Lightbox - LED circuit question

[anthony]

I know that this will be very basic for some, but not me! I am making a light source for my light box. I have bought some LEDs to have a play with and so read a little on wiring the circuit. However, I have done my order but without getting any resistors. So, before I start I wondered if someone can tell me that I will destroy my LEDs without an appropriate resisitor or that maybe it will be OK.

I saw a little battery case with an on/off switch. That seemed handy so my power is going to be a 9v battery. My LED's are white, 1800mcd (maplin NR73Q). The important figures for these seem to be 20mA (max 30mA) and VF 4V. I was going to use 4, maybe 6 of them.

I really don't know what I am doing but wondered ...............

If I ran 4 in series from the 9v battery would it work at all?

If I had two circuits in parallel, each with 2 LEDs in series - would that work without a resisitor?

If my second suggestion is workable then I have another question.......

How does the wiring go?

Do I just run a wire from the +ve of the battery to the anode on the LED, then from the cathode of that LED to the anode of the next, then from the cathode of that LED to the -ve on the battery (but do that for each series)

Don't be shy to say that I am completely wrong (well I know I am not right) but using the V=IR for resistance suggested a low value needed for two pair LEDs in paralell and I wondered if I could dispense with the resistor(s) altogether. If anyone feels inclined to offer a little bit of electronics knowhow/principles in layman talk then that'd be cool!

Cheers

Anthony

PS I am tring to avoid buying a 20p resistor with a fiver postage! and there are no elctronics shops near me.

[Eddie]

Short answer is, use a resister. if you let me know what resister value you need I might have one that I can post to you.

Eddie

[PhillyDee]

ebay is probably your best bet for resistors.

[Parallax]

To run 1 LED from a 9v battery :-

The VF of the LED is 4v so you need to drop 5v (9v minus 4v) across a resistor.

The current is 20mA so from R=V/I, R=5/.02 which equals 250 ohms.

250 ohms is not a prefered value so you need to use 220, 240 or 270 ohms.

4 LED's in series probably wont work - there wont be enough voltage across each LED.

2 circuits in parallel each with 2 LED's in series MIGHT work but there will be 4.5v across each LED so it may blow them.

Your wiring thoery is correct.

Steve

[anthony]

Cheers Guys

Ebay is my next port of call!

Cheers Eddie.

Using a web based calculator I'd need a couple at 56ohms to run 4 LEDs as 2 pair in parallel. But let me look at ebay first.

I'll let you know how I get on.

Anthony

[anthony]

Ordered a selection of ebay!

Anthony

[Eddie]

8)