Maker Pro
Maker Pro

Question about dependence of current in a circuit.

mannok

Jan 8, 2015
5
Joined
Jan 8, 2015
Messages
5
Firstly, sorry for my poor English and I know that this is a idiot question in electronic. I am a fresh beginner.

1st question, Imagine that I have a portable battery which can output 5v/2A. What does it means? Is it means this battery will force itself to output 5v with 2A? Or just means this battery will output 5 volt and can provide 2 amp at maximum?

2nd question, for those AA battery in the market state that they can output 1.5v but they doesn't stated the output current. Then what is the current they output? Assume I have a LED which consume 3v to light up, then I connect 2 1.5v battery to become 3v and supply power for the LED. Then what is the current in the circuit? Is the resistance of the LED constant? Why the LED won't blew up? << (I have tried, the LED work perfectly)

3rd question, I want to buy a motor that consume 1 - 6v and the reference current is 0.35-0.4A. What is the meaning of reference current? Is it means that the motor will regulate the current pass through itself to 0.35-0.4A? If I supply the power to the motor by the portable battery stated in 1st question (output 5V/2A), will the motor blew up because of the current supply (2A) is much larger than the reference working current stated by the motor (0.4A)?

4th question, I have a device that should be powered by 5v/1.2A via micro USB. Will I damage the device if I use 1st question portable battery as power source? If no, what is the current output by the battery? If the current is 1.2A then is it means the device can regulate the circuit current to 1.2A even the power source can output 2A? If the answer is YES, how about I change another power source that output 10A? Will the device be damaged? (because I have tried to supply power to a LED by a laboratory DC power supply, I keep increasing the supply current, the LED blew up finally.)

Sorry for those long questions, I am a newbie. I know the basic theory of KCL, KVL and ohms law, it will be appreciated if anyone can explain the answer to me by these laws.
Thousand Thanks!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 

Harald Kapp

Moderator
Moderator
Nov 17, 2011
13,700
Joined
Nov 17, 2011
Messages
13,700
Welcome to electronicspoint.

1st question, Imagine that I have a portable battery which can output 5v/2A. What does it means? Is it means this battery will force itself to output 5v with 2A? Or just means this battery will output 5 volt and can provide 2 amp at maximum?
The latter: 5V, current from 0A to 2A, depending on the load.


2nd question, for those AA battery in the market state that they can output 1.5v but they doesn't stated the output current. Then what is the current they output? Assume I have a LED which consume 3v to light up, then I connect 2 1.5v battery to become 3v and supply power for the LED. Then what is the current in the circuit? Is the resistance of the LED constant? Why the LED won't blew up? << (I have tried, the LED work perfectly)
The max. current from a battery depends on many parameters, the main parameter being the internal resistance of the battery. Due to the chemical or physical processes within a battery it can pump only a limited amount of current. The more current you draw, the less voltage you will see at the terminals. There is no general answer as to the limit. This depends on the size of the battery, the battery's chemistry, its age etc. You'll have to look this up in the battery's datasheet (example) or read this rather general information.
An LED will require a series resistor for proper current limiting. It will work without resistor due to the limited current supplied by the battery, but this is no good design. Read this ressource for in-detail information.

Want to learn more about batteries -> batteryuniversity (I'm in no way connected with this site).


What is the meaning of reference current?
That depends on how it is defined by the motor's datasheet. I don't know a universal definition for this value. I'd assume this is the current drawn by the motor at a defined voltage and a defined load. A motor's current will change with the load even if the voltage supplied is constant.

Will I damage the device if I use 1st question portable battery as power source? If no, what is the current output by the battery? If the current is 1.2A
As long as the voltages match and the power source can deliver at least as much current or more as the load, you'll be fine. The load wil draw only as much current as it needs, 1.2 A in this case.
Don't worry, this is a common misunderstanding by beginners. The current rating of a power source is the max. current is can deliver. This does not mean that it will force this current through the load.
Your LED blew up because the current wasnot limited to a safe level, see above reply.
 

mannok

Jan 8, 2015
5
Joined
Jan 8, 2015
Messages
5
Welcome to electronicspoint.


The latter: 5V, current from 0A to 2A, depending on the load.



The max. current from a battery depends on many parameters, the main parameter being the internal resistance of the battery. Due to the chemical or physical processes within a battery it can pump only a limited amount of current. The more current you draw, the less voltage you will see at the terminals. There is no general answer as to the limit. This depends on the size of the battery, the battery's chemistry, its age etc. You'll have to look this up in the battery's datasheet (example) or read this rather general information.
An LED will require a series resistor for proper current limiting. It will work without resistor due to the limited current supplied by the battery, but this is no good design. Read this ressource for in-detail information.

Want to learn more about batteries -> batteryuniversity (I'm in no way connected with this site).



That depends on how it is defined by the motor's datasheet. I don't know a universal definition for this value. I'd assume this is the current drawn by the motor at a defined voltage and a defined load. A motor's current will change with the load even if the voltage supplied is constant.

As long as the voltages match and the power source can deliver at least as much current or more as the load, you'll be fine. The load wil draw only as much current as it needs, 1.2 A in this case.
Don't worry, this is a common misunderstanding by beginners. The current rating of a power source is the max. current is can deliver. This does not mean that it will force this current through the load.
Your LED blew up because the current wasnot limited to a safe level, see above reply.
Thank you very much for the answer and explanation!!!!!!!!!!!!!!!!!!!!!
In addition for the last answer. I want to know is there any safe level of current for motor? Why the motor will not be damaged because of drawing too much current like the LED? Why we should limit the current for LED but not for motor?
 

BobK

Jan 5, 2010
7,682
Joined
Jan 5, 2010
Messages
7,682
LEDs are a special case where the curve relating voltage and current is very steep. I.e. a small change in voltage can cause a large change in current. Most other devices do not work that way. Additionally, the LED voltage at it's rated current is not very precise, it will vary over several tenths of a volt, and varies with temperature.

Bob
 

Bluejets

Oct 5, 2014
6,901
Joined
Oct 5, 2014
Messages
6,901
Motors regulate current drawn by back emf.
Overloading a motor can cause it to draw excessive current and burnt out.
 

mannok

Jan 8, 2015
5
Joined
Jan 8, 2015
Messages
5
Thank you Bobk and Bluejets.
One more question, will the resistance of the motor/LED vary at different voltage supply?
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
The resistance wont vary, but the current through the component will vary with supplied voltage
( tho LED's again can be a special case compared to other components)
 
Top