LED connection in series

[Delludom]

Can I connect 3mm LEDs and 5mm LEDs in a series connection? Thanks in advance for your help

[Zero999]

Can I connect 3mm LEDs and 5mm LEDs in a series connection? Thanks in advance for your help

3mm and 5mm are just different LED packages. It's what's inside, which is important.

What's the power supply voltage?

What colour are the LEDs? Do you know their forward voltages?

If the power supply voltage is higher than the sum of the LED forward voltages, then it's fine to connect them in series and the forward current though all of the LEDs will be the same.

[Delludom]

Thanks..
I'm giving 220v
I need to connect 33 LEDs of 5mm and 2 LEDs of 3mm.
This is the information I have...  I am making this for Christmas thank you for your reply

[ogden]

Thanks..
I'm giving 220v
I need to connect 33 LEDs of 5mm and 2 LEDs of 3mm.
This is the information I have...  I am making this for Christmas thank you for your reply

Best gift for your parents will be you are being still alive. To everybody asking elementary advice regarding 220V build my advice is: don't. Stay away from mains until you are knowledgeable enough. You anyway have not enough time to 1) learn everything needed regarding build and safety 2) do actual build.

[james_s]

Just use a power supply. Those "wall wart" types are widely available and inexpensive. They provide a nice safe isolated output from whatever your local mains voltage is.

[Zero999]

Just use a power supply. Those "wall wart" types are widely available and inexpensive. They provide a nice safe isolated output from whatever your local mains voltage is.

Good idea.

24V is a fairly standard voltage.

The LEDs can be connected as 5 stings of 6 and one string of 5, with suitable current limiting resistors.

[tooki]

Thanks..
I'm giving 220v
I need to connect 33 LEDs of 5mm and 2 LEDs of 3mm.
This is the information I have...  I am making this for Christmas thank you for your reply

Yyyyeahhhh…. don't even THINK about running those LEDs directly off mains.

Just use an off-the-shelf power supply down to 24V like Hero999 says.

[Zero999]

Also go to a proper distributor, to get a decent brand. Don't get the cheapest ebay junk you can find.

https://export.rsdelivers.com/product/xp-power/vel18us240-eu-ja/xp-power-18w-plug-in-power-supply-24v-dc-750ma/1217153

[james_s]

Or look at thrift stores, junk sales and other such sources, there are millions of orphaned power bricks from broken or obsolete equipment. I don't know that I've ever bought one new.

[mariush]

LEDs have various forward voltages depending on the color and chemistry used to create them.

You have here a list of forward voltages for various led colors: https://en.wikipedia.org/wiki/Light-emitting_diode#Colors_and_materials

So for example, most red leds will have an average forward voltage of 1.7v , most yellow and orange leds will have a forward voltage of around 2.1v, and white or blue leds require around 3v

You can chain multiple leds (put them in series) as long as the sum of their forward voltage doesn't go above the voltage of your power supply.  So for example, with a 12v power supply (a cheap wallwart adapter) you could have 12v / 1.7v = 7.05v , so basically 7 leds. With 3v leds you'd want to stick to maximum 4 leds.   
You'd probably want to chain only 6 leds though, because if you make a very long string, you may not have 12v near the end of the string due to losses in the thin wires you're going to use , so better to leave some room.
Now it's not enough to just put them in series, because leds don't behave like incandescent light bulbs. if you just connect them to the power supply, they'll burn up.
You have to limit the current going through the leds to a reasonable value, and the easiest way to do that is using a simple resistor.
For 3mm and 5mm, most leds have a recommended current of maximum 20mA (0.02A) but really for a xmas tree, you won't need more than 10mA

you can determine what resistor to put in series with your 6 leds using Ohm's law formula -  I'm going with 6 red leds and a 12v power supply but you can change the numbers... I'm going to limit them to 10mA because it's enough

the formula is like this  :  Voltage equals Current x Resistance or  V = IxR

In your case  voltage is the power supply voltage, minus the sum of the forward voltages of your leds. The current is the value you want to limit the leds at, which is 10mA (0.01A) in my example

So :   12v  - ( 6 leds x 1.7v)  = 0.01 A x Resistance  ... and from this > Resistance  =  (12v - 6x1.7) / 0.01  = (12v - 6x1.7) x 100 =  1.8 x 100 = 180 ohm 

This is actually a E24 value which means it's very easy to find, but if you had come up with a different number, you can round up or down to the nearest E24 or E96 value - slightly smaller resistor value means a bit more current through the leds, bigger value resistor means a bit less current so slightly less bright .

Now you ca make your tiny strings of a few leds + resistor and then you simply connect them to your two wires (positive and negative of your power supply). Easiest would be to cut a bit of insulation on each of those two wires and solder your tiny string of leds + resistor between the two wires and then use heatshrink or electrical tape to cover the point where you made the connections.
 

edit: also if you don't want to spend money on a power supply but you have a phone charger or usb power bank or anything with micro-usb connector, you can buy one of these boards which have contacts for wires so that you could connect wires from a set of batteries like 4AA batteries for example, but they also have a micro usb connector, so you could plug a phone charger or usb power bank in them...

here's a couple of sellers

1: https://www.ebay.com/itm/2A-DC-DC-Boost-Step-Up-Voltage-Converter-Micro-USB-2-24V-To-5-28V-Adj-Regulator/112931387130?hash=item1a4b3c36fa:g:h2kAAOSwHgday3gf:rk:1:pf:0
2: https://www.ebay.com/itm/DC-DC-Boost-Step-up-Regulator-Converter-USB-2-24V-to-3-3-3V-5V-9V-12V-24V-Module/322369192609?epid=2118019319&hash=item4b0eb382a1:g:9EoAAOSwpqRazhQN:rk:2:pf:0

So that can boost up to 28v but with just 5v at the input, it probably can't.
It can probably do 12 ..15v just fine, at up to around 1A ... and that would be just fine if you use a 5v 1..2A phone charger
Basically, if you use a 5v 1A phone charger, you have around 4.8w (because the tiny board isn't 100% efficient) at 12v , which means the maximum current would be 4.8w / 12v = 0.4A  .. so if your strings of leds use 10mA each, then you could have up to 40 such led strings on your long string of leds  ( so with my example of 6 red leds per tiny string, up to 240 leds in total)

[james_s]

It's pretty easy to determine the forward voltage, many multimeters have a diode check function that will do it, or you could hook up a single LED in series with a 1k or so resistor to a 9v battery and measure the voltage across the LED.

[Zero999]

It's pretty easy to determine the forward voltage, many multimeters have a diode check function that will do it, or you could hook up a single LED in series with a 1k or so resistor to a 9v battery and measure the voltage across the LED.

The meter diode check often only works up to 2V, so a 9V battery and 1k resistor is better. Watch out though, as the maximum reverse voltage rating for some LEDs is below 9V, although I've personally never destroyed an LED by subjecting it to 9V reverse bias, so it could be the manufacturers covering themselves.

[james_s]

My Fluke meter goes up higher than that, but it varies from one meter to another.

I wouldn't worry about damaging an ordinary LED with a 9V battery so long as the current limit is reasonably low. You only need to test a sample anyway, once you know the forward drop of that color you can assume the others of its type are about the same.