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LEDs at less than forward voltage ; no resistor?? Answered

I'm new to playing around with LED's, so please forgive the ignorance.  Right now I am trying to make a homemade lamp of sorts.  I am using white LEDs with the following specs per the seller.

FW current - 25 mA
FW supply - 3.3V (typical), 3.6V (max)

Tested one LED with two AAA batteries and no resistor.  I was told by radio shack guy that since supply voltage was less than FW supply I would be fine, and the LED would pull the amperage it needed.  Lit up and looked fine to me.

I then found a 9V DC wall wart, and decided I wanted to run possibly up to 12 LEDs using 4 parallel strings of 3 LEDs in series (9.9 volt drop across each series).  I bought a multimeter and checked the DV voltage from the power supply, and it ran consistently between 9.1 - 9.2 volts.  The thought with the 9V wall wart was that the voltage supply would be lower than the FW supply, thus I would not need resistors, simplifying and reducing # of connections needing soldering (new to that as well).

My question is, will this 9V work for my system?  I've tested 3 LED's in series, and also 2 parallel strings of 3 LEDs in series, and both lit fine.  I've read posts where people say that the lights will not light up or will be dim if the FW supply is not provided.  As mentioned, they seem fine to me, and as this lamp is planned for more of a mood lighting, I don't need it much brighter.  Is the only downside a dimmer light?  Will it last just as long? 

My main concern is putting the time and effort into constructing this thing, and then having the LED's fail for some reason I don't understand, and having to rewire it all.  I've thought about buying a 12V supply and using a resistor, but would rather not unless there is a good reason to do so.

Thanks in advance for any input!

5 Replies

Downunder35m (author)2015-10-22

You need resistors otherwise the LED's won't survive.

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Thank you for the feedback, but can you expand on why they won't survive?

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Google and Wikipedia will give you plenty of stuff to read on LED's and how to wire them - or why not to wire them in certain ways.

To put it simple: No LED is identical,
No LED is able to limit the current going through it,
No LED should be used without a current limiting resistor.

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caitlinsdad (author)brumachine822015-10-22

1. Look up any online "LED calculator" to input the components you have and power source 9 or 12 v for it to design a circuit for you.

2. Think of the resistor as the "fuse" in a circuit to to throttle a safe amount of current so you don't blow out your LEDs. Voltage spike can come from when you plug in the adapter or if one LED shorts out. Resistors are cheap insurance for your build.

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nazmo76 (author)2015-10-22

I don't have an answer but here is my thought:

All your LEDs are different, even though theroically they have all the same properties,

in your circuit some will receive more than the maximum forward voltage and some less than it

(a while ago I built a lamp like that for growing peppers but it didn't last because some leds were diying) so you are better using some resistors, though I don't know how it can be achieved pratically...

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