How to know if i should put a resistor?

Question

I have a 12v board circuit and i was going to put atleast 7-10 LED's attatching them together parrellel wise, i was wondering if i need a resistor or if the led's will take too much power, im a pretty newbie here so help would be appreciated.

Answer 1

You waste power running the LED's in parallel with a resistor. Run them in series using voltage drops. I.e. if each LED has a forward voltage Vf of 2 volts, place 6 in series to equal the 12 volts supply. You use less components too. If you want to run 12, run two parallel strings of (6 Series). The voltage is important, however current moreso. They generally have a maximum forward current of 20ma. When you place 6 in series it will still only draw 20ma (Current in a series circuit is always the same, but voltage is not). If you run them in parallel with a current limiting resistor you will have a cumulative effect of current. i.e. 10 LED's in parallel will consume 200ma current, plus the loss in the resistor.. (Voltage in a parallel circuit is the same, however current is different.) Hope this helps.

Answer 2

A LED will almost always need a resistor or it will burn. Never count on internal resistance in a battery to do the trick since you'll never know how large it will be. It changes with remaining capacity and type of battery. A LED typically needs 10mA and have a voltage drop of about 2.7 V. So you can estimate the needed resistor by subtracting the voltage drop over the LED from the supply voltage and dividing by 10 mA. For one LED R=(12-2.7)/0.01 Ohms=930 Ohms. A 1 kOhms resistor would be a good choise.

I suggest you use one resistor for each LED. It will distribute the heat.

If you still insist on having only one resistor you will need a heavy duty one. The power over the resistor will for 10 LED:s get (12-2.7)*0.01*10 W = 0,93 W. You'll need at least a 2 W rating to keep heat at a comfortable level. The power rating on the resistor is how much it can handle continuously without burning. So if you choose a 1W rated resistor instead it will be able to handle the power but it will be VERY hot. The resistor value for 10 LED:s gets R=U/I=(12-2,7)/(0.01*10)=93 Ohms. But like I said I don't recommend it. If one LED brakes down current over the other LED:s is increased and also the voltage drop over each LED can vary somewhat making different currrent run through each LED. Small resistors are very cheap. So you can use many of them without exceeding your budget.

Also now a days you can buy LED:s with built in resistors for a given supply voltage. The total cost will go up but it will give a cleaner  design.

Answer 3

Would depend on what's driving the LEDs.

If is simply a battery/power pack, and the LEDs are rated 12V, then you should be able to connect them direct between pos and neg.

If they shine too bright then you might need consider a resistor in series to lower the current.

If they shine too dim, then the EMF of your power source might not be enough.

If you are using any form of IC output to light the LEDs then you will need to use a resistor for each LED to pull it up to pos or down to neg, depending on whether the IC o/p goes neg or pos respectively.

Answer 4

Several wrong answers.

You ALWAYS need a resistor with a LED (unless it has one built in).

You cannot put them in parallel. It does not work.

LEDs work from a current. And you develop that with a resistor. If you try to drive them with a voltage, you will get burned out LEDs or ones that don't light, or a lot of variation in brightness.

In this case, I'd put several in series with one resistor, that works OK. If the VF spec of the LEDs is 2 volts, then put 4 in series with one resistor. The 4 add up to 8 volts, which leaves you with 4 volts across the resistor.

If your current is 20mA (different LEDs require different currents, it is how you adjust the brightness) then the remaining 4 volts and 20ma gives you 4/0.02 = 200 ohm. Power in the resistor is 80mW, small so a 200 or 220 ohm 1/4 watt resistor would work.

Make another string of 4 LEDs and 1 resistor if you need more.

The above example is for a 2v 20ma LED, different values will give different results. Try to keep at least 3-4 volts across the resistor.

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Answer 5

From  Italy  with  my  bad  English  ( sorry )

    

each  led   need   a 1000 Ohm  serial  resistor  .

if  you  need  7   LED   you will  need 7  resistor  too..

Never  connect  LEDs  in serial   because  they  are  

not  so  similar    as  we  could  imagine ..

Also  with serial  connection  of  LED s  in case  of  fault

of  a  single  LED   , all  the  chain will be  off.

the  power  of  the resistor    :

10mA  in a  resistor of  1000 Ohm     P = I^2 x R  = 0.1W

so  0.1 W  is  not  a big  power

if  you  use  10   LEDs    the  total power  is  1 W

Ohm