ampere is a measure of how many electrons move past a point every second.