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led n00b here, need a little help Answered

hey guys i am new to electronics and figured id start with leds, my first project will be to make a torth from a penguin caffinated mints tin using some domestic batteries, some Super bright (17000mcd) 3v Green LEDs and 10 resistors only described as "Used to operate a 3.3v LED on a 5v Power Line. " and the same for the 10 12v resistors. now can anyone tell me. when would i need a resistor in my circuit? (i know the one for the 12v line would be perfect for arraching to a car battery and not killing the led.) but the 5v resistors? when would i use them? when i make this torch i will be using 2 or 3 aa batteries, 2 aa batteries make 3v and these leds are meant for 3.3 so im assuming i wont need a resistor however if i used 3 aa (4.5v) would i need the 5v resistor or would it be to much? im sorry im confusing thanks


"Since you know these terms you can remember that electricity flows easily from the anode to the cathode but not the other way around." quoted from one of the above instructables, this is backwards thanks

electrons (negative current) flow from K to A he direction of electricity flow (positive current) is opposite to that of electrons

yea thats confusing (not that it bothers you when pikachu attempts to wake you up)

I'm pretty sure that I'm right about the anode cathode thing. Just google it.

also can i ask when making an led that blinks, why is a microchip needed? is this so that when the capacitor is charged the circuit is broken?

not necessary a chip. any circuit that switches on its own as wanted is ok this circuit can be 555 / 2 transistors / frequency divider from line ac voltage / thermal relay / relay / motor powered mechanical switches and other possibiitis

It's not exactly a microchip. It's an integrated circuit, and I believe the one you are referring to is a 555 timer. A 555 is normally used to make an astable circuit with pulses. By changing the capacitance and resistance of the circuit, you change the frequency (ie. How quickly the led blinks, The pitch of the noise from the speaker, etc.).


9 years ago

wait, just remembered why i got confused, i wasnt told the current rating of my leds just the operating voltage and the brightness

guess a current. 20 mA is ok for most leds. power them up and see if brightness is ok and they dont heat after long time

on the surface

when you connect a led to the power source it takes exactly the volts it needs. since the source has higher voltage there is a problem

if you dont put a resistor the source will force the extra volts into the led as well and burn it

the resistor takes on itself the extra volts so that the led does not get them

but even with the correct volts we need to set the correct current for the leds. too high current will burn them same as too high voltage

the current depends on the voltage that the resistor gets and on its resistance. A (current) = V (voltage) / R (resistance)

for example we want to run 3 V 20 mA led on 5 V
we need to throw away 2 V and need the current to be 20 mA
2 V / 20 mA = 0.1 Kohm = 100 ohm
so we need 100 ohm (brown black brown gold) resistor

thumb rule : if the led and source voltage are exactly the same its NOT ok to run the led on it without resistor. put atleast some small resistor or use another led / supply. its still not recommended cause if the source voltage lowers alittle the led goes off

exception : its ok to connect leds without resistor to low power sources (like alkaline batteries other than duracell / energizer) if the voltage is the same or differs in less than 0.5 volt. most alkaline (not rechargeable !) batteries have small resistance of their own which acts as resistor

you can run more than 1 led together on 1 resistor. the leds share the source voltage between them and the resistor takes what remains. the max amount of leds on one resistor is such that the source voltage is enough for all of them

for example lets run 10 2.4 V 20 mA leds on 12 V. the max leds that can share 12 V are 4 (we could 5 but we must use a resistor and drop some volts on it). so we'll make 2 series of 4 leds each and 2 extra leds

resistor for each of the 4 led series :
12 - (4 X 2.4) = 2.4 V
2.4 V / 20 mA = 0.12 Kohm = 120 ohm (brown red brown gold)

resistor for the 2 extra leds in series :
12 - (2 X 2.4) = 7.2 V
7.2 V / 20 mA = 0.36 Kohm = 360 ohm. the closest resistors are 330 ohm (orange orange brown gold) and 390 ohm (orange white brown gold)

this explanation is not accurate if you look into detail but i hope it gives good start and common sense

there are online wizards that can calculate the correct resistor

it all makes sense now i know all this physics but i just coulnt apply it practically, (by the way the interneal resistance is the emf-tpd)


9 years ago

ok so say i have the resistance of the resistor, i know the supply voltage, how can i find out how many volts the resistor will "absorb"

There are several Instructables on using LEDs - the search box is at the top-right of the page.


9 years ago

also when using 3 leds in series i do not need more resistors do i? (i am doing higer physics in school but we never did led specifics)