Say you have six LED's in a circuit how do you calculate resistor size if the LED's have different forward voltages. I see 0.02A being used a lot as a sort of standard amperage but wont this affect brightness of some LED's in circuit.Already checked search.
If you put them all in series, some will be brighter, some will be darker than they should be. The same current will run through all the LEDs remember. It would be better to run them in parallel..
H20-g33k New to this and have read in a few places you should avoid parallel circuits but i'll try it that way.Can you just put resistors on the higher powered LED's to even things out or do you need to put a resistor on each of them Ta.
I must admit, my knowledge of electronics is sketchy, but you should have some sort of current limiting resistor for each led. The problem with having lots of leds in parallel is that it tends to be more wasteful.
Dont fancy all these resistors so it seems the easiest way for a first attempt is to do this is in series and to stick to the one colour and don't mix them up.Thanks.
Not at all... Most LEDs indeed need 20mA, putting them in series with the right resistor will not change their (manufacturer specified initial) luminosity... Just add the forward voltages of the diodes in series, substract this of the supply voltage, and divide then by 0.02 to obtain the correct resistor. Of course, if you try to use LEDs of let's say 200, 1000 and 5000mCD together you will have severe differences in brightness, whether you connect them in series or parallel. CD
Thanks H2O-G33K one more question.If the total Vf of LED's exceeds the line in voltage then I suppose the remaining LED's to be connected should have another supply and another resistor to suit.
I have 4 white leds from Radioshack wired up in series with each other without a single resistor, and it worked out perfectly. As long as the leds have identical characteristics, then they should have same brightness when connected in series. I think the forward voltage for each led was about 3.3V, so if you divide 12V from power supply by 4 you get 3 volts for each led, a value which is within the safe operating range for the leds.
With your replies and a bit of studying old Kirchhoff i think i'm ready now to come face to face with a soldering iron.Only one thing!how do you close a thread,is it that Close Current Tag or what ?
The forward voltage of any led depends on the current flowing through it, shown below for a Kingbright blue. It's not a single value like 3.3V or 3.7V. That's the forward voltage at one particular forward current, sometimes the current that gives a certain brightness, sometimes the maximum safe current. The Kingbright is a 3.7V @ 10mA, but it's 4V @ 20mA. Same led. If you only allow the leds 3V, the current will be very low and they won't shine like they could. And without a few spare volts and a resistor, there's nothing you can do to increase the brightness to the spec mcd. There's always somebody may want to add something, even after a year or three.
