Do I need a resistor here?

MillenniumFalsehood · Post by **MillenniumFalsehood** » Thu Mar 22, 2007 8:42 pm

Okay, I've heard that you always need a resistor. Well, if I have three white LEDs that rate 3.4v and a 9v power source, the LEDs don't overheat, and they still glow brightly, do I really need a resistor?

jwrjr · Post by **jwrjr** » Thu Mar 22, 2007 10:40 pm

For what you describe, no, you don't need a resistor. (Stupid power-user trick - there is effectively a resistor in the battery. That's why if you draw a lot of current from the battery, the voltage will go down and the battery will get warm.) But if you try that with an AC adapter and the adapter voltage is higher than the led voltage, the result will be different. Explosively different.

TREKKRIFFIC · Post by **TREKKRIFFIC** » Fri Mar 23, 2007 10:26 am

Good to know. I always thought in a series circuit like the one described there had to be at least one resistor soldered to the cathode of the first LED in the string.

I have PL E I'd like to build with flashing running lights in the saucer but I don't want to have to install a circuit board to do that. I was thinking about hooking up a red self-flashing LED followed by a regular non-flashing green LED and a non-flashing white LED in the same series. They would all be powered by a 9V battery. I read in another thread that if you run non-flahing type LED's in a string with a flashing LED first in the sequence that all the LED's will flash. I hope this is true. Can anyone confirm this ?

.

BERT aka MODEL MAKER · Mon Mar 26, 2007 11:13 pm

How many 5 mm LEDS can be run on a 9 volt battery without any resistors

just the battery and the wires to the LEDS ?
Same question using 3 mm LEDS.
Thanks

veedubb67 · Post by **veedubb67** » Tue Mar 27, 2007 11:03 am

Depends on the forward voltage of the LEDs and whether you're wiring them in series or parallel. White LEDs have different voltage requirements than colored ones.

BERT aka MODEL MAKER · Tue Mar 27, 2007 2:30 pm

just white ones and so that they all go on at the flip of the switch. which way is the best to wire them.

jwrjr · Post by **jwrjr** » Tue Mar 27, 2007 10:25 pm

You can run 2 white leds in series from a 9 volt battery (use a 100 ohm or higher resistor). Theoretically there is no limit to how many leds you run in parallel ... it just depends on how long you want the battery to last.

BERT aka MODEL MAKER · Tue Mar 27, 2007 11:47 pm

THANKS

just wanted to see what could be done with what.