# how do I determine volts for my leds?



## Sickie Ickie (Jun 20, 2006)

Okay, this is kind of backward from the normal question of how to hook up leds.

I have some red leds I salvaged from a toy. Problem is I have no idea how many volts they take to light up. I've killed leds before by making the wrong guess. Any ideas?

Obviously they are not written on the leds. Am I safe in attaching 2 of these in series and then attaching a 1.5 to start- or would that blow them out?

kind of an impossible question, I know- but if anyone can help me out I'd appreciate it.


----------



## Moon Dog (Oct 3, 2006)

Can you measure the voltage from the toy you took them out of?

How many batteries did the toy take and in what configuration?


----------



## NecroBones (Feb 10, 2006)

That's probably safe to start. Most normal red LEDs (not the ultrabrights, which have a higher voltage requirement) have a forward voltage of about 1.7 or so. You can usually safely put 2V through most red, green, and amber standard LEDs.

If you try two in series and they don't light, you know you're under the voltage requirement. At 1.5 volts (a single AA battery for instance), you may not be able to get just one LED to start up. Testing with small batteries is often a good idea anyway since if you over-do the voltage, they don't supply enough current to explode the LED in your fingers.


----------



## Sickie Ickie (Jun 20, 2006)

Moon Dog said:


> Can you measure the voltage from the toy you took them out of?
> 
> How many batteries did the toy take and in what configuration?


The toy is long gone and the simple answer to both questions is I don't know.


----------



## Moon Dog (Oct 3, 2006)

Since the batteries most toys take are rated at 1.5V, like NecroBones mentioned, I'd start there.

Granted the toy most likely had a small power supply inside, I'd still start there.


----------



## Sickie Ickie (Jun 20, 2006)

Well, the leds are actually from a small gopher boppin' game that had many leds and many batteries. I'll try the two inseries and a 1.5 tonight (AA) and see what happens. Thanks guys!


----------



## DarkShadows (Feb 6, 2006)

Most leds are around 2 - 3 volts with out resistors. To work with 12 volts, pick yourself up a 470 ohm resistor for each led


----------



## DeathTouch (Sep 6, 2005)

You could probably get away with putting a 330 ohm in line witha 9v batter.


----------



## Otaku (Dec 3, 2004)

I usually use a 150 ohm resistor with 2 or 3 LEDs in series on 9 volts.


----------



## gmacted (Jan 18, 2006)

Standard LEDs have a typical forward voltage drop of ~0.7V and a forward current of 20 mA.

To connect one led and a 9V battery, you would need the following:

V = I/R --> R = V/I

R = (9V - 0.7V)/0.02 = 415 ohms (390 ohm 1/4 watt is a standard resistor and would work just fine)

To connect two leds and a 9V battery, you would need the following:

R = (9V - 0.7 - 0.7)/0.02 = 380 ohms (390 ohm 1/4 watt is a standard resistor and would work just fine)

The more LEDs you add, the smaller the ohm value you will need. You need to keep the forward current as close to 20 mA as possible.


Ultrabright LEDs usually have a larger forward voltage ( ~2-3V) and a forward current of 20 mA (current usually doesn't change) and will require a smaller value resistor.

Hope this helps.


----------



## Sickie Ickie (Jun 20, 2006)

Thanks guys. My head spins when I see 

"V = I/R --> R = V/I

R = (9V - 0.7V)/0.02 = 415 ohms (390 ohm 1/4 watt is a standard resistor and would work just fine)"

LOL I did the old trial and error method. Seems 2 AA will work the 2 red leds just fine. Thanks for all the help guys!


----------



## gmacted (Jan 18, 2006)

Sickie Ickie said:


> Thanks guys. My head spins when I see
> 
> "V = I/R --> R = V/I
> 
> ...


It's really just simple math.


----------



## Sickie Ickie (Jun 20, 2006)

Simple math for an educated brain. Note that I didn't get my double masters in Math for a reason.


----------



## gmacted (Jan 18, 2006)

Sickie Ickie said:


> Simple math for an educated brain. Note that I didn't get my double masters in Math for a reason.


I would agree that the concepts behind the formula can be considered complicated, but plugging in the numbers can easily be done with a calculator. That's what I meant by simple math. I meant no disrespect.


----------



## Sickie Ickie (Jun 20, 2006)

Miscommunication gmacted. No disrespect thought.


----------



## mizu5588 (Oct 10, 2007)

Here is what you need. Just plug your specs in and it gives you your schematic.

http://led.linear1.org/led.wiz


----------



## mizu5588 (Oct 10, 2007)

Of course source voltage is your battery that you plan on using.


----------



## Sickie Ickie (Jun 20, 2006)

That works great if you have a choice on what type of led to use, but if you already have the leds and they are not marked, this gives no help.

Besides, I already solved my delima. Thanks guys.


----------



## UnderMan (Aug 23, 2007)

The best way to determine the voltage is to wire a 100 ohm resistor in series to it and measure the voltage across the LED. The LED will use whatever voltage level it needs to glow and the resistor will take the rest.


----------

