Wiring two LED's with a 5v power supply, what type/how many resistors do I need?

[cfrea]

Hi guys,

I am looking at getting two 5mm LED's. My power source is 5V.

In order to wire them, do they need to be in parallel or series in this case? Additionally, what type of resistor would I need to use in this application?

Here are some specs of the LED:

Reverse Voltage:5.0 VDC
Forward Voltage: 3.2min-3.6max

What I don't understand is the difference between the two and if I need to wire it via series or parallel

Thanks!

[mariush]

If you put LEDs in series the forward voltage adds up, so 6.2v is more than 5v, which makes it almost sure the LEDs won't light up.
So you either use a led driver to boost the voltage while limiting the current to your desired value (for best efficiency, but probably not worth it in your case), or you connect the LEDs in parallel, each with it own resistor to limit current.
Figure the current you want in Amps (0.01A = 10mA) and the you take an average forward voltage (let's say 3.4v in your case, but  keep in mind the forward voltage lowers as the temperature of the LED goes up) and now you can easily forward the resistor value from the elementary formula V = IxR 
So  V (input) - V (led) = I x R   which means  R  =  (Vin - Vled) / I  ,,, in our case  (5-3.4) / 0.01= 160 ohm  which is not a standard value so you could go with a 150 ohm (and allow more than 10mA to go through led) or with 180 ohm but let slightly less current go through led at least initially.
With 180 ohm , you'd have I = (5v - 3.4v) / 180 =  0.0088 A or 8.8 mA  but as the LED warms up the forward voltage could drop down to about 3.2v and then you'd have I = (5v-3.2v) / 180 = 0.01A or 10mA

One other thing worth knowing is the power wasted in the resistor, so that you'd know what resistors to use. Formula is P = IxIxR ... for our 180ohm and 0.01A numbers we have P = 0.01x0.01x180 = 0.018w or 18mW which means you can safely use cheap standard through hole 0.125w rated resistors , or even tiny cheap smd resistors (0603 and up) which typically are rated for 50mW and up.  Generally, you don't want to dissipate more than around 75% of a resistor's rating

This is also worth knowing when you want to figure if it's worth moving from cheap resistors to a led driver, to get better battery life. In our example with 10mA leds and 3.2v forward voltage and 180 ohm resistors,  we'd use  2 x 10mA x 3.2v = 64 mW on light, and 2 x 18mW = 36 mW on resistors as heat, so our circuit is only about 64% efficient.
A led driver IC could in theory raise this efficiency at around 90%, which means only around 5mW or so would be lost in the led driver circuit, which depending on what battery you choose, may extend the time the leds stays on.
Here's an example of such a chip, cheap and simple to use and still easy to solder by hand: http://uk.farnell.com/microchip/mic2287cyd5-tr/pwm-wled-driver-1-2mhz-sot23-5/dp/2509928
The datasheet has an example circuit in the first page, where they show how to use it to light up 3 white leds in series. The efficiency can go as high as 85%, compared to the 64% you get with resistors. Doesn't sound like much, but the energy savings can make a huge difference when the LEDs are more powerful, let's say 100mA or something like that.

It takes in up to 5.5v and can produce up to 34v with a switch current of 500mA , which means you can put in series as many leds as you wish as long as the sum of their forward voltages doesn't go over 34v. 

[cfrea]

Thank you,

I think I for the most part understand now lol.

Based on my calculation (correct me if I'm wrong) if my LEDs pull 20mA then i essentially need two 80ohm resistors correct?

Here are the two LED's Im looking at purchasing btw with the specs:


http://www.ebay.com/itm/232007964491
http://www.ebay.com/itm/222171347580

[cfrea]

BTw,  I guess an 80 ohm resistor is not common so I'd go with an 82 right

[mariush]

There's usually little brightness variance as you increase the current, the increase is not linear with the current.  At 18mA the led will probably be 99% as bright as at 20mA.

I would just use 100 ohm resistors, that will get you about 15-18 mA which should be very close to the maximum and very bright.

Also keep in mind those eBay leds are sometimes rejects, leds which fail some kind of automated measurements (amount of brightness for current, minimum or maximum forward voltages etc) so in such cases it would be smart to not use them to their maximum ratings if you want longer life. Probably not the case with such easy to manufacture leds, it's more common with 10w+ led assemblies but again, it's not uncommon.
 

[cfrea]

That's very helpful to know. Didn't realize these may be QC rejects. Do you have a recommendation on a shop where I could get more reliable/QC passed LEDs?

Thanks again for your help!

[mariush]

As you're in US, check out Digikey or Mouser.  There's also Newark, which is the US website of farnell.com, they're great but you'd have to filter out leds which are sold from their warehouses in Europe (you'd pay extra for shipping)

Digikey : 
indicators (usually small current, more directional light, like you found on ebay) : http://www.digikey.com/product-search/en/optoelectronics/led-indication-discrete/524729
lightning (higher current)  color :  http://www.digikey.com/product-search/en/optoelectronics/led-lighting-color/525607  or white : http://www.digikey.com/product-search/en/optoelectronics/led-lighting-white/525606

Don't get scared by the surface mount leds on first pages, they're the cheapest in volume so it's normal to see them. You can filter by lots of things, like see only through hole leds , by view angle (how wide the beam is), color, forward voltage, you can also enter quantity so that you get prices for buying only 10 or 25, not rolls of 3000 leds or just one.

Newark : http://www.newark.com/led-products  - they split the leds into high brightness (over 75mA) and standard (below 75mA) and then each of these are further split into white / single color / multiple colors