Powering LEDs

[malex]

I am currently using 4 AA batteries in series (6v) to power LEDs in series and also a resistor of 340 ohms to prevent too much current drawn.

Now, when I connect 3 LEDs in series and then the resistor in series, they lights up and all is good.  When I try to put 4 in series none of them light up.  I used my voltmeter to check and the voltage drop is 1.7v for each LED when 3 are connected (they light up) and 1.45v when 4 are connected (none of them light up).

Lastly, when I check the voltage drop across the resistor I get .43v drop with 3 LEDs and 0v (i've checked this many times) when 4 LEDs are connected.
 
The voltage drop across the LEDs are higher than the resistor, which doesn't make sense according to the physics I've learned.

The LEDs were bought from a bucket at a store so I don't specifically know the specs of them.  I assume they're pretty standard red coloured LEDs.

So far I have checked:
Correct polarities used
Each LED was checked for malfunction
LED was placed before and after the resistor in different combinations

Any ideas?

[AndrewM]

Diodes, LEDs included, have a standard forward voltage drop (that 1.7V you are seeing with the 3 in series).  If less voltage is supplied than the required standard voltage drop, they don't behave properly (ie light up).  If too much voltage is supplied, they burn out, which is why you should have the resistor in series, extra voltage above the 1.7 per LED gets consumed (turned into heat) by the resistor.  In your case you need a little more voltage to run 4 LEDs, so add another battery to bump it up to 7.2V total (the resistor will use up the .4 volts extra) or get rid of the 4th LED.

[malex]

Ah, I see.


But will the LEDs still behave like a diode but just not light up?

[benji]

each led should have 1.7 volts across it to open (pass current)

you need at least a 4*1.7==6.8 volts battery

so how in the world a 6 volts would be enough for 4 leds?

and yea they wll light up,leds are diodes, they just light when current is running through em,
the more current (less resistor value), the more light .

[alessio136]

each led should have 1.7 volts across it to open (pass current)

you need at least a 4*1.7==6.8 volts battery

so how in the world a 6 volts would be enough for 4 leds?

and yea they wll light up,leds are diodes, they just light when current is running through em,
the more current (less resistor value), the more light .

do you safe power if you connect 4 diodes in serie in stead of connecting four of them in parallel, each with his own resistor?

I gues so becouse you will have only a direct current passing throug them, but I'm not sure...

[benji]

yea sure it will save ya power and cost
the only dis of serial wiring is that it needs a higher voltage source in order to open up the 4 leds

assuming its not easy to change source voltage as current