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I have to make a battery powered lamp for a school project but having no experience or knowledge whatsoever in electronics I really need help! I'm using a halogen GU10 bulb which is 35W, 240V, toggle switch and 2 AA batteries. Can someone please explain how I would wire the light bulb to a toggle switch and batteries? Also, do I need a resistor anywhere? Apparentely halogen bulbs aren't really suitable for lamps, so should I change to a superbright LED? Thanks!

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## 9 Replies

Goodhart (author)2008-04-27

Depending on "which" LED you use, if you are using only one, you will need a resistor, like in the standard diagram that follows, BUT it does depend on the capacity of the specific LED you use

Since LEDs have very low resistance, you must have an external resistor to limit the current or they will burn up. To find out the value of the resistor needed, we use Ohm's Law.

V=IR (voltage = current * resistance)
(Vsource - Vled) = IR

The specs on my 5mm blue LED for instance, say it can operate at 3.2-3.6v at 20mA current. Since we know V & I, we can solve for R.

V/I = R
(6V - 3.2V) / 0.02A = R
140ohm = R

jemily (author)2008-04-27

ooh okay, I understand now! thanks for the info & diagram! It's rather coincidental that I'm using a 5mm blue LED too. Just one question: why is it 6V and not 3V? I thought AA batteries were 1.5V each - does this mean that I don't need a resistor like tech-king said? Thanks to tech-king and guyfrom7up for your help too!

NachoMahma (author)2008-04-27

. Just plug 3 into the formula, instead of 6. You'll have to forgive Goodhart; he's getting old.

Goodhart (author)2008-04-27

Oooops......Getting ? LOL

jemily (author)2008-04-27

haha! So i guess i'll have to use more than 2 batteries, because otherwise i'll get a negative resistance? ty.

NachoMahma (author)2008-04-27

. You don't need a resistor. You won't get max brightness, with less than the 3.2-3.6V, but it shouldn't affect you too harshly - 3V should be close enough.

Goodhart (author)2008-04-27

....if the batteries are completely fresh :-)

guyfrom7up (author)2008-04-27

goodharts diagram looks pretty good halogens run off ac, so that'd be hard

tech-king (author)2008-04-27

you should change to an led. a halogen bulb that uses 240v may not work off 3V. in theory, if you use white leds, you wont need a resistor.