Help! Why don't these LED's work!

Roady

Roady

Roady

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Hi guys, I have a very poor understanding of electronic principles but why don't these work?!

LEDsplice.jpg


I spliced an LED onto another cable with a LED to try and drive 2 LED's for HDD activity lights from my mobo. Only one of them lights up! Fixable (change to 'lower power requirement' LED's) or give up what I'm trying to do?! :confused::eek::(
 
Are you using resistors to control the current through the LEDs? If not, and they aren't 5v or 12v type LEDs then you are likely to blow them.

And yes they have to be wired correctly as mentioned.
 
No resistors, they're these so listed as 3.3v

Nowhere could I found the actual voltage outputted by motherboards front panel LED headers, very few list it, the ones which do list it as 2.2v, 5v and 3v!

They came pre assembled, I assumed they'd be the same so matched white to white, blue to blue!
 
Just double check you have the cathode and anode legs the right way around. positive to the long leg, ground to the short leg. If you don't have the different size legs to check the anode (+) will be the smaller part that angles down inside the led.
 
Ah so the resistors will be in the LED package itself or on the lead somewhere as those are designed to run off the front panel connector. If not then that would be a bad design.

All the LEDs are the same product but you've hacked into them to join them?

So if you've not rewired the LEDs directly and just joined their wires blue to blue and white to white there should be nothing stopping them working. Unless there is no resisstor and Lamptron assume that all front panel headers are below the Vf of the LED, which seems unlikely.
 
cypto said:
Just double check you have the cathode and anode legs the right way around. positive to the long leg, ground to the short leg. If you don't have the different size legs to check the anode (+) will be the smaller part that angles down inside the led.
Click to expand...

Thanks cypto, will check that later!

Tealc said:
All the LEDs are the same product but you've hacked into them to join them?

So if you've not rewired the LEDs directly and just joined their wires blue to blue and white to white there should be nothing stopping them working. Unless there is no resisstor and Lamptron assume that all front panel headers are below the Vf of the LED, which seems unlikely.
Click to expand...

Yup just hacked into them to splice them together, soldered and then heatshrink... Was quite impressed with my work until it came to test them lol!

I can't see any evidence of a resistor on the cables or the LED side, is there some way to determine the voltage of the LED's so I can buy some resistors? Or easier to start again and buy LED's & resistors to suit?
 
If the resistor is built in it'll appear as a tiny black square inside the LED package.

A bit like this..
SSL-LX5093ID-5VSSL-LX5093HD-5VSSL-LX5093HD_zps647baa5e.jpg


A multimeter (in diode mode) across the LED will show you it's forward voltage but I'm unsure how well this would work with a LED with resistor in the package as that will work with the meter's internal resistance and modify the value. Adding a 330 Ohm resistor to a 1.9v Green LED added only 0.1v to the Vf so perhaps not a big issue.

You can pick up LEDs for around 5-10p each, resistors are around 1p each (when buying 100). You will need to determine the voltage of the source you are connecting to so you can calculate the correct resistor.
 
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Thanks Tony, pretty sure my LED's don't have those, I'll 'reverse engineer' one of them this weekend and see if I can spot a resistor hiding somewhere!

will connect my multimeter to my mobo header to see what I'm working with, I'll be very surprised if it's not a standard 3.3v or such?! So I probably need to look at a pair of 1.5v LED's with resistors? Any idea what rating I'd need?
 
Blue LEDs have typically a 3-3.5v forward voltage. A typical ultra/super/hyper bright 3mm or 5mm will have a maximum current rating of 30mA. I usually aim for 20mA with my LED lighting projects.

I've never measured my front panel connections but I'd expect they might be 5v. 3.3v isn't really enough for Blue or White LEDs. I don't think I've measured a blue LED either as I've not used one in ages.

So if we take a Blue LED with Vf of 3.5v you need to drop 1.5v across the resistor at 20mA. That's 75 Ohms. If the Vf was 3v you'd need a 100 Ohm resistor. V=I x R. Or rather R = VS - Vf / Current in Amperes.

If you need any help, or parts let me know Haydn.
 
As always Tony, many thanks for your help and for sharing your wealth of knowledge!

I just about follow what you're saying so I'll be putting it to the test this weekend! Wish me luck! ;)
 
Only just getting back to this (too many other things to do! :()

Measured the voltage as 1.55v which doesn't sound right... Not sure how I measure amperage with the multimeter I have either!

Although I think that would explain why only 1 of the 3v LED's lit up?
 
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