Is it safe to put 2 LEDs in series as indicator for a Nerd60 board

[Owntrolf]

Im thinking of like hotgluing 2 leds under my clear zealio stabs and wiring them in series to light up the stabilizers to show that my caps lock is on. Is it even safe to do because usually its one LED only right?

[bmmcwhirt]

If you have the appropriate resistor for the power draw of the two LED in series yes. They can even be in parallel if done correctly.

This simple guide should help.

http://www.ledsupply.com/blog/wiring-leds-correctly-series-parallel-circuits-explained/

There are also numerous calculators out there for calculating what resistor to use based on the specs of the diode/LED, just google 'led current limiting resistor calculator'

[Owntrolf]

So I can wire it in parallel without using resistors then right?

[vvp]

No you cannot wire it in parallel without resistors. LEDs always need a resistor when powered from a constant voltage power source.

The only case when you can wire LEDs without resistor is when you used a special LED driver chip (a constant current source) to power the LEDs.

[bmmcwhirt]

VVP is correct. When LED are wired in parallel the resistor value will be different than when they are wired in series but is still necessary. That value is determined by a formula using the current draw, forward and reverse voltage of each diode.

http://ledcalc.com/

[bmmcwhirt]

Let's make this a little more clear with an actual example.

We'll use this LED for our example:  http://www.mouser.com/ds/2/216/WP710A10QBC-D-68125.pdf

Vs = 5V (this is the power voltage, I'm using 5V as thats typical for USB)
Im = 20mA (you can calculate this but mouser provides this or you can see this is the test current in the data sheet)
Vf = 3.3V (forward voltage which will be our voltage drop)

So if I go here ( http://ledcalc.com/#calc ) and select parallel I use these values
Supply Voltage: 5
Voltage Drop: 3.3
Desired Current: 20
# LED: 2

This tells me I want a 47ohm resistor

In series that value changes.
Supply Voltage: 5
Voltage Drop: 1.65 (3.3/2)
Desired Current: 20
# LED: 2

This says I need a 100ohm resistor.

Hope that helps.

[unoab]

In series that value changes.
Supply Voltage: 5
Voltage Drop: 1.65 (3.3/2)
Desired Current: 20
# LED: 2

This says I need a 100ohm resistor.

This is wrong, voltage drop in series for these LEDs is 6.6v (3.3+3.3), so no value of resistor will allow you to achieve 20mA of current off of a 5v supply.  Based off the datasheet, you could maybe get them to draw ~1-2mA at 2.5v being supplied to each LED with no resistor for a very dim light (~5-10% the luminous intensity expected if driven at ~20mA).

[bmmcwhirt]

In series that value changes.
Supply Voltage: 5
Voltage Drop: 1.65 (3.3/2)
Desired Current: 20
# LED: 2

This says I need a 100ohm resistor.

This is wrong, voltage drop in series for these LEDs is 6.6v (3.3+3.3), so no value of resistor will allow you to achieve 20mA of current off of a 5v supply.  Based off the datasheet, you could maybe get them to draw ~1-2mA at 2.5v being supplied to each LED with no resistor for a very dim light (~5-10% the luminous intensity expected if driven at ~20mA).

Yes, I wasn't going to go into why this calc does it the way it does but it calculates a slightly higher resistor value than needed.
If you use this for series it's more accurate:  http://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor
Use the Iforward of just one LED as you get a slightly higher resistor value but you can put in the total If. Keep in mind that 2 5V resistors in series with a 5V supply is a bad idea and parallel is the better option. Even 3.3V LED pull more than the 5V in series but they will still work just not good on the LED and you wont get maximum brightness.

I was using the 5V LED just as an example and it was a bad one.

Owntrolf, stick with parallel as your going to have fewer issues to deal with in this case as long as all the LED in parallel are identical on that branch you only need to have the Current and Voltage drop of one LED and use the first ledcalc with parallel settings. Also do not be tempted to use one resistor for both LED when in parallel your using 1 resistor per LED.

[bpiphany]

Just to be clear, if no one mentioned it. You should never connect LEDs in parallel without one resistor per strand. Things are unlikely (not improbable) to go wrong with low power LEDs, but read up on thermal runaway. In short, if one LED starts getting hot it may suck up more and more of the total current and finally burn, then your second LED immediately gets handed all the current and meets the same destiny as the first..

[bmmcwhirt]

Just to be clear, if no one mentioned it. You should never connect LEDs in parallel without one resistor per strand. Things are unlikely (not improbable) to go wrong with low power LEDs, but read up on thermal runaway. In short, if one LED starts getting hot it may suck up more and more of the total current and finally burn, then your second LED immediately gets handed all the current and meets the same destiny as the first..

Yea not sure we exactly covered that but always always one resister per led no less!

Thanks for pointing that out bpiphany.

Just to explain what he means, no tow LED are exactly alike and like all electronic components they have a +/- tolerance. If one LED is drawing more current than the other it will heat up which can cause a variety of bad things, the worst being a short that could cause damage to other components. All the calculators I linked earlier should be generating the one resister per led. If not I apologies and you should not use that, but I'm pretty sure they do.

Again, thanks bpiphany.