Arduino Forum

Using Arduino => General Electronics => Topic started by: chetan0412 on Jun 05, 2013, 12:30 pm

Title: internal resistance of atmega 328
Post by: chetan0412 on Jun 05, 2013, 12:30 pm
I am using voltage regulater IC 7805 to give supply to arduino but it gives higher current supply.I want to know the internal resistance of atmega 328 so that I use resistance in between the digital pin and 7805 to lessen the current
Title: Re: internal resistance of atmega 328
Post by: AWOL on Jun 05, 2013, 12:44 pm
The device will draw as much current as it needs.
Title: Re: internal resistance of atmega 328
Post by: majenko on Jun 05, 2013, 12:54 pm
The current rating of any power supply, or voltage regulator, is the maximum current it can provide.  A device connected to that power supply will only take as much current from the supply as it requires.  If it tries to take more than the supply can provide you risk breaking the supply, or the voltage "drooping" to a level that causes the device to stop working properly.

A constant-voltage supply never forces current.

You can think of it as a well, with the device being a bucket.  You can't put a bucket down the well that's bigger than the well opening, and you will only draw as much water as will fit in your bucket.  The well doesn't squirt water out at you.
Title: Re: internal resistance of atmega 328
Post by: Grumpy_Mike on Jun 05, 2013, 01:00 pm
Quote
I want to know the internal resistance of atmega 328

No you don't want to know this. It is not a constant value it varies during operation of the device.

Quote
so that I use resistance in between the digital pin and 7805 to lessen the current

No all you will succeed in doing is making it not work at all.
You can reduce the current a small bit by reducing the voltage, but if you reduce it too much it will stop working and you can't feed in voltages higher than the supply voltage.
Title: Re: internal resistance of atmega 328
Post by: chetan0412 on Jun 05, 2013, 01:06 pm
Thanks to all for fast reply......

chetan
Title: Re: internal resistance of atmega 328
Post by: fungus on Jun 05, 2013, 01:26 pm
Ohms law really works!

Title: Re: internal resistance of atmega 328
Post by: pito on Jun 05, 2013, 02:02 pm
Quote
Ohms law really works!

Do you mean: 5V/0.0095A (16Mhz/5V, active mode supply current) = 526ohm :) :)
Title: Re: internal resistance of atmega 328
Post by: fungus on Jun 05, 2013, 02:05 pm

Quote
Ohms law really works!

Do you mean: 5V/0.0095mA (16Mhz/5V, active mode supply current) = 526ohm :) :)

0.0095mA?
Title: Re: internal resistance of atmega 328
Post by: pito on Jun 05, 2013, 02:07 pm
Corrected: A
Title: Re: internal resistance of atmega 328
Post by: fungus on Jun 05, 2013, 02:18 pm

Do you mean: 5V/0.0095A (16Mhz/5V, active mode supply current) = 526ohm smiley smiley

Yes!

Disclaimer: Amps may go down as well as up.
Title: Re: internal resistance of atmega 328
Post by: MarkT on Jun 05, 2013, 10:37 pm

I am using voltage regulater IC 7805 to give supply to arduino but it gives higher current supply.I want to know the internal resistance of atmega 328 so that I use resistance in between the digital pin and 7805 to lessen the current

Ohms law doesn't apply to active devices, in general.  It applies to conductors, semiconductors (if uniform, no PN junctions), electrolytes.
Batteries are basically made of plates and electrolyte and the internal resistance there is largely due to the ohmic resistance
of the electrolyte I think (although this is by no means constant with time / temperature or age).
Title: Re: internal resistance of atmega 328
Post by: fungus on Jun 05, 2013, 10:51 pm

Ohms law doesn't apply to active devices, in general.

They don't obey the laws of physics?

Title: Re: internal resistance of atmega 328
Post by: pito on Jun 05, 2013, 11:58 pm
Ohm's law and Maxwell's eq etc are not fundamental laws of physics. They are only an approximation to make EE happier. Instead of solving Schroedinger's equation for metals/semiconductors we simply use Ohm's law.. :)
Title: Re: internal resistance of atmega 328
Post by: Grumpy_Mike on Jun 07, 2013, 12:06 am
Quote
Ohm's law and Maxwell's eq etc are not fundamental laws of physics.

I think you will find they are unless you are taking a radical view of the word fundamental if so then neither is Sshrodinger.
Ohms law embodies the definition of the units of volts amps and ohms.