Electronics > Beginners

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

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bob21:

--- Quote from: gf on February 04, 2020, 08:32:19 pm ---Which current limiting resistor values did you use for each these measurements?
--- End quote ---

I'm sorry, but what do you mean?

If you mean what was limiting the current, I hooked each led string back up to it's original AA pack and measured using the newly purchased Uni-T clamp meter using the clamp. I checked each string individually as if i had never changed anything (ie. reconnected to each 2 x AA cell pack) and I also took readings with them all hooked up as per my rubbish diagram with the purple dot and 2 green dots where I have labeled them Lines 1-4 and Circuits A-B.

There was nothing else added to limit current. Or do you mean something else?


--- Quote from: gf on February 04, 2020, 08:32:19 pm ---What do you mean with "5 Wraps"? Do you mean 5 lines (of the particular lamp type) connected in parallel?

--- End quote ---

I mean wrapping the wire round and through the clamp as Brumby illustrated a few posts back.

gf:

--- Quote from: bob21 on February 04, 2020, 08:58:24 pm ---If you mean what was limiting the current, I hooked each led string back up to it's original AA pack

--- End quote ---

OK, using the original battery pack - w/o resistors.
What is the voltage across the LED string (or across the battery) under load? Still about 3V?


--- Quote ---I mean wrapping the wire round and through the clamp as Brumby illustrated a few posts back.

--- End quote ---

All clear.

I just noticed:

--- Quote ---I switched to '2A~' and with the entire circuit hooked up...

--- End quote ---

Why AC and not DC? (particulary since it is connected to the battery pack)

bob21:

--- Quote from: gf on February 04, 2020, 09:45:40 pm ---OK, using the original battery pack - w/o resistors.
What is the voltage across the LED string (or across the battery) under load? Still about 3V?
--- End quote ---

I didn't check this, I only checked the current using the clamp.


--- Quote from: gf on February 04, 2020, 09:45:40 pm ---Why AC and not DC? (particulary since it is connected to the battery pack)

--- End quote ---

That is he dial setting for this model meter... it also has the DC sign on it, but I don't know how to type it you just press the blue select button  to change to DC.


 


Ok, I have worked out some [possibly very incorrect, and stupid] numbers! :)

Please don't shoot me, I am still learning  :-+ I didn't even know what a resistor was/did 4 days ago, so bare with me :)

Let's start with the individual calculations of running each circuit in parallel.

Line 1 - 10 LEDs = Total Consumption off a 2 x AA pack putting out 2.92V = 41mA read from Uni-T clamp

Resistor needed: 6V-3V = 0.041 x R ==> R = 3V / 0.041 = 73 Ohms
Wattage of resistor needed: 0.041 x 0.041 x 73 = 0.122W - so an R82 (or R68?) 0.5W should be correct for this?

Line 2 - 10 LEDs = Total Consumption off a 2 x AA pack putting out 2.92V = 36mA read from Uni-T clamp

Resistor needed: 6V-3V = 0.036 x R ==> R = 3V / 0.036 = 83 Ohms
Wattage of resistor needed: 0.036 x 0.036 x 83 = 0.107W - so an R82? 0.5W should be correct for this?

Line 3 - 6 LEDs = Total Consumption off a 2 x AA pack putting out 2.92V = 37mA read from Uni-T clamp

Resistor needed: 6V-3V = 0.037 x R ==> R = 3V / 0.037 = 81 Ohms
Wattage of resistor needed: 0.037 x 0.037 x 81 = 0.110W - so an R82 0.5W should be correct for this?

Line 4 - 15 LEDs = Total Consumption off a 2 x AA pack putting out 2.92V = 41mA read from Uni-T clamp

Resistor needed: 6V-3V = 0.041 x R ==> R = 3V / 0.041 = 73 Ohm
Wattage of resistor needed: 0.041 x 0.041 x 73 = 0.122W - so an R82 (or R68?) 0.5W should be correct for this?


BUT: We want 1 + 2 in Series and 3 and 4 in Series:

1 + 2 = Circuit A
3 + 4 = Circuit B

Circuit A

Total consumption = 77mA

Resistor needed: 6V - 3V = 0.077 X R ==> R =3V / 0.077 = 38.96 Ohm (So 39 Ohm)
Wattage of resistor needed: 0.077 x 0.077 x 39 = 0.231W - So an R39 0.5W should be sufficient for Circuit A

Circuit B

Total consumption = 78mA

Resistor needed: 6V - 3V = 0.078 X R ==> R =3V / 0.078 = 38.46 Ohm (So 39 Ohm)
Wattage of resistor needed: 0.078 x 0.078 x 39 = 0.237W - So an R39 0.5W should be sufficient for Circuit B

So, going back to my rubbish diagram, I need to add 2 x R39 0.5W resistors where the green dots are, and they shouldn't get hot because they aren't under much load?


 


Am I correct in thinking that we don't need to limit the PSU? Each individual line of LEDs is run in series, so a failure would take out the whole line? Would then not the entire 6V be dumped on the remaining 2 lines.... or am I getting this totally wrong?

Edit: Oops, I typoed one of the resistor values as R9 instead of R39, fixed.

gf:

--- Quote from: bob21 on February 04, 2020, 10:00:33 pm ---BUT: We want 1 + 2 in Series and 3 and 4 in Series

--- End quote ---

Do you really mean a true series connection, or do you just mean daisy-chained behind each other, but electrically still connected in parallel?

When two strings are connected in series then the current through both strings will be the same, and the voltage drops will add up. This mean, you would need to combine strings 1+4 and strings 2+3 each, since these are the pairs with (almost) equal current consumption. If one string has as voltage drop of say 3V, then two strings in series will have a voltage drop of 6V. When the PSU voltage is 6V as well, this would leave no headroom for an additional voltage drop across a current limiting resistor :-(

That's one reason why I asked
"What is the voltage across the LED string (or across the battery) under load? Still about 3V?"

If the actual voltage drop across the LED strings happens to be less than 3V, then we gain some headroom for the resistor in case of a series connection. And generally it is of course better to know the actual voltage drop instead of guessing "roughly 3V" for the calculation of the current limiting resistor.

Btw, I'm still also wondering whether your LED bulbs (or their sockes) possibly have a tiny current limiting resistor built-in, or whether they include only the LED chip and nothing else? Can you see anything in the bulbs which looks like a SMD resistor?

bob21:
I did have a quick look for individual resistors on each LED a few days back and didn’t notice anything. It’s late here now, but tomorrow I will check:

1. Each LED in each line for individual resistors
2. The voltage drop, ie hook up each individual string to its AA pack and take a voltage reading before and after turning on and post back. How would be best to test this? Probe at start and end of string? Each string only has 2 wires. If you removed 1 LED, the entire string would fail.

I’m not sure what you mean by daisy chained. If you mean line 1 pos to psu pos, then line 1 neg to line 2 pos, then line 2 neg to psu neg, then yes. This is how they are connected. Same for 3 and 4.

If I need to change the config of the strings so that the numbers are better, I can do this. My end goal is safety first and longevity of the LEDs second.

Watch this space.

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