Author Topic: Hooking up 4 x 3v LED string lights to a 6v PSU  (Read 10314 times)

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Offline bob21Topic starter

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Hooking up 4 x 3v LED string lights to a 6v PSU
« on: February 01, 2020, 05:01:57 pm »
Hi,

This is a total n00b question, and one which I am sure has probably been answered but I am not exactly sure what it is that I am searching for, so searches turned up a lot of results that didn't seem to fit.

So here goes:

I have 3 x cheapo LED string lights. The kind you get from bargain stores that run on 2 x AA cells, so totaling ~3v DC per set. Like this: https://imgur.com/a/CrOQbYI

I want to hook all 3 of these up to a PSU I have here that is 9v DC. So the question is: can I just splice all 3 together and connect to the PSU? Or is this the worst idea ever?

My logical head says the load will just balance and each string of LEDs 'should' get its required ~3v. But then I wouldn't be posting this question if I wasn't totally sure it wouldn't catch fire to the house, cat and everything in the vicinity  :-//

Will this work? If not, what is the best way to go about this? If I need to grab some components and solder something, I can do this. But I wouldn't have a clue what I would need to buy and solder to what.

Any input would be very much appreciated

Edit: Changed title to reflect current aims.
« Last Edit: February 05, 2020, 09:44:57 am by bob21 »
 

Offline TerminalJack505

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #1 on: February 01, 2020, 06:46:45 pm »
Are there some components other than the switch in the base?  I can't tell from your picture.  It is likely that there is some type of boost circuit in there, such as a Joule thief.  How you proceed will depend on what type of circuit is being used to drive the LEDs.
 
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Online tunk

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #2 on: February 01, 2020, 07:00:43 pm »
Looks very much like a LED string I have and there's no electronics inside.
If it's 9Vdc +/-0.1V, then in principle it should work if you connect the
three strings in series, i.e. 3+3+3V. As a precaution you could add a low
value resistor in series, maybe 5-10ohm.
 
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Online Zero999

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #3 on: February 01, 2020, 07:43:35 pm »
Those LED strings rely on the internal resistance of the batteries and LEDs to limit the current.

What voltage does the 9V power supply actually measure, when you connect a meter to it?
 
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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #4 on: February 01, 2020, 09:35:54 pm »
Thanks all for the swift responses!

Are there some components other than the switch in the base?  I can't tell from your picture.

The picture I linked to was just an example. I have checked the 3 lights I am using, and they seem to be just a switch between the cells and the string. Like tunk's. They are cheapo units from B&M, Home Bargains, Poundland etc.

Looks very much like a LED string I have and there's no electronics inside.
If it's 9Vdc +/-0.1V, then in principle it should work if you connect the
three strings in series, i.e. 3+3+3V. As a precaution you could add a low
value resistor in series, maybe 5-10ohm.

Series isn't possible, the lights move away from each other in different directions. Sorry I should have mentioned this a bit better. All battery units are within a few cm of each other, but the other end of each string is not. It's a bit like this: https://www.dummies.com/wp-content/uploads/312189.image0.jpg

Is there no hope of a parallel connection? I have small ABS boxes that can house some components if needed. Basically, the whole idea of this is that the lot is plugged into a smart plug at the end. It's a bind turning on and off each lot every time I use them. I want to just ask Alexa to do it instead and they chew cells if I leave them on too.

Those LED strings rely on the internal resistance of the batteries and LEDs to limit the current.

What voltage does the 9V power supply actually measure, when you connect a meter to it?

I hoard old PSUs so I have some choice. I've just checked the 9V PSU I originally mentioned and it's reading at: 8.9v and is supposedly 1.7A. Please note my meter is a cheapo and not a nice Fluke as I would like one day ;)

I have others that may be better suited, these are all the others below 12V:

6V~300mA x 2
8V~400mA
5V~1A
4.5V~1A
6V~800mA
5V~800mA
 

Online tunk

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #5 on: February 01, 2020, 09:49:42 pm »
When I said series I meant:
- power supply positive (to resistor) to positive first string.
- negative first string to positive second string
- negative second string to positive third string
- third string negative to power supply negative
This should work with your setup; assuming the PSU is 9.0V or below,
and the strings are identical.

Edit: that 8V PSU may be even better, again assuming it's really 8V.
« Last Edit: February 01, 2020, 09:53:01 pm by tunk »
 
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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #6 on: February 01, 2020, 10:29:48 pm »
 :palm: my error, I understand what you mean now.

The strings aren’t identical in appearance (some are small globes some are large, some are stars), but they may have the same amount of LEDs etc. I will need to check. They are certainly all powered by 2 x AA cells. I suppose worst case, they were very cheap, but I know that’s not the point.

I think it’s probably worth just giving it a crack with the resistor as you suggest and the 8V PSU.

Do you mean something like this for the resistor? eBay: 202893151256

 

Online Zero999

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #7 on: February 01, 2020, 11:31:26 pm »
Yes those will do but the price is silly. Rapid Electronics sell a pack of 100 for £0.94, including VAT. Delivery is a lot more, so unless you're ordering other things to make the value up to enough to qualify for free delivery, the rip-off eBay price might work out cheaper.
https://www.rapidonline.com/truohm-cr-025-6r8-carbon-film-resistor-0-25w-pack-of-100-62-0324
 
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Online tunk

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #8 on: February 01, 2020, 11:37:05 pm »
If it's really 8V then it should in theory work without a resistor.
I guess you have to test it, wire it up and connect it for a second
or two and see if it works.
 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #9 on: February 02, 2020, 03:11:32 am »
If all 3 strings are electrically the same (or very similar), then this simple series connection should work fine - but I'll always endorse the addition of a current limiting resistor for cheap protection.

In this case, what I mean by "electrically the same" is that the current drawn by each when running off the 3V of batteries is much the same for all 3 strings.  When connected in series, the same current will pass through all 3 strings and if there is a notable difference in the stand-alone current draw of each, you might encounter some undesired effects - such as differences in brightness.
« Last Edit: February 02, 2020, 03:13:08 am by Brumby »
 
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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #10 on: February 02, 2020, 09:38:55 am »
Yes those will do but the price is silly. Rapid Electronics sell a pack of 100 for £0.94, including VAT. Delivery is a lot more, so unless you're ordering other things to make the value up to enough to qualify for free delivery, the rip-off eBay price might work out cheaper.
https://www.rapidonline.com/truohm-cr-025-6r8-carbon-film-resistor-0-25w-pack-of-100-62-0324

Yes, I see what you mean. I wonder if CPC have something, they offer free delivery after about £8. I am sure I could find something to bulk that out.

If it's really 8V then it should in theory work without a resistor.
I guess you have to test it, wire it up and connect it for a second
or two and see if it works.

This morning I have checked a few things.. small curve ball. I was just checking exactly what the 8V PSU outputted, then I noticed it's actually AC  |O:

 > 8V~400mA is AC not DC, so is the 6V~800mA

Edit: I also have another set of small globes that I could add and use the 9V?

So the highest DC voltage I have (sub 12V) is the 9V 1.7A, or then it's the 6V~300mA which is an old Jabra charger

If all 3 strings are electrically the same (or very similar), then this simple series connection should work fine - but I'll always endorse the addition of a current limiting resistor for cheap protection.

In this case, what I mean by "electrically the same" is that the current drawn by each when running off the 3V of batteries is much the same for all 3 strings.  When connected in series, the same current will pass through all 3 strings and if there is a notable difference in the stand-alone current draw of each, you might encounter some undesired effects - such as differences in brightness.

I have measured the strings and counted the LEDs:

1. Large Globes: 

  a. Total wire length: ~176cm
  b. No of LEDs: 10
  c. Space Between LEDs: ~16cm
  d. Just a switch between cells and leds?: Yes

2. Small Globes:

  a. Total wire length: ~190cm
  b. No of LEDs: 10
  c. Space Between LEDs: ~15-16cm
  d. Just a switch between cells and leds?: Yes

3. Stars:

  a. Total wire length: ~140cm
  b. No of LEDs: 6
  c. Space Between LEDs: ~24cm
  d. Just a switch between cells and leds?: Yes

I am not so concerned if some of the LEDs aren't as bright as some of the others, as long as they're light up 'reasonably'. I'm kinda thinking it'll be worth just hooking them up to the 6V and seeing what happens.

Update:

I connected the LEDs in series as tunk said using the 6V 300mA PSU that I mentioned above. And unfortunately there wasn't even a flicker of light :( I realise 6V is less than 8V, but I would have expected something.

So, before doing something else, or reattaching all the disconnected AA cell boxes, I tried connecting each of the lines directly to the PSU. Low and behold, they are working, evenly lit, and brighter than they were with fresh AA's.... on a 6V~300mA PSU... my brain is hurting. Each AA pack was outputting almost 3V, yet a 6V PSU with them connected in parallel is brighter. I really don't understand how this works. With that said, it's working. So, Is it safe to leave like this?
« Last Edit: February 02, 2020, 10:24:59 am by bob21 »
 

Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #11 on: February 02, 2020, 11:04:02 am »
So, Is it safe to leave like this?

NO!
 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #12 on: February 02, 2020, 11:16:16 am »
By having each string connected to 6V, you are overdriving the LEDs something extreme!!!

Connecting the strings in parallel means they each get 6V - which is far too much!  Each string does not know about the others - they just see the voltage provided by the power supply.  At 6V, they will be sucking an excessive amount of current for them to survive for very long - but they will be bright!

Just connect one of the strings to the 6V supply and you should see the same, excessive brightness.  Adding more strings in parallel does not change this, as long as the power supply can handle the current drawn.

The power supply, on the other hand, sees the current going out as being the sum of the current for each individual string - and as long as this is within its capability, then it will pour this current into the strings.


If you really want to use this 6V supply, then use ONLY 2 strings and put them in series.  If you get a fourth string, then you can make a second set of two strings in series and connect that second set in parallel to the first.
 
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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #13 on: February 02, 2020, 11:51:50 am »
Please, don't worry - This is why I asked the question :) I have not left them plugged in like that. I checked and updated the post.

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?


 


I also have a 4.5V 1A PSU - or would that still overdrive them too much? What if I found a 3V PSU? Would parallel be ok then? I guess not...?

I am not trying to be obtuse, I am just trying to learn :D

Edit: Just noticed an error in my drawing where the strings connect to the PSU - pos and neg incorrect, have updated drawing
« Last Edit: February 02, 2020, 12:35:16 pm by bob21 »
 

Offline mariush

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #14 on: February 02, 2020, 12:48:52 pm »
You have to understand a very simple thing:  LEDs are current limited, not voltage limited.

This means the amount of current that goes through the LED must be limited, otherwise the LED will be damaged, will blow up.

So once the voltage is above the forward voltage of a LED (a value which varies depending on the chemicals used to make the LED, and it's between around 1.7v for red leds, 2..2.2v for amber / green leds and 2.8v..3v for blue/white leds) the led turns on and lets any amount of energy flow through it. If you let too much energy go through it, the led overheats and gets damaged.

You can limit the amount of current going through the led with various methods, but the most simple one is to put a resistor in series with the led.
So for example, if you have a single white led with 3v forward voltage which can consume up to 20mA safely and you want to power it with 5v from a usb charger (power bank, phone charger, doesn't matter), you want to pick a resistor in such a way that the led will only "see" 20mA go through it. So you use the formula:

Input voltage - Forward voltage of led = Current x Resistance value.

so if we put the values in our example in the formula:  5v - 3v = 0.02A (20mA) x R  ===> R = 2v / 0.02 = 100 ohm

Now in your string of leds powered by 2 AA batteries.
You have to understand that there's a difference between a battery powered circuit and a circuit powered by a power adapter (doesn't matter if it's AC or DC).
A battery has an internal resistance, it can only push so much current before the battery itself overheats and the voltage drops. So you have to imagine the battery as a power adapter PLUS a resistor in series with the power adapter.

In the case of AA batteries, these typically can output up to around 1-2A (2000mA) of current ... so you can look at that chain of leds and count how many leds are there.... i count about 20 leds.
So if you had ideal batteries that can output continuously 2000mA of current, these 2000mA of current would be evenly split across the 20 leds, so each led would get 100mA.
So you already have an inherent current limitation without adding a resistor to each led, to make sure it doesn't blow up.

You also have to keep in mind that the wires are not ideal, they have some resistance... so you see that chain of leds has 20 leds and it's maybe let's say 2 meters long... that means that between the + and - poles of the battery there's going to be 4 meters of wire.
The resistance of the wire will vary depending on the thickness of the wires... let's say that those wires are AWG24 ... if you look at an AWG table, you can determine that's a resistance of around 85 mOhm per meter of wire : https://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes  so with 4 meters of wire, you have around 0.4 ohm  of resistance in the wires alone.  Keep in mind that the resistance value is for copper, while these cheap led strips may use a mix of copper and steel wire, or steel and aluminum wires, because steel and aluminum is cheaper... so let's go with 0.5 ohm of resistance in the wires alone.

So the battery behaves like there's a resistance inside it, and the wires also have some resistance, and all this resistance prevents the battery from pushing too much current in the chain of leds, so those 10-20 leds will receive maybe a bit more current than what it's recommended when the batteries are fully charged, but the leds will survive because the wires will behave like a heatsink, sucking the heat from the leds and dissipating it over the long wires.

This design works for cheap stuff like those 10-20 leds powered by 2 batteries sold for 1-2$, because nobody will request refunds or complain if they die 1-2 months after Christmas.
If some led dies, the other will continue to work because they're all in parallel, but keep in mind that the rest of the leds now receive more current, so they're more "stressed" and the risk of another one failing increases. If too many die, the other will also die, because there's no resistor for each individual led, to protect it.

However, if you use a power adapter, this power adapter will have much lower internal resistance, instead of let's say 50 ohms of internal resistance, the power adapter will have maybe 5-10 ohm of internal resistance, so this resistance along with the resistance of the wire will not be big enough to limit the current going to the leds and the leds will blow up.

In chains of leds designed to be powered with an adapter, you will often see a tiny resistor at the base of each led, or in the socket where the led is placed.
Or, you may see a resistor hidden inside the wire going to groups of 3 leds, for chains of leds designed to be powered by 12v (3 x 3...3.2v = ~ 10v, 12v is used to account for losses in the long wire all the way to the last led)

So how can you convert that 10-20 led chain of leds (in parallel) powered by 2 batteries to work with DC adapter.
Well, you have to add the resistance that's hidden in the battery.

Pick the amount of current you want for each led and determine what voltage you're gonna use to power that chain of leds where the leds are all in parallel.
Let's go with 5v and 10mA for each led that's white and has a forward voltage of around 3v ... so if you have 20 leds on the chain, you would have 20x 10ma = 200mA or 0.2A

So going back to that formula : 5v - 3v = 0.2 A x R  so R  = 2 / 0.2 = 10 ohm

So if you add a 10 ohm resistor to this chain of leds, you can power them from a 5v adapter, and they should not consume more than 200mA. In reality they'll consume a bit less, because those meters of wire also have a tiny resistance (less than 1 ohm)

If you want to power 3 such chains from the 5v adapter, you can put one 10 ohm resistor at the start of each chain, and connect them in parallel to your adapter. Each chain will only consume up to your calculated current value (the 200mA in my example).

If you want to be more efficient you can re-arrange the leds to  have groups of leds in series, then these groups you can place in parallel .

For example, let's say you have a 12v power adapter, or you want to power some leds from a computer power supply.
You know that the forward voltage of a single white led is around 3v, so you can realistically only put 3 leds in series (3 x 3v = 9v) because 4 leds will be exactly 12v, and your power supply may not always be exactly 12v.
So you can make a group of 3 leds and put a resistor in series with this group of 3 leds and that resistor will protect the group of 3 leds:

12v (input voltage) - 3 leds x 3v (forward voltage of led) = 0.01A (10mA current for the group of leds) x R  => R = 3v / 0.01 = 300 ohm, so I'd use one of the standard values like 270 ohm (which would give a tiny bit more current through the group) or 330 ohm (a bit less current)

Now, you can take these bits of 3 leds + resistor and connect them in parallel , as many as you want, and the total power consumed will be number of groups x ~ 10mA (current for each group of leds plus resistor)

 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #15 on: February 02, 2020, 01:08:52 pm »
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?


 


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.
« Last Edit: February 02, 2020, 01:13:13 pm by Brumby »
 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #16 on: February 02, 2020, 01:17:53 pm »
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.
 
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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #17 on: February 02, 2020, 01:52:22 pm »
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?
« Last Edit: February 02, 2020, 02:46:43 pm by bob21 »
 

Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #18 on: February 02, 2020, 02:45:34 pm »
If I understand correctly, a 300mA 6V PSU should be dishing 150mA to A and B.
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?
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?
Need? - Not really - but that would be the "better" way, in my book.
« Last Edit: February 02, 2020, 02:48:41 pm by Brumby »
 
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Offline gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #19 on: February 02, 2020, 02:49:36 pm »
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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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #20 on: February 02, 2020, 02:57:37 pm »
If I understand correctly, a 300mA 6V PSU should be dishing 150mA to A and B.
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?
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?
Need? - Not really - but that would be the "better" way, in my book.

Ok, understood. So I need to work on the assumption that each series could receive the full 300mA load. Like if there is a loose connection or all LEDs in a series fail.

In which case, that would be a max of 15mA per LED

6V - 3V = 0.015 (15mA) x R  ===> R = 3V / 0.015 = 200ohm

So I need to place a 200ohm resistor on each series for the best possible safety?


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).


How not good are we talking? Broken LEDs or Fire?

These things are near furniture which Is why I am trying to make this as safe as possible. If the LEDs just fail, they were cheap. If they're going to burn the house down, that's different.
 

Offline mariush

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #21 on: February 02, 2020, 03:34:36 pm »
First be sure that your 6v 300 mA is actually 6v.  And is it DC or AC? How are you measuring the voltage?

If it's AC voltage, you'll have to convert that to DC using a bridge rectifier and a capacitor.
You'll get a DC voltage with a peak of around 1.415 x Vac - ~ 1.5v ,,, so if you have 6v AC power adapter, you'll get around 7v DC.
The current will be lower, around 0.62 x Iac ... so the 6v AC 0.3A becomes ~7v DC 0.2A

If it's DC voltage... some cheap power adapters will have a slightly higher output voltage when there's no load (or very low load). For example, that 6v adapter may be 6v when you measure using a multimeter because the multimeter is a very low load, it consumes less than 1mA measuring the voltage. However, if you connect something that consumes a bit more power (like an incandescent 6v bulb for example), the voltage may go down a bit, let's say to 5.5v

If your chain of leds has no resistors or anything to control the current going through them, you WILL damage the leds.
Such a chain will such as much current as the power supply can provide so you'll want to limit the current in some way.
If the chain is basically multiple leds in parallel like I assumed in my previous post, a single resistor at the start of the chain could be enough.

You can use that formula: Voltage input - Forward voltage of leds = current x resistor.

With 20 leds in parallel, each consuming 10mA, you have 200mA or 0.2A
So with 6v DC and 3v for the led, you have

6v - 3v = 0.2 x R => R = 3 / 0.2 = 15 ohm

What I forgot to mention is that you also have to be aware how much power is lost in the resistor, so that the resistor won't burn.
The formula for that is Power =  Current x Current x R   

In the above example, the power is  0.2 A x 0.2A x 15 = 0.6 watts, so you'd want a resistor rated for at least 1w otherwise the resistor will be super hot and may even burn up.

If you have 2 chains each with 20 leds in parallel... you could connect them together to form a single chain of 40 leds in parallel.
BUT, you'll have to keep in mind how will the extra long wire will affect things and you'll also have to adjust the resistor to account for that.
So for 40 leds, you'd have R = (6v-3v) / 0.4 A = 7.5 ohm (you'd probably want to use standard 6.8 or 8.2) , and the power on resistor P = 0.4 x 0.4 x 7.5 = 1.2 watts,  so a 1w resistor is no longer suitable, you'd have to use a 3w rated resistor

The extra long wire could cause some of the leds towards the farthest end to be less bright than the leds closer to your power supply.

 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #22 on: February 02, 2020, 03:55:48 pm »
So I need to work on the assumption that each series could receive the full 300mA load.

I'm going to stop you right there.

You seem fixated on the 300mA rating of the supply.  Forget about that while you play with all the other numbers.

The 300mA ONLY comes into consideration when you work out everything else and come up with the total current that your circuits will want to draw.  Then all you need to do is compare that load current with the 300mA. 

... and here is the ONLY time you use the 300mA rating of the supply:

If the load current <= 300mA  then all is well.
If the load current > 300mA  then you will be overloading the supply.


The load circuit determines the current drawn.  For a particular circuit, the current it will draw from a supply of a given voltage will be the same, whether that supply is rated at 300mA or 300,000A.  If that circuit requires 1mA, then that's all that a power supply will put out.
« Last Edit: February 02, 2020, 04:00:57 pm by Brumby »
 
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Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #23 on: February 02, 2020, 05:39:40 pm »
So I need to work on the assumption that each series could receive the full 300mA load.

I'm going to stop you right there.

You seem fixated on the 300mA rating of the supply.  Forget about that while you play with all the other numbers.

The 300mA ONLY comes into consideration when you work out everything else and come up with the total current that your circuits will want to draw.  Then all you need to do is compare that load current with the 300mA. 

... and here is the ONLY time you use the 300mA rating of the supply:

If the load current <= 300mA  then all is well.
If the load current > 300mA  then you will be overloading the supply.


The load circuit determines the current drawn.  For a particular circuit, the current it will draw from a supply of a given voltage will be the same, whether that supply is rated at 300mA or 300,000A.  If that circuit requires 1mA, then that's all that a power supply will put out.

You'll have to forgive me for being as thick as two short planks when it comes to this. I am trying to digest as quickly as I can though, so forget the 300mA for now. Got it. The only reason I was going back to the 300mA is because I logiced that if 300mA / 41 LEDs = 7.31mA, then I could not use the below calculation with a single LED current of 10mA, as the final currrent draw would be 410mA, which is overloaded.

First be sure that your 6v 300 mA is actually 6v.  And is it DC or AC? How are you measuring the voltage?

If it's DC voltage... some cheap power adapters will have a slightly higher output voltage when there's no load (or very low load). For example, that 6v adapter may be 6v when you measure using a multimeter because the multimeter is a very low load, it consumes less than 1mA measuring the voltage. However, if you connect something that consumes a bit more power (like an incandescent 6v bulb for example), the voltage may go down a bit, let's say to 5.5v

If your chain of leds has no resistors or anything to control the current going through them, you WILL damage the leds.
Such a chain will such as much current as the power supply can provide so you'll want to limit the current in some way.
If the chain is basically multiple leds in parallel like I assumed in my previous post, a single resistor at the start of the chain could be enough.

You can use that formula: Voltage input - Forward voltage of leds = current x resistor.

With 20 leds in parallel, each consuming 10mA, you have 200mA or 0.2A
So with 6v DC and 3v for the led, you have

6v - 3v = 0.2 x R => R = 3 / 0.2 = 15 ohm

What I forgot to mention is that you also have to be aware how much power is lost in the resistor, so that the resistor won't burn.
The formula for that is Power =  Current x Current x R   

In the above example, the power is  0.2 A x 0.2A x 15 = 0.6 watts, so you'd want a resistor rated for at least 1w otherwise the resistor will be super hot and may even burn up.

If you have 2 chains each with 20 leds in parallel... you could connect them together to form a single chain of 40 leds in parallel.
BUT, you'll have to keep in mind how will the extra long wire will affect things and you'll also have to adjust the resistor to account for that.
So for 40 leds, you'd have R = (6v-3v) / 0.4 A = 7.5 ohm (you'd probably want to use standard 6.8 or 8.2) , and the power on resistor P = 0.4 x 0.4 x 7.5 = 1.2 watts,  so a 1w resistor is no longer suitable, you'd have to use a 3w rated resistor

The extra long wire could cause some of the leds towards the farthest end to be less bright than the leds closer to your power supply.

The PSU is a 6V DC and I checked it by switching my Multimeter to '20' on the DC side and the reading was 6.01V.

Because I think the LEDs at the end of a super long chain of 40 would suffer (as you've suggested), I think it may be better to place 2 resistors at the green dots and have the 2 x chains connected individually as per the last picture I posted (unless you mean this isn't possible?). If it is possible, I need to find 2 resistors - one for each green dot, right?

I am just repeating this so you can see my thinking, I want to make sure I understand.

So with each LED consuming 10mA:

Circuit A - 20 LEDs: 6v - 3v = 0.2 x R => R = 3 / 0.2 = 15 ohm
Circuit B - 21 LEDs: 6v - 3v = 0.21 x R => R = 3 / 0.21 = 14.28 (so safe to just use 15?)

For the resistors losing power and getting hot, I would like them to be as cool as possible as these are near a sofa. So:

Circuit A - 20 LEDs: 0.2 x 0.2 x 15 = 0.6 watts (as you've said)
Circuit B - 21 LEDs: 0.21 x 0.21 x 15 = 0.66 watts (still ok to use 1W and remain cool?)

So I need to place 2 x 15 Ohm Resitors rated at least 1W at each green dot point in my diagram?

Just a side question, would it be possible to use a 15ohm @ 3W on these smaller chains to keep the heat down even more? Or will this not work? I am thinking about minimum heat due to proximity to fabrics.

Now going back to the PSU mA rating - can this load cope with it? 10mA x 41 = 410mA, so no...

So is it not at all possible to just adjust the calculation so it fits within the confines of the PSU (Yes I know I am meant to be forgetting this... but this was my thought process)

If I have a 300mA PSU and 41 LEDs, I do not want each LED to draw more than 7.31mA. So recalculating the above using that value:

Circuit A: 7.31mA x 20 = 146.2mA or 0.146A
Circuit B: 7.31 x 21 = 153.51mA or 0.15351A

Circuit A - 20 LEDs: 6v - 3v = 0.146 x R => R = 3 / 0.146 = 20.54 ohm (so safe to just use 20?)
Circuit B - 21 LEDs: 6v - 3v = 0.15351 x R => R = 3 / 0.15351 = 19.54ohm (again, just use 20?)

For the resistors losing power and getting hot:

Circuit A - 20 LEDs:  0.146 x 0.146 x 20 =  0.426 watts - so 1W well and truly safe?
Circuit B - 21 LEDs: 0.15351 x 0.15351 x 20 = 0.494 watts - again, 1W well and truly safe?

So this is why I kept going back to the 300mA.. 41 LEDs x 7.31mA = 299.71mA

Question is, is it wise to run it at it's limit? Would it not be better to take it down a notch and say, for example:

Each LED 7mA

Circuit A: 7mA x 20 = 140mA or 0.14A
Circuit B: 7mA x 21 = 147mA or 0.147A

Circuit A - 20 LEDs: 6v - 3v = 0.140 x R => R = 3 / 0.140 = 21.4285 ohm (so 22ohm?)
Circuit B - 21 LEDs: 6v - 3v = 0.147 x R => R = 3 / 0.147 = 20.4081 (so 21ohm?)

For the resistors losing power and getting hot:

Circuit A - 20 LEDs:  0.140 x 0.140 x 20 =  0.392 watts - so 1W surely not getting hot at all now?
Circuit B - 21 LEDs: 0.147 x 0.147 x 21 = 0.453 watts - again, 1W well and truly safe?

Now we come back to the 41 total LEDs each drawing a maximum of 7mA = 287mA so we aren't running the PSU to the max.

So I need to put this on the 21 LED string: https://www.reichelt.com/gb/en/metal-oxide-resistor-1-w-5-22-ohm-1w-22-p1799.html?PROVID=2788&wt_guka=92495611608_406948187429&PROVID=2788&gclid=EAIaIQobChMI15_ok7Oz5wIVSLTtCh3rvgI9EAQYASABEgJIjfD_BwE&&r=1

Can't seem to find 21 ohm 1W resistors easily, so would a 22ohm just work on both? Would be way easiser to grab one pack.


If I haven't understood it now, I don't think I ever will.
« Last Edit: February 02, 2020, 05:45:08 pm by bob21 »
 

Offline mariush

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #24 on: February 02, 2020, 06:58:52 pm »
You can slightly adjust the resistor values to get something easy to buy. You'll just get a bit less or a bit more current through the leds.

Reuse the formula just put R instead of current, for example

Circuit B - 21 LEDs: 6v - 3v = 0.21 x R => R = 3 / 0.21 = 14.28 (so safe to just use 15?)

Rewrite it like this

Circuit B - 21 LEDs: 6v - 3v = Current x 15 => Current = 3 / 15 = 0.2A (200mA / 21 leds = 9.52mA per led)

The PSU is a 6V DC and I checked it by switching my Multimeter to '20' on the DC side and the reading was 6.01V.

Yes, well that's no load.
Repeat the measurement with 20 leds connected to it and see if you still get 6v. You may find out that the closer you are to its maximum advertised output of 300mA, the voltage may go down a bit. So you may see 5.5v at 250mA or something like that.

The power supply may be conservatively specified, you may find out that it can actually output 500mA while the voltage goes down to around 5.5v ... not saying it would a good idea to use such a power supply for long time above its advertised capabilities... it's up to you.
If you go above 300mA, it may or may not output more, but there's no guarantees.
The power supply may lower the voltage a bit (so the total power will be the same, for example 6v x 300mA = 1800mW or 5.5v x 325 m = ~1800mW, same amount of power)... The power supply may have some protection mechanism built in, where it would constantly reset itself for brief moments of time.

Yes, you can use resistors rated for higher wattage than needed. You can also parallel resistors to spread the heat across a larger surface. When you parallel two resistors, you get half the resistance but 2x the area to dissipate heat.
So for example, instead of using a 10 ohm resistor, you could have two 22 ohm resistors in parallel ending up with 11 ohm

You can use 3w resistors but they're big and ugly.  I'd look into paralleling 2 1w resistors first.


 
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