# Help: LED Circuit Diagram

#### Oxley

Joined Sep 10, 2017
4
Drafted this circuit diagram, any notes you have on this before I attempt to build it would really be appreciated.
The end game is to be able to run, 13x Red LED's 5mm 2-2.2 Volt 20mA, as well as, 14x Blue LED's 5mm 3.2-3.5 Volt 20 mA, from a wall socket. Thank you in advance for any advice you have to offer.

#### DickCappels

Joined Aug 21, 2008
7,722
The power supply a 40m constant current supply?

It might work but unless you know for sure, maybe you want to put some current sharing resistors. Otherwise, one string is likely to hog most of the current. I would be prepared to insert a resistor dropping 3 or 4 volts in series with each string to improve current sharing.

#### Alec_t

Joined Sep 17, 2013
12,206
The series resistors will need to be different for the two strings, because one string has an extra LED.
Is the LED driver a constant (adjustable) current type or a constant (adjustable) voltage type?

#### Oxley

Joined Sep 10, 2017
4
The power supply a 40m constant current supply?

It might work but unless you know for sure, maybe you want to put some current sharing resistors. Otherwise, one string is likely to hog most of the current. I would be prepared to insert a resistor dropping 3 or 4 volts in series with each string to improve current sharing.
It is a constant current supply - https://www.1000bulbs.com/product/89633/LEDF-TC1120035015C.html

#### Oxley

Joined Sep 10, 2017
4
The series resistors will need to be different for the two strings, because one string has an extra LED.
Is the LED driver a constant (adjustable) current type or a constant (adjustable) voltage type?
It is a constant current supply, https://www.1000bulbs.com/product/89633/LEDF-TC1120035015C.html,
If i added a 110 ohm resistor to the shorter parallel would it even out the power demand? Or would it be easier to add a redundant red led to the shorter strand so that the demand is even?

#### DickCappels

Joined Aug 21, 2008
7,722
I don't think you should connect a 350 ma constant current source to two parallel strings is 20 ma LEDs because they would not last long.

That power supply will increase its output voltage until the LEDs draw 350 ma.

You need a lower current power supply or you would need to drop more voltage across resistors.

#### Alec_t

Joined Sep 17, 2013
12,206
I agree, that power supply is insuitable.
Personally, I'd probably use a readily-sourced 12VDC 1A fixed voltage supply. The LEDs could be arranged in two groups, each group consisting for example of four parallel strings, three of the strings having in series 2 reds, 2 blues and a resistor; the fourth string having in series 1 red, 1 blue and a resistor.

#### Oxley

Joined Sep 10, 2017
4
I agree, that power supply is insuitable.
Personally, I'd probably use a readily-sourced 12VDC 1A fixed voltage supply. The LEDs could be arranged in two groups, each group consisting for example of four parallel strings, three of the strings having in series 2 reds, 2 blues and a resistor; the fourth string having in series 1 red, 1 blue and a resistor.
So this is more along the lines of what I should use?
https://www.1000bulbs.com/product/100980/LEDF-ACA12V12H10M.html
really appreciate your all the help.

#### WBahn

Joined Mar 31, 2012
26,398
What is that block box that is somehow producing 39.9 V on Line 1 and 37.7 V on Line 2?

Unless it is a circuit that produces two, independent 20 mA currents, you have a problem.

You don't know what the individual forward voltage drops are going to be. Based on the limits given, they could be anywhere between

Line 1: 36.2 V to 39.9 V
Line 2: 34.4 V to 37.7 V

So it's possible that Line 1 will have less voltage on it than Line 2.

If you just connect them together (if that black box is just a node tying them together) then one of the lines (probably Line 2) is going to hog all of the current that the driver can deliver, at least until it fails.

If you don't have separate current sources (even if your driver is a constant current source, you can't assume that it will split evenly, because it won't), then you need appropriate current limiting resistors.

#### DickCappels

Joined Aug 21, 2008
7,722
So this is more along the lines of what I should use?
https://www.1000bulbs.com/product/100980/LEDF-ACA12V12H10M.html
really appreciate your all the help.
That looks like a good choice for your application provided that you can re-organise your strings so each needs less than 12 volts including a volt or two for the current limiting resistors.

For example 7 strings with with an average voltage of 10.5 volts, running 20 ma through each string. That should give you something one to two volts for each current limiting resistors, but you would need to actually design the strings to know for sure.