http://i38.tinypic.com/xdxsuv.gif

1.
i know that a diode is short circuit when the currect threw it is positive
and open circuit other wise

but the graph of it says
when the current goes from zero till infinity
the voltage is zero (which is fine)

but
that on current 0 the voltage goes from zero till minus infinity

what does it mean " zero till minus infinity"?

i cant see how it means open circuit

2.
why a battery gives a constant voltage no matter the current?

 

When (conventional) current flows through a diode from anode to cathode there is a voltage drop across the diode. This volt drop is fairly constant with only a slight increase when the current is increased.
What about reverse (zener) breakdown?

It is a bad idea to think of the normal operating behaviour of a diode as being short circuit or open circuit.

The graph is an ideal representation, real diodes dont behave exactly like that.

Zero = the integer number between +1 and -1, (1 + (-1) = 0

till = Norwegian/Swedish for the English word "to"

minus = denotes a negative number

infinity = a very large number

Exactly, it does not.
You only think this because your basic understanding and terminology of a diode is wrong.

Only an ideal thoretical battery works like that.
For a real battery, the terminal voltage will drop as the current drain increases.

JimB

__________________
Experience is directly proportional to the value of the equipment ruined.

 

He's talking about an ideal diode, when it is fowrard biased the voltage across a diode (a real one) is +/-0,7V for a silicon diode, and it is fairly constant as JimB said, when it is reverse biased the current is so small (units of nA) that it is considered an open circuit, and you will measure any voltage (untill it reaches its breackdown voltage which is also the minimum voltage it needs to conduct in reverse ).

__________________
"A person who never made a mistake never tried anything new"
Albert Einstein

 

i use only ideal components
"zero till minus infinity"
i ment i cant understand the physical meaning of this part in the graph

 

Zero to minus infinity simply means that the voltage *may* be anything between
zero and minus infinity. When the voltage across the diode is minus, the diode
does not conduct. A few examples:
v=-1, i=0
v=-2, i=0
v=-100000, i=0
So the voltage can be minus anything and the current will always be zero.

They seem to be saying that when the voltage across the diode is zero
it conducts (and the amount of current would depend on the external circuit).

They should have specified that 'v' is the voltage across the diode itself,
not the input voltage v to the circuit as it is drawn.