Can we power the arduino uno board using the 5vdc external power source to 5v pin?
Yes.
Add a diode from 5V (anode) to Vin (cathode) to avoid reverse driving the 5V regulator and possibly damaging it.
Diode 1N4001 can do the work,not sure about this.
How about zener diode,to protect the board from over voltage.
1N4001 is fine. See page 10:
5.1V Zener to ground, can't hurt.
CrossRoads:
Add a diode from 5V (anode) to Vin (cathode) to avoid reverse driving the 5V regulator and possibly damaging it.
If that was a problem, then you could not operate the UNO from USB, as that is precisely what the USB port does - feeds +5V to the 5V line!
Arduino uno produce 1 ampere current from the transistors.if we want more than that,
The best way to supllier the 5vdc from external sources.
zainimechanical:
Arduino uno produce 1 ampere current from the transistors.if we want more than that,
The best way to supllier the 5vdc from external sources.
??
Sorry everyone, there is a lot confusion on this page.
The drawn sketch is wrong. Vin is the higher voltage (the input for the voltage regulator).
The Arduino Uno 5V is not able to produce 1A.
Yes, you can use 5V to power the Arduino Uno, but there are a few things you have to know:
In some cases the voltage regulator was blown. You can avoid that with an extra diode (from output (5V) to input of voltage regulator). The Arduino Uno is protected to prevent current flowing into the USB bus into the PC. With a standalone ATmega chip the 5V can put current into the USB bus of the PC.
I use a number of times external 5V to power Arduino boards. My external 5V are protected against high currents. I do have at least the diode over the voltage regulator, and sometimes I remove the voltage regulator. I didn't have a problem yet.
Peter_n:
Sorry everyone, there is a lot confusion on this page.The drawn sketch is wrong. Vin is the higher voltage (the input for the voltage regulator).
The Arduino Uno 5V is not able to produce 1A.
Yes, you can use 5V to power the Arduino Uno, but there are a few things you have to know:
In some cases the voltage regulator was blown. You can avoid that with an extra diode (from output (5V) to input of voltage regulator). The Arduino Uno is protected to prevent current flowing into the USB bus into the PC. With a standalone ATmega chip the 5V can put current into the USB bus of the PC.I use a number of times external 5V to power Arduino boards. My external 5V are protected against high currents. I do have at least the diode over the voltage regulator, and sometimes I remove the voltage regulator. I didn't have a problem yet.
The 7805 smd version should supply up to 1 amp peak but very briefly until it begins melting 
cjdelphi:
The 7805 smd version should supply up to 1 amp peak but very briefly until it begins melting
Actually, by design it simply shuts down instead, as does the LM1117. No such pyrotechnics
{I nearly mis-spelled "shuts"!}
See Reply #5 here from Arduino Team re: powering from external 5V:
http://forum.arduino.cc/index.php/topic,82046.msg758170.html
Paul__B:
cjdelphi:
The 7805 smd version should supply up to 1 amp peak but very briefly until it begins meltingActually, by design it simply shuts down instead, as does the LM1117. No such pyrotechnics
{I nearly mis-spelled "shuts"!}
Actually, when hot the current slowly drops along with voltage or quickly with excessive current, but it does not shut down like you describe..
CrossRoads:
See Reply #5 here from Arduino Team re: powering from external 5V:
Yes, interesting; the original thread relates to problems not with the regulator, but to the FTDI chip.
My reading of it all is that some people have had some problems some times with various boards, but they really do not have the foggiest idea as to what is happening. 
I repeat - if there was a problem with back-feeding the on-board regulator, how come the USB supply (which may well be capable of 2 amps) causes no such problem?
The problem is of course, second-hand reports. Who knows what "external regulated 5V supplies" were in fact used? The "take home message" is that using the on-board regulator - which is in fact quite rugged in itself - is at least predictable and reliable, as largely, is a USB port.
{Aside: I have not been getting the forum "Broken!" page for a day or two!}
The USB supply is fused with 500mA PTC fuse, so 2A coming into the board is doubtful.
The datasheet for the LM1117 regulator has the protection diode from output to input for cases where there is high capacitance, or where the input is shorted and the output driven.
So I would suggest the onboard regulator is not all that rugged - the MC32669 regulators never had these issues. The LM1117 did.