Hi all, I'm hoping you can help me with a project I'm in the middle of. Hopefully this is the correct place to post. I tried searching the forums and found some relevant info but I still don't understand exactly why this isn't working.

I'm trying to light a Vf 3.00 white Osram LED mounted on a starboard with solder pads I've wired to a couple AA batteries. I'd like to use this to light a Rose encapsulated in Epoxy (for my wife from her Grandmothers funeral). The datasheet for this LED lists output at 350mA and 700mA.

I've previously built a custom LED fixture for my aquarium using fixed current drivers and detailed instructions so I'm not completely new to how to wire these up but I don't have any type of background in electronics to please bear with me and my ignorance.

I'm using 2x AA batteries in series. They both read 1.59v or thereabouts. My multimeter is not the best.
I can light a blue LED leftover from my aquarium fixture with these batteries no problem. I don't remember if the blue led is actually 3v or if it's less.

When the whites wouldn't light I thought maybe I needed more than 3 volts so I added a third AA to the series. Still nothing. I have three of these LED's so I tried another one thinking maybe I had a dud. The 2nd one wouldn't light either.
Shouldn't 4.5v light a 3V led??

I wondered if my connections were wrong so I measured for continuity to check my soldered connections and all looks good. I have a jumper wire to complete the circuit and if I disconnect it I loose continuity.

I have some resistors I was planning to use after making sure the LED's worked for me but I'm not sure which to use for the best battery life, etc. Any help on that would be great as well.

Thanks folks, appreciate any help you can offer since I'm literally "in the dark" on this one.
Greg

 

Welcome to AAC!

I'm using 2x AA batteries in series. They both read 1.59v or thereabouts.

What is the voltage of the series batteries while connected to the white LED? Do you have a datasheet or part number for the LEDs? AA batteries aren't going to last long with a 350mA load.

I can light a blue LED leftover from my aquarium fixture with these batteries no problem. I don't remember if the blue led is actually 3v or if it's less.

White LEDs these days are usually blue LEDs with a phosphor to make white light. My low power OSRAM white LEDs specify a forward voltage of 3-4.1V. They light at lower voltages, but they're dim.

When the whites wouldn't light I thought maybe I needed more than 3 volts so I added a third AA to the series. Still nothing. I have three of these LED's so I tried another one thinking maybe I had a dud. The 2nd one wouldn't light either.
Shouldn't 4.5v light a 3V led??

Have you done a diode check? Your cheap DVM might use high enough of a voltage to turn on white LEDs. White LEDs have a low reverse breakdown voltage and many have another LED antiparallel to the white.

 

You didn't mention if you double checked the polarity.

Also please post the data sheet.

 

Shouldn't 4.5v light a 3V led??

It should, but probably not for long.

 

What is the voltage of the series batteries while connected to the white LED? Do you have a datasheet or part number for the LEDs? AA batteries aren't going to last long with a 350mA load.

Thanks for helping me out! I'll check the voltage while connected tonight. LED info below:

Manufacturer Product Number	LST1-01F06-4070-00
Description	LED COB OSLON WHT STARBRD 4000K

I kind of expected the batteries to die quickly but I thought a resistor would help with that. Don't need anywhere near the full output of these LED's.

Have you done a diode check? Your cheap DVM might use high enough of a voltage to turn on white LEDs. White LEDs have a low reverse breakdown voltage and many have another LED antiparallel to the white

I'll try the diode check. Didn't realize this was a thing. I think my DVM uses a 9V so fingers crossed.
Thanks so much!! I'll report back.

 

You didn't mention if you double checked the polarity.

Also please post the data sheet.

If by checking the polarity you mean switching the wires back and forth a few times to see if anything happens...lol....then yes. I did double check the first time I connected to the batteries. I'll try to attach the data sheet.
Thanks!

 

Also there are four pads on that starboard so it is possible to use the wrong two.

 

Thanks for helping me out! I'll check the voltage while connected tonight. LED info below:

Manufacturer Product Number	LST1-01F06-4070-00
Description	LED COB OSLON WHT STARBRD 4000K

I kind of expected the batteries to die quickly but I thought a resistor would help with that. Don't need anywhere near the full output of these LED's.



I'll try the diode check. Didn't realize this was a thing. I think my DVM uses a 9V so fingers crossed.
Thanks so much!! I'll report back.

The diode range of a multimeter puts a current through the diode and reads the voltage. If it supplied 9V it would damage the diodes being measured.

That said, it may not be able to read as high as 3V on an LED. I tried mine ant it lit several white LEDs dimly, but gave no voltage reading, which seems odd to me.

 

Also there are four pads on that starboard so it is possible to use the wrong two.

I'm leaning towards this as my issue. I used the positive and negative on the right side for the battery connections and the left side for the jumper. I wonder if direction matters?

 

You only need to use 2 of the pads, if you jump the other 2 you will short out the battery.

 

side for the jumper.

What jumper? The two + terminals and two - terminals are most likely connected together and both to the LED. There is no jumper involved. If you put a jumper between a + an - that explains quite well why they do not light!

Show us a picture of your wiring.

 

Uh oh...I assumed from my last project that I'd need to connect the two unused pads to complete the circuit. The instructions for the last LED in a circuit called for one of these:

 

I'll disconnect and try tonight.
Thanks again for the help. You're all invaluable resources that deserve serious props!

 

Remove the jumper.
Recheck the polarity.
Reconnect the batteries. (only 2)

 

Yep, I'm a moron. Cut the jumper. LED bright as can be.
Thanks all, really appreciate it.
Any suggestions for a resistor or should I just base on the brightness I need?

 

With the jumper shorting the battery then its max output current is now lower than a new battery. Without a series resistor to limit the current then a new battery might burn out the LED. With a new 3V battery, try 10 ohms, 5.6 ohms, 3.3 ohms and do not use less than 1 ohm.

 

I have some resistors I was planning to use after making sure the LED's worked for me...

LEDs require series current-limiting resistors (or a constant current supply of the appropriate current). "Testing" LEDs without one almost ensures they fail.

 

Any suggestions for a resistor or should I just base on the brightness I need?

We have no idea how bright they are, what the specs are, or what brightness you want. So yeah, you should just find a resistance that gives the brightness you want.

 

The max continuous current for the LED (when its metal star is cooled properly) is 700mA!

 

So yeah, you should just find a resistance that gives the brightness you want.

Perhaps explaining how to calculate LED resistor values would be more helpful than "use whatever works."