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Hooking up 4 x 3v LED string lights to a 6v PSU

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Brumby:

--- Quote from: bob21 on February 02, 2020, 11:51:50 am ---So, let me just try to summarize what I think you've said. I have drawn a crappy diagram too.

I can use the 6V PSU with 2 x 3V LED lines in series. Then if I were to get a fourth set (I actually already have one) and connect to the  3rd in series, I can then connect this in parallel with 1 and 2?


 


--- End quote ---

Yes, that's the idea.

But even then, there is a weakness in the setup - and mariush has gone to a lot of trouble writing it up.

In short, it's the current that needs to be controlled for LEDs and series resistance (whether internal resistance of the source of power, resistance of the wiring or a separate resistor) will play an important part in that.  Too little resistance and the LEDs will draw too much current - and die early.

Take a moment and consider this chart of current vs voltage for an LED.  Don't worry too much about the actual volts on this chart, it will be for a particular type of LED (not yours) - but the same general trend will be found on any LED (including yours!).

This LED would be sold as operating at 20mA with 2.7V applied:

Notice what happens to the current when the voltage creeps up by just 0.1V ... the current is 2.5 times the rated current.  With the voltage up by just 0.3V, it is 6.5 times higher!!  Letting 130mA run through an LED that is meant to handle 20mA is going to kill it a lot quicker.

But, yeah, it will be bright.

Brumby:
You will also notice from that chart that when you drop the voltage by 0.2V from the nominal 2.7V, the current drops down to practically zero.  When that happens, you won't get any visible light output.  (Well, that isn't strictly true, but it won't be particularly useful and you might have to darken the room to see it.)

LEDs be crazy.

bob21:
Wow, thanks so much for such a time consuming write up Mariush, and thank you to you also Brumby for all the info. I totally appreciate the time people are taking to help me with this.

I am still re-reading everything and trying to get my head round it, but, I have added a 4th line to the circuit like this:


 


So if I understand correctly, each parallel circuit (let's call them A and B) has almost the same amount of LEDs.

If I understand correctly, a 300mA 6V PSU should be dishing 150mA to A and B. 150mA / 20 = 7.5mA, so each LED is receiving 7.5mA - or am I totally off target here?

I think I am definitely going to grab some resistors to make this safer, so I have also marked the diagram with coloured dots.

Would I just need one resistor where the purple dot is? Or would I need 2, where the green dots are? I am thinking the answer to this is that both A + B need one?

Would the aforementioned resistors from eBay / Rapid still be suitable?

I'm really sorry if it seems like I'm not getting this. But I'd like to say again how much I appreciate you putting up with me  ;D

Update:

Ok, I have re-read all of this and I think I understand I need a resistor where the green dots are, at the beginning of each series.

I have (lets's say) 20 white 3V LED's on each series chain (A and B), and they should get no more than about 20mA.

The PSU is 6V 300mA running 2 parallel circuits, so 150mA a circuit. So looks like I only have 7.5mA available to each LED with this PSU assuming all are working, so lets limit to that.

6V - 3V = 0.0075 (7.5mA) x R  ===> R = 3V / 0.0075 = 400 ohm

So I could use the 'common' 330 ohm resistors, place where each green dot is in the diagram and still be ok?

Brumby:

--- Quote from: bob21 on February 02, 2020, 01:52:22 pm ---If I understand correctly, a 300mA 6V PSU should be dishing 150mA to A and B.

--- End quote ---
No.

This is a very common misconception.  The current rating of a supply is the maximum current it is rated to provide.  The actual current drawn will be a function of the load.  If the load only takes 100mA (for example), then that is all the supply will dish out.  This is the same for any and all power sources that you would generally come across.**  Ohms Law describes this for the simple case of a resistive load.

** This is the "constant voltage" type.  There are supplies that operate on a "constant" current or current limiting basis - but we're not talking about these just here.


--- Quote ---Would I just need one resistor where the purple dot is? Or would I need 2, where the green dots are?

--- End quote ---
One where the purple dot is would be enough to provide basic protection - but one for each of the green dots would be considered a better option as it would allow independent "finding the operating voltage" for each arm.


--- Quote ---I am thinking the answer to this is that both A + B need one?

--- End quote ---
Need? - Not really - but that would be the "better" way, in my book.

gf:
Line 3 (6 LEDs) and line 4 (15 LEDs) in series is not a good idea, since the two lines likely require different drive currents (assuming the LEDs are all the same type).

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