when i used USB supply for Arduino, It work well.I Want to know when arduino use external supply ,what is its working voltage and working current? if i use the usb power , i have two power source, it is nonsense. anybody can shed a light ?

i used arduino to control one toy motor with L298N,  the motor  only want 3-5V voltage . I just have a  AC/DC adaptor which can output 9v 1A ,So i used it to supply the arduino and L298N. because the motor is small ,i used a 78L05 to get 5v  for L298n Pin4 and connect the adaptor output directly to arduino external powerin .  but the arduino seem not to work and blink and the outpin (should high) have no result. After five or later minutes, it work and not blink.  i think maybe the adaptor not provide enough current for arduino,but the adaptor output is 1A and 9v , is not enough to driver a small motor and arduino??

i just have one AC/DC adaptor,the output  is 12v and 1.5A . if i used it as arduino decimal external power ,
does the arduino can edure ? or i will see a fire and smell the smoke?

thanks again. so i have to use a seperate voltage supply or the arduino Voltage input to driver motor. another hint is to connect one or two 0.1uf capcitor  on motor termials. now i know that the output voltage pin of arduino only indicate its onboard "powering" and have not capacity to drive others. thanks.

maybe i havenot express my idea clearly. i said the supply for motor is the Arduino voltage output pin. (The 5v output pin.) It isn't the same as the supply for Arduino, it's just arduino's output.Maybe the Opto-isolator can  produce some effects?

thanks for reply . I will deal with it like what you teached me to use a capacitor to accross the motor terminal. But there is another problem about voltage.
 Because my motor is so small ,  the voltage supply of arduino is enough. it is possible to use it directly and not use seperate votage supply ? if i use an Opto-isolator ,it maybe play an role as seperate voltage supply. But should i use two Opto-isolator  to isolate the PIN4 and Pin9 of L298?

other more ,anybody can teach me how to post a image?

thanks for your reply above all , i will try it . maybe i should do like that:

1. use seperate Volatage supply
2. L298 pin9 -- connect arduino 5v output  and  add a 100nf capacitors  to GND.    
           pin4  --connect seperate 5v and  add a 100nf capacitors  to GND.  
           pin2,pin3 -- connect motor , add  4 diode to release the current in motor.



maybe is it right? and the volatage GND can share the Arduino GND?
            

i use the ir receiver to control the motor. but i got some problem. when the motor run ,the arduino will not response to ir signal again . i use the L298N to control the motor.
 and L298N voltage is directed connected to the Arduino 5v supply, not used any other component. maybe i should use a seperate battery and not the arduino 5v output pin ??

the L298N : pin4.pin   ----- Arduino  5v output
                 pin5,pin7  ------Arduino pin5,pin7  TTL
                 Pin6        -------Arduino pin6 Ena
                 pin1,8,15 --------Arduino GND


 Ir ----    pin1 ------arduino 5v output
             Gnd   --------Arduino GND
             data  ----arduino pin2