Author Topic: Hooking up 4 x 3v LED string lights to a 6v PSU  (Read 10312 times)

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Offline gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #25 on: February 02, 2020, 08:14:25 pm »
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

Well, it's a matter of the desired brightness. You can even go down further if you don't need that much brightness. Just try different currents and check visually. But don't exceed the maximum current given in the datasheet of the LED chips! And, since the thermal conduction of the bulbs is likely unknown, I'd even stay quite below the maximum current given in the data sheet, in order to prevent overheating the LED chips. Overheating is one of the major causes for a premature death of LED chips.

For the calculation of the current limiting resistor I would also measure the actual forward voltage of the LEDs at the desired operating point, and I'd also measure the PSU voltage under the actual load (it may drop when loaded, and even drop significantly if it gets overloaded and a current limiter happens to joins in).

Quote
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.

Is it a SMPS or a traditional PSU?
I.e. does it provide clean DC (filtered, or even stabilized), or just pulsating DC directly from the rectifier?
Do you read any voltage with the multimeter switched to AC mode?

[ Note, in case of pulsating DC, the calculation of the value and power dissipation of the current limition resistor is no longer that simple, but requires integration of the rectified sine wave over the time interval where the waveform is larger than the LED's forward voltage. ]
« Last Edit: February 02, 2020, 08:16:27 pm by gf »
 
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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 #26 on: February 02, 2020, 09:35:40 pm »
Well, it's a matter of the desired brightness. You can even go down further if you don't need that much brightness. Just try different currents and check visually. But don't exceed the maximum current given in the datasheet of the LED chips! And, since the thermal conduction of the bulbs is likely unknown, I'd even stay quite below the maximum current given in the data sheet, in order to prevent overheating the LED chips. Overheating is one of the major causes for a premature death of LED chips.

Is it daft to say, as bright as possible? I obviously want them to look good, but if they're going to burn out then they will of course have to come down a bit. As mentioned before, I don't so much care if they die at this point (as this is a learning curve for me) but what I would care about is if they caught fire. So if by 'death' you mean they go off and never come on again ergo 'you've lost £3' then I'm annoyed, but not that much vs them overheating catching fabric and setting a blaze to the place because I didn't keep a constant eye on them with a fire extinguisher in hand.

Is it a SMPS or a traditional PSU?
I.e. does it provide clean DC (filtered, or even stabilized), or just pulsating DC directly from the rectifier?

The PSU is a traditional PSU. It's an old Jabra brick Model: ACGN-22B

The label says: Output 5-6V max 5W (6V 300mA)

Do you read any voltage with the multimeter switched to AC mode?

Yes, I have just checked and I read 13V AC when red probe is to pos and black/common probe is to neg. I don't get a reading if I swapped them round (in DC I would see a negative value ie. -13, I didn't see this).

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.

Using a friends meter, which cost a bit more than mine, but is still no Fluke I did this:

1. Took a voltage reading with nothing attached to PSU: 6.19V
2. Took an Amp reading by connecting the red probe to the unfused 10A port and switching the dial to 10A: it fluctuated .44-.49 so WAY over the quoted measurement if I understand that .3 would equate to 300mA.

I then repeated, by hooking up all 41 LEDs for max load as the above diagram (Circuits A and B comprised of 1+2 for A and 3+4 for B):

1. Took a voltage reading by bearing the cable at the point the purple dot is shown and equivalent on the neg wire. The reading was: 5.93V
2. Took an Amp reading using the same method as above, all the LEDs go out upon probe contact and the reading is the same: .44-.49

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)

Got it, thanks, that makes sense.

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.

Well, it seems from the testing I have just done that you have hit the nail on the head. It's way over stated spec. I don't want to use something or create something unsafe.

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

Will this generate more heat than a single 3W?

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


This may be less of a problem, because I could end up housing this in a small ABS box that's non-offensive. But I will keep both methods in mind.

So what's the verdict on this PSU? It's not fit for this task....? Any recommendations for a replacement?
 

Offline Damianos

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #27 on: February 02, 2020, 11:10:47 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?


From this point and next, it cannot be assumed that the LEDs consume the same current.
Explaining this as a kid: in the midpoint of the branch B "arrives" a current equal to 6*ILED but "departs" a 15*ILED, how is this possible?

It is late at night and I am not able to post now some thoughts about possible solutions.
 
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Offline gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #28 on: February 03, 2020, 12:01:28 am »
Took an Amp reading using the same method as above, all the LEDs go out upon probe contact and the reading is the same: .44-.49

Hmm, did you hook up the Amp meter in parallel to the LEDs, i.e. shorting the LEDs with the meter?
In this case you would of course measure a higher current than flowing through the LEDs w/o the meter attached.
For current measurements, the meter must be connected in series (and for voltage measurements in parallel) to the DUT.
 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #29 on: February 03, 2020, 02:15:45 am »
Took an Amp reading using the same method as above, all the LEDs go out upon probe contact and the reading is the same: .44-.49

Hmm, did you hook up the Amp meter in parallel to the LEDs, i.e. shorting the LEDs with the meter?
In this case you would of course measure a higher current than flowing through the LEDs w/o the meter attached.
For current measurements, the meter must be connected in series (and for voltage measurements in parallel) to the DUT.
Oh dear....

For your typical multimeter:
Voltage measurement - the meter is just peeking in on the circuit:


Current measurement - the meter becomes part of the circuit

Yes, this means you have to break the circuit to do a current measurement - which is one of the things nobody likes.  This also introduces a voltage drop - called "burden voltage" - in the circuit which would not be there in normal operation.  This will affect the numbers and it can cause problems.  Just be aware of the burden voltage for your meter and the range it is set on.  (It is usual that this varies between ranges on a given meter - and, of course, it can vary between meters.)
« Last Edit: February 03, 2020, 02:18:33 am 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 #30 on: February 03, 2020, 02:30:39 am »
Yes, this means you have to break the circuit to do a current measurement ...

This is when you use a typical multimeter.  There are other options using a clamp mechanism.

Using the clamp style, AC current measurement is easy, which is why you see these everywhere.  DC requires a different approach which is a bit more involved.

The most commonly cited meter is the UNI-T UT210E.  This is a clamp-style meter which has DC current measurement - and it is not expensive!

All you have to do is place the clamp over ONE conductor and it will measure the DC current.  Resolution is 1mA, but accuracy is less as you need to deal with the fluctuations in the reading due to other magnetic fields.  This meter would still be quite OK for the sort of measurements you are doing.
« Last Edit: February 03, 2020, 02:39:35 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 #31 on: February 03, 2020, 06:47:43 am »
Yes, that’s how I tested. So the Uni-T looks like a must have for the toolbox.

Ok, so how does this sound for a plan:

1. Order the Uni-T
2. Use to test the draw of the entire circuit
3. Reattach each AA cell pack to its respective string and test the current draw of each so we know what they each draw (if this is possible)

Would this provide helpful information?

I’ve only ever used a non-clamp cheapo meter just to test cells and continuity etc. so I welcome something better for the toolbox.
« Last Edit: February 03, 2020, 07:10:18 am by bob21 »
 

Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #32 on: February 03, 2020, 06:56:21 am »
The clamp goes around ONE wire.  The meter does not have to touch any wires at all.  All clamp meters work on the magnetic field generated by flowing current.

Don't put both wires coming from the supply through the middle.  If you do, the magnetic field from one wire will be cancelled out by the other.
« Last Edit: February 03, 2020, 07:00:43 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 #33 on: February 03, 2020, 07:12:26 am »
Just edited my post because I noticed you said attach to one wire, but I did actually think it would have to make contact. This sounds awesome! Amazon have them and my friend has prime. Watch this space ;-)
 

Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #34 on: February 03, 2020, 07:31:02 am »
Here's one example of wrong and right...



The same rule applies for DC as it does for AC.
 
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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 #35 on: February 03, 2020, 08:20:08 am »
And that seriously works for small wires? Wow. I thought it would have clamped in the jaws, glad you pointed that out :D

Just waiting to hear back from my friend to see if I can transfer money and use his prime. If it’s a no go (can’t see this being the case though) are there any high street alternatives? Or is this pretty much one of those price to functionality gems that no one else produces?

Edit: ok, scratch that. Got green light for amazon so Uni-T ordered, but also a neighbour who works for a telecoms company has lent me this: https://www.screwfix.com/p/kewtech-kt203-ac-dc-clamp-meter-400a/4048P?kpid=4048P&ds_kid=92700048793290430&ds_rl=1244066&gclid=Cj0KCQiApt_xBRDxARIsAAMUMu8DA0ohmKcjeAFfc-vQHwNb4LW314LOOg-5oqWlW5T9buupCpmoQWUaAnAnEALw_wcB&gclsrc=aw.ds that I can borrow until my Uni-T arrives. Is this suitable? It seems to mention DC current? Nevermind. It's completely lifeless... I guess that's why he didn't mind lending it to me  ::) I'll wait for the Uni-T.
« Last Edit: February 03, 2020, 09:28:26 am by bob21 »
 

Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #36 on: February 03, 2020, 10:31:43 am »
Nevermind. It's completely lifeless... I guess that's why he didn't mind lending it to me  ::) I'll wait for the Uni-T.

Oh.

Seems it does do DC current via clamp so, yes, it will be able to measure DC current without having to cut into wires.

The lowest range, however, is 40.00A (the maximum value displayable on a particular range) - which means the resolution (last digit) is 10mA.  This should be enough for you to get some idea of what's going on, but the Uni-T has a 1mA resolution on the lowest range which will provide more information.


Here's a trick: if you create several loops of wire, the current reading shown on the meter will be multiplied by the number of loops.  Here is an example of 5 loops.  The meter reading will be 5 times higher than the actual current flowing - because there are 5 wires carrying current through the clamp.



So to get the correct current in this example, you divide the displayed value by 5.  Using the 30.00 A value displayed in this image, the actual current flowing through the wire would be 6.00 A.


Change the number of loops and you change the number you divide by.
« Last Edit: February 03, 2020, 10:37:37 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 #37 on: February 03, 2020, 11:30:45 am »
Makes sense. Thanks :) I am guessing using a mean calculation with more wraps like that is better for smaller wires?

I actually can't wait for this Uni-T to arrive now.... want to get stuck back in. :D
 

Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #38 on: February 03, 2020, 12:38:48 pm »
I am just grabbing some other stuff for another project from CPC, and they have a ton of resistors. Is it worth me grabbing a small array for the toolbox? Maybe some of these:

https://cpc.farnell.com/search?st=22ohm%203w
https://cpc.farnell.com/search?st=22ohm%201w
https://cpc.farnell.com/search?st=10ohm%203w
https://cpc.farnell.com/search?st=10ohm%201w

I also want to connect the wires with plugs so it won't be a right faff to move the whole lot. I was thinking drill 3 holes in the ABS box, mount something like this: https://cpc.farnell.com/cliff-electronic-components/fm686803/circular-receptacle-pin-3-way/dp/CN16599?st=male%20female%20plug then have the corresponding plug on the PSU and lines. I can't seem to find these things sold in male and female sets, anyone know what they are called?
 

Online Zero999

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #39 on: February 03, 2020, 01:00:28 pm »
A kit of resistors is always a nice thing to have. At bare minimum I'd recommend an E12 kit of resistor values from 10R to 1M, preferably 0.5W metal film, but carbon film is fine for most applications.

Note, resistors come in standard values to make manufacture and specifying them easier. See the Wikipedia article.
https://en.wikipedia.org/wiki/E_series_of_preferred_numbers
 
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Offline Brumby

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #40 on: February 03, 2020, 01:28:59 pm »
A kit of resistors is always a nice thing to have. At bare minimum I'd recommend an E12 kit of resistor values from 10R to 1M, preferably 0.5W metal film, but carbon film is fine for most applications.
+1 for this.

You will end up using just a few values from this on a regular basis (that you will end up getting more of those values), a few more every now and then and most will be sitting there 10 years from now.  BUT - it's always worth having a range on hand if you are going to be experimenting and for the price they go for, it's a no-brainer.
 
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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 #41 on: February 03, 2020, 02:31:38 pm »
Thanks both. Do you mean something like this? https://cpc.farnell.com/unbranded/mf0w2ffe012kil/resistor-kit-0-5w-1-e12/dp/RE03416

It's quite a lot to drop on a starter project though.

Also, @Zero999 is "Through-hole Resistor with metal oxide film element and high safety standard high purity ceramic core." what you mean by metal film?


Ok, for like a third of the price of that kit, I can get these:

Code: [Select]
MCKNP03WJ0100AA9	MULTICOMP PRO	MULTICOMP PRO  Resistor, Wirewound, 10R, 5%, 3W
MOR01SJ0100A10 MULTICOMP PRO MULTICOMP PRO  Resistor, 1W 5% 10R
MOR03SJ0220A19 MULTICOMP PRO MULTICOMP PRO  Resistor, 3W 5% 22R
MF50 10R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 10R, 50 Pack
MF50 12R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 12R, 50 Pack
MF50 15R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 15R, 50 Pack
MF50 18R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 18R, 50 Pack
MF50 22R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 22R, 50 Pack
MF50 27R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 27R, 50 Pack
MF50 33R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 33R, 50 Pack
MF50 39R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 39R, 50 Pack
MF50 56R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 56R, 50 Pack
MF50 68R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 68R, 50 Pack
MF50 82R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 82R, 50 Pack
MF50 100R MULTICOMP PRO MULTICOMP PRO  Resistor, 0.5W 1% 100R, 50 Pack
MFP1-12R JI TT ELECTRONICS / WELWYN TT ELECTRONICS / WELWYN  Resistor, 1W 5% 12R
MOR01SJ0150A10 MULTICOMP PRO MULTICOMP PRO  Resistor, 1W 5% 15R
MFP1-18R JI TT ELECTRONICS / WELWYN TT ELECTRONICS / WELWYN  Resistor, 1W 5% 18R
MOR01SJ0220A10 MULTICOMP PRO MULTICOMP PRO  Resistor, 1W 5% 22R
MCKNP01WJ033KA10 MULTICOMP PRO MULTICOMP PRO  Resistor, Wirewound, 0R33, 5%, 1W
MFP1-39R JI TT ELECTRONICS / WELWYN TT ELECTRONICS / WELWYN  Resistor, 1W 5% 39R
MFP1-56R JI TT ELECTRONICS / WELWYN TT ELECTRONICS / WELWYN  Resistor, 1W 5% 56R
MOR01SJ0680A10 MULTICOMP PRO MULTICOMP PRO  Resistor, 1W 5% 68R
MFP1-82R JI TT ELECTRONICS / WELWYN TT ELECTRONICS / WELWYN  Resistor, 1W 5% 82R
MOR01SJ0101A10 MULTICOMP PRO MULTICOMP PRO  Resistor, 1W 5% 100R

I've gone for 1% variance and MOR where I can.

Just gotta find those plugs now ;) Looks like eBay wins for that: 223071120027
« Last Edit: February 03, 2020, 03:24:57 pm by bob21 »
 

Online Zero999

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #42 on: February 03, 2020, 04:51:58 pm »
Yes, that looks suitable. Repeat that pattern up to 1M and you'll have a nice stock.

I personally have E24 from 10R to 1M and E12 from 1R to 10R and 1M to 10M, but it's a bit overkill for the hobbyist.
 
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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 #43 on: February 04, 2020, 05:47:25 pm »
It's here!  :-DMM

I switched to '2A~' and with the entire circuit hooked up I get:

1 wire: ~0.158
5 wraps: ~0.820

This is measured from 1 of the wires of the PSU using the clamp, before the series/parallels start (at the purple dot).

I did notice when doing the single wire, it was quite an exact immediate number. With 5 wraps it took a while to climb and stabilise.

More to follow. Doing this around making dinner etc.

Edit:


Ok, got it:



Line 1 (10 small globes)


Single wire: 41mA
5 Wraps: 198mA

Line 2 (10 Large Globes)

Single wire: 36mA
5 Wraps: 256mA

Line 3 (6 White Stars)


Single wire: 37mA
4 Wraps: 146mA (could only manage 4 wraps as wire very short before 1st star)

Line 4 (15 Globes)

Single: 41mA
5 Wraps: 245mA


Lines 2 and 4 seem a bit off when dividing the wrap figure (it doesn't give exactly the single wire figure). Whereas 1 and 3 are almost spot on. Is this normal?

I suppose the important thing to note is that the total figures are adding up for the single wires:


Sum of singles: 155mA, and is 158mA measured from PSU wire.

But I have probably screwed up my wrapping a bit:

Sum of wraps: 881mA (obviously taking into account that line 3 needs to be 182mA for this calc), and is 820mA from PSU wire

Do I need to retest?

Also, I noticed that the draw from 1+2 (41+36) is almost the same as 3+4 (37+41)... so circuit A is pulling 77mA and circuit B is pulling 78mA..... does this mean they are [kind of] correctly balanced?

Edit2: Btw, an extra thanks to Brumby for the DMM recommendation. This thing is great. It's about 1/3rd the size of the old piece of junk I had it's auto ranging is something I didn't have before along with a zero and hold button etc which is awesome and it's super intuitive to use.

Edit3: Just got to pop out, when I get back I'll see if I can work out what resistors I need for each circuit.
« Last Edit: February 04, 2020, 07:30:53 pm by bob21 »
 

Offline gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #44 on: February 04, 2020, 08:32:19 pm »
This is measured from 1 of the wires of the PSU using the clamp, before the series/parallels start (at the purple dot).
[...]

Which current limiting resistor values did you use for each these measurements?

What do you mean with "5 Wraps"? Do you mean 5 lines (of the particular lamp type) connected in parallel?
 

Offline bob21Topic starter

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #45 on: February 04, 2020, 08:58:24 pm »
Which current limiting resistor values did you use for each these measurements?

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?

What do you mean with "5 Wraps"? Do you mean 5 lines (of the particular lamp type) connected in parallel?

I mean wrapping the wire round and through the clamp as Brumby illustrated a few posts back.
« Last Edit: February 04, 2020, 09:02:47 pm by bob21 »
 

Offline gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #46 on: February 04, 2020, 09:45:40 pm »
If you mean what was limiting the current, I hooked each led string back up to it's original AA pack

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.

All clear.

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

Why AC and not DC? (particulary since it is connected to the battery pack)
 
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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 #47 on: February 04, 2020, 10:00:33 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?

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

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

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.

« Last Edit: February 04, 2020, 11:05:21 pm by bob21 »
 

Offline gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #48 on: February 04, 2020, 11:18:49 pm »
BUT: We want 1 + 2 in Series and 3 and 4 in Series

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?
 
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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 #49 on: February 04, 2020, 11:38:06 pm »
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.
« Last Edit: February 04, 2020, 11:43:40 pm by bob21 »
 


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