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Backwards LEDs??

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ibbob50

New Member
Hi guys, it's my first post in here so take it easy on me! I dabble in electronics a bit, so I know a thing or two, but I'm more of a mechanical bike/car guy!!

Here's my issue:

I ordered a bunch of LEDs from Hong Kong. I have purchased a number of times before from the seller without issue... until today.

Upon opening the package I was greeted by my order. Then I went to test them (just for fun) with a 2.2v battery set up.

I touched the positive leg of the LED (longer one/anode) to the battery, and the negative leg of the LED (shorter/K/cathode) to the negative of the battery... and nothing.

So I flipped it around and bam! It worked!

Why are my LEDs backwards? I'm pissed because I know I'm going to have to wire them up remembering to do it backwards to make these things light up.

Here's what I bought:

50 PCs 1W 8mm 140° StrawHat ORANGE LED 180,000mcd@300mA - eBay (item 390113432112 end time Dec-01-09 19:48:13 PST)

Anyone? Anyone?
 

DirtyLude

Well-Known Member
I have no idea, but I've got a bunch of LEDs with the short wire as anode. They are all Chinese cheapies as well.
 

MikeMl

Well-Known Member
Most Helpful Member
Hi guys, it's my first post in here so take it easy on me! I dabble in electronics a bit, so I know a thing or two, but I'm more of a mechanical bike/car guy!!

Here's my issue:

I ordered a bunch of LEDs from Hong Kong. I have purchased a number of times before from the seller without issue... until today.

Upon opening the package I was greeted by my order. Then I went to test them (just for fun) with a 2.2v battery set up.

I touched the positive leg of the LED (longer one/anode) to the battery, and the negative leg of the LED (shorter/K/cathode) to the negative of the battery... and nothing.

So I flipped it around and bam! It worked!

Why are my LEDs backwards? I'm pissed because I know I'm going to have to wire them up remembering to do it backwards to make these things light up.

Here's what I bought:

50 PCs 1W 8mm 140° StrawHat ORANGE LED 180,000mcd@300mA - eBay (item 390113432112 end time Dec-01-09 19:48:13 PST)

Anyone? Anyone?

Where is your current limiting resistor?
 

trevor261

New Member
Sometimes these torch LED's have a Zener diode across the LED. I think it is there to maintain a (relatively) constant voltage across the LED and prevent damage to the LED from reverse bias. The manufacturer might have swapped the flat side and the long-lead side of the LED to represent the cathode and anode of the Zener rather than the LED.
 

ibbob50

New Member
Long story short, the LEDs are for creating an array that will function as a turn signal for a motorcycle. The are to be wired up in series in groups of 6. With a source voltage of 13.4v, 6 LEDs = 13.2v. The rest is scrubbed off with a resistor.

They are only to be turned on for at the very most 1 minute (blinking), so I hope heat isn't an issue. The LEDs are set in aluminum passenger pegs that are then filled with epoxy encapsulating compound. I hope that's enough of a heat sink.

Here's the explanation I got from the seller:

Yes, it is right. Please look at the diagram on
ebay, the shorter pin is positive. Because my 0.5W
and 1W 8mm, 10mm led use the same shape and lead
frame so we make the 1w led up backward.
Thanks

What I take from that is that they make them up backwards to differentiate between 0.5w and 1w LEDs.

Wow you spent a fortune, why not 13$ for 100 normal ones?
The LED normally need 3.3V btw and a resistor, not a 2V short.

Thanks Birdman...

I'm aware I need a resistor. They're in the mail too. The LED's typical forward voltage is 2.2v, not 3.3v (unless I've completely overlooked something. In which case, please show me.)

I suppose, "why not $13 for 100 normal ones?" is a valid question considering you have no clue what they're going to be used for.

I'm working on my second set of these LED passenger peg/blinkers, and I'm pulling out all the stops to make them the best they can possibly be. It was worth it to me to spend the money and end up with a unique finished project that safely and brightly signals to other drivers my intent to change lanes. (Yes, I know the stock signals work great too.)

If you would all like to see my first set of pegs in action, click this link (and stop laughing at my shoes):

YouTube - LED passenger pegs cbr600rr
 

picbits

Well-Known Member
I can't work out how 300ma @ 2.2v - 1 watt personally.

I've used Luxeon Star 1W LEDs and they need a shedload of heatsinking when being driven at their full 1W rating.
 

audioguru

Well-Known Member
Most Helpful Member
The LEDs are not 1W. They are 0.66W.
The LEDs are not 140 degrees. They are 100 degrees.
The LEDs are not 2.2V each. They are from maybe 2.0V to 2.4V and will either instantly burn out or not light because you do not have enough voltage across the current-limiting resistor to limit the current properly.
I don't think your battery charges at only 13.4V. Maybe from 13.8V to 14.4V.
 

ibbob50

New Member
The LEDs are not 1W. They are 0.66W.
The LEDs are not 140 degrees. They are 100 degrees.
The LEDs are not 2.2V each. They are from maybe 2.0V to 2.4V and will either instantly burn out or not light because you do not have enough voltage across the current-limiting resistor to limit the current properly.
I don't think your battery charges at only 13.4V. Maybe from 13.8V to 14.4V.

Good lord... I am not cut out for this forum!

You guys, I'm just going from the specs I see in front of me on the screen, given to me by the seller.

Cut me some slack. Just a little. If you read my first post, you'll note I'm handy, but not an electronics wiz. All I asked for was help on why my LEDs appeared to be built backwards.

My last set of pegs worked out fine... they lit up, and did not burn out instantly...

Obviously (we all knew this) the specs on the LEDs have been exaggerated, as most specs are on Chinese gear. I know they're not 1w Luxeon material... trust me.

Anyways, I got my answer on why they're constructed "backwards". Thanks for your help.
 

DirtyLude

Well-Known Member
People don't mean to be rude here, they just state things as they see them. It's a technical forum so people tend to point out technical details rather than be too wordy with their posts. Also, it pays for most of the people here to be detail oriented, many are engineers and to them the difference between a 1W LED and a 0.66W LED is a serious design difference.
 

ibbob50

New Member
People don't mean to be rude here, they just state things as they see them. It's a technical forum so people tend to point out technical details rather than be too wordy with their posts. Also, it pays for most of the people here to be detail oriented, many are engineers and to them the difference between a 1W LED and a 0.66W LED is a serious design difference.

Thanks man, understood.

I'm not an engineer. I'm a kid with a bike. Just trying to learn about this stuff as I go.
 
Last edited:

tcmtech

Banned
Most Helpful Member
As a kid with a bike just take some of the forums members to simply be grouchy old men past their prime, city folk who dont have the ability to actually do anything cool, people with OCD and no sense of humor, disgruntled office workers who hate where their life went, or just clueless goofs who dont know any better. For every member that wants more information there will be another who thinks you give out too much as is. :rolleyes:

However everyone else is Okay after that! So just say what you think you understand and shrug off all complaints and criticism that follows.:)
And if they get too wound up over what your doing have some fun with it and drive them over the edge then sit back and have a good hard laugh while imagining that person sitting in some cubicle in some office building having a complete meltdown over what you said! :D:D

Try and match the combined LED's voltages to be just under 12 volts being thats what your system voltage is rated at. Then factor the series dropping resistor to give the maximum allowable current at around what ever your peak charging voltage is which should be around 14.4 volts or so.
 

ibbob50

New Member
...Try and match the combined LED's voltages to be just under 12 volts being thats what your system voltage is rated at. Then factor the series dropping resistor to give the maximum allowable current at around what ever your peak charging voltage is which should be around 14.4 volts or so.

Thank you! Very useful information.
 

audioguru

Well-Known Member
Most Helpful Member
It is very confusing when you buy cheap Chinese junk that has spec's that does not make sense. They were even made with the leads backwards.

I think you need to measure the charging voltage of your bike then string 5 LEDs in series and in series with a current-limiting resistor that limits the current to about 200mA then measure the forward voltage of all your LEDs . Then calculate a suitable current-limiting resistor for each series string of 5 or maybe 6 LEDs.
 

ibbob50

New Member
I completely agree with you Audioguru. The reason they gave me for building them backwards was this:

They have a (supposedly) 0.5w LED that has the exact same dimensions/frame. So, to differentiate between their 0.5w and 1.0w LEDs, they built the 1.0w LEDs backwards.

I know, I know... cheapo Chinese stuff. I'm still excited though!

Thank you for the advice everyone. You don't know how much I appreciate all the useful info.

I've been planning this build using this online LED series calculator:

LED series parallel array wizard

Thank you again guys!
 

audioguru

Well-Known Member
Most Helpful Member
The LED design wizard does not know the range of forward voltage for your LEDs and does not know the range of charging voltage of your bike.

You need to measure these spec's and arrange the LEDs so that they all light without burning out.
 

ibbob50

New Member
The LED design wizard does not know the range of forward voltage for your LEDs and does not know the range of charging voltage of your bike.

You need to measure these spec's and arrange the LEDs so that they all light without burning out.

Will do. Thanks!
 
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