Resistors

[Decadence]

Ok...im doing my first small electronics project. I dont know much at all. I know that an LED im using has a max voltage of 2.0V and a typical voltage of 1.7V. Im using a 9V battery so i figure that i will need a resistor to block out the remaining 7-7.3V. Now I dont know which resistor to pick out. What colors of the stripes...how many ohms...how many watts and so on. Any help would be greatly appreciated.

-thanks

 

[fsahmed]

Semiconductor devices do not work in the ashion ui have in mind!!! To save the led from burning out, u have to limit the current using the resistor, the excess voltage will be sunk on the resistor automatically, the resistance value will determine the current that flows through the circuit whenever a voltage above 2v is applied, since the led will act as a short circuitted path once it has the voltage across its terminals. The actual current handling capability will be given in the datasheet of the led,

however, for 9V, a resistor of 8.2k would bring the current down to about 1mA which, if the LED is like the one usually found in appliances for indications, should be enough to drive the led yet keep it from blowing. U can experiment by using a potentiometer to reduce the resistance till u get the desired brightness & dont smell any odor that shouldnt be coming from circuits.

An 8.2k resistor with 5% tolerance would have the bands from left to right:

Grey Red Red Golden

The tolerance band is more spaced out than the bands depicting the actual value.

 

[Decadence]

i just tried the resistor you suggested but it seems much too strong. I didnt even get a flash out of it. What can you suggest thats less resistance?

 

[David Bridgen]

A normal red l.e.d. has a maximum forward current of 20mA.

With a 9V supply a resistor of 390 ohms will allow about 18mA, which is fine.

 

[Decadence]

what does a 390 Ohm resistor look like (what are the band colors?)

 

[David Bridgen]

Orange white brown.

A quarter watt resistor is suffiient.

 

[Decadence]

I have a white brown red...would that be ok?

The LED seems a little dull when hooked up with that.

 

[zachtheterrible]

A few words of advice:

Figure out how the bands on a resistor work, it will be indespensible if you plan on getting into electronics. Just go to google and type in 'resistor color code'

If all you are doing is lighting up the LED with the 9 volt, consider using 3 volts. It will last a many times longer than a 9 volt battery

Once you find the nominal current that your LED needs, here is the calculation to find what resistor to use:

R=V/I
where:
R= Resistance
V= The voltage that you will apply to your circuit
I= The current that you WANT to flow through your LED (lets say 20ma in this case)

R=9V/.02A (20ma)
So R= 450 ohms.

390 ohms like dave said is plenty close.

 

[Decadence]

Thanks for the advice. I found a resistor that is 470 ohms and all seems to be workin well with it.

 

[David Bridgen]

Ok Decadence. 470 ohms is fine.

Since I was preparing another reply when you posted I shall put it here anyway.

I have a white brown red...would that be ok?

That's 9100 ohms and allows a very small current, that's why the l.e.d. isn't very bright.

Once you find the nominal current that your LED needs, here is the calculation to find what resistor to use:

R=V/I
where:
R= Resistance
V= The voltage that you will apply to your circuit
I= The current that you WANT to flow through your LED (lets say 20ma in this case)

R=9V/.02A (20ma)
So R= 450 ohms.

You need to subtract the l.e.d's forward voltage from the supply, which gives 7.3.

7.3/20 = 0.365k

 

[Decadence]

Thanks alot everyone. You guys really helped.

 

[TV-Engineer]

As already said, you really should try to learn resistor colour codes and how they work. Once you know that, it'll stick in your mind and be easy from that point forward.

This site is good for finding out the values of resistors you have until you've learnt the code:

**broken link removed**

 

[zachtheterrible]

You need to subtract the l.e.d's forward voltage from the supply, which gives 7.3.

7.3/20 = 0.365k

forgot about that

 

[fsahmed]

I am sorry, I dont know under what influence did I put the 'k' & went on explaining the color code for it as well, I had meant 82 Ohms, for 100mA current. Sorry again.

 

[ODIN'S DEN]

NICE!!

As already said, you really should try to learn resistor colour codes and how they work. Once you know that, it'll stick in your mind and be easy from that point forward.

This site is good for finding out the values of resistors you have until you've learnt the code:

**broken link removed**

[/NICE THNKX]