Author Topic: Hooking up 4 x 3v LED string lights to a 6v PSU  (Read 10306 times)

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Online gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #50 on: February 05, 2020, 12:25:37 am »
How would be best to test this? Probe at start and end of string?

Measuring both ends is a good ieda, since there might be a voltage drop along the string
(at only 40mA I would not expect too much, though).

Quote
Each string only has 2 wires. If you removed 1 LED, the entire string would fail.

Only if you destroy the cable during removal. The LEDs of a string are not connected in series, but in parallel, therefore the removal of a LED is not supposed to break the circuit. Only the current consumption would decrease.

Note, if the LEDS of a string were connected in series, then you would need a voltage of about 3V*10=30V in order to drive 10 LEDs - too much for 2x AA cells (unless a switching regulartor would be used to boost the voltage).

Quote
I’m not sure what you mean by daisy chained.

With daisy chained I mean concatenated behind each other, i.e. the second string connected to the end of the first one (+ to +, and - to -), which is electrically a parallel connection.

Quote
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.

OK, you really mean an electrical series connection.

Again, this would only be possible for strings 1+4, which draw the same amount of current, and for strings 2+3 which also draw the same amount of current. If you connect two strings in series then the same amount of current flows through both ones.

If you would connect strings 1+2 in series, then you'll either need to drive string 1 with a lower current than 41mA or strings 2 with a higher current than 36mA, since there is "only one current" flowing through both strings then. The whole amount current which leaves string 1 enters string 2, since there exist no path where the current could escape. At each connection point in the circuit, the sum of incoming and outgoing currents is the same (Kirchhoff's law).
 
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Online gf

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Re: Hooking up 3 x 3v LED string lights to a 9v PSU
« Reply #51 on: February 05, 2020, 12:46:35 am »
[...]
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?
[...]

If the PSU provides just pulsating DC, then one consequence to be expexted will be flickering of the LEDs.
 
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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 #52 on: February 05, 2020, 08:29:06 am »
If the PSU provides just pulsating DC, then one consequence to be expexted will be flickering of the LEDs.

Well in that case I would have to guess that the DC does not pulsate. The LEDs definitely do not flash when hooked up to this. They are solid on.

Measuring both ends is a good ieda, since there might be a voltage drop along the string
(at only 40mA I would not expect too much, though).

This might be tricky as I’ve have to start stripping other ends of cables which I ideally wanted to avoid, but what I can do this morning is take a voltage reading before and after each string is turned on (when connected to its cell pack) and provide the voltage drop figures.

Only if you destroy the cable during removal. The LEDs of a string are not connected in series, but in parallel, therefore the removal of a LED is not supposed to break the circuit. Only the current consumption would decrease.

Note, if the LEDS of a string were connected in series, then you would need a voltage of about 3V*10=30V in order to drive 10 LEDs - too much for 2x AA cells (unless a switching regulartor would be used to boost the voltage).


Ah, I understand now. Thanks. :-+

With daisy chained I mean concatenated behind each other, i.e. the second string connected to the end of the first one (+ to +, and - to -), which is electrically a parallel connection.

OK, you really mean an electrical series connection.


I have both, 1 and 2 are electrically in series, as are 3 and 4. Then these are in parallel to the PSU to share the 6V. 

Again, this would only be possible for strings 1+4, which draw the same amount of current, and for strings 2+3 which also draw the same amount of current. If you connect two strings in series then the same amount of current flows through both ones.

If you would connect strings 1+2 in series, then you'll either need to drive string 1 with a lower current than 41mA or strings 2 with a higher current than 36mA, since there is "only one current" flowing through both strings then. The whole amount current which leaves string 1 enters string 2, since there exist no path where the current could escape. At each connection point in the circuit, the sum of incoming and outgoing currents is the same (Kirchhoff's law).

Got it. So I need to do some reorganising: 1+4 in series, 2+3 in series, both connected in parallel to PSU.

Just out at the moment, will provide voltage drop readings for each unit in a bit.

Edit: Changed title to reflect current aims.
« Last Edit: February 05, 2020, 09:45:35 am by bob21 »
 

Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #53 on: February 05, 2020, 09:44:11 am »
More info:

With a 2 x AA cell pack start reading of 2.93V, I hooked up each string of lights and checked the voltage again from the point where the pack was reconnected to the wires.

Line 1: 2.86V
Any visible resistors in LED heat shrink? Difficult to tell, as opaque, but my guess is no, it's too thin. Ie. it only looks as wide and long as the LED legs

Line 2: 2.85V
Any visible resistors in LED heat shrink? Difficult to tell, as opaque, but my guess is no, it's too thin. Ie. it only looks as wide and long as the LED legs

Line 3: 2.87V
Any visible resistors in LED heat shrink? Definitely not, the heat shrink is transparent, and it's just the LED legs and wire.

Line 4: 2.86V
Any visible resistors in LED heat shrink? Definitely not, the heat shrink is transparent, and it's just the LED legs and wire.

The size of each LED heat shrink part is the same size in 1 and 2 as in 3 and 4. I know resistors can be tiny, but I honestly don't believe there's any in there. The heat shrink would be bigger or show the outline of an additional component, it's tight against the LED legs and wire.
 

Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #54 on: February 05, 2020, 11:45:03 am »
Having worked out some more numbers (again, please do not shoot me, these might all be junk!), we are now using:

Line 1 + 4 = Lets call this Circuit C
Line 2 + 3 = Lets call this Circuit D

Updated Diagram:


 


Pink and Purple dots are resistor locations? Is this correct?

C's total draw is 82mA

6V - 3V = 0.082 x R ==> R = 3V / 0.082 = 0.246 Ohm

We only have 22 or 27 Ohm resistors, so can we use a 22 for this?

D's total draw is 73mA

6V - 3V = 0.073 X R ==> 3V / 0.073 = 41.09 Ohm

We only have R39 and R47

What is the formula to adjust a resistor value and work out what the resultant current will be?

Is it:

Circuit C: R = 3V x C ==> 3V / R22 = C ==> 0.136mA so if using a R22 resistor on C, it will get 136mA not 82mA.. problem.
Circuit D: R = 3V x C ==> 3V / R39 = C ==> 0.076mA so if using a R39 resistor on D, it will get 76mA not 73mA... no big deal?


So D is pretty ok at this point - how to go about C? I thought about using 2 x R47's for 1/2 R so 23.5R:

R = 3V x C ==> 3V / R23.5 = C ==> 0.127mA so if using a 2 x R47 resistor on C, it will get 127mA not 82mA.. still a problem.

Am I over thinking this, could use some wisdom :)
 

Online gf

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #55 on: February 05, 2020, 06:09:17 pm »
C's total draw is 82mA
6V - 3V = 0.082 x R ==> R = 3V / 0.082 = 0.246 Ohm

I think you still didn't get it. If you put two components in series, then the currents don't add up.

41mA flow from the PSU into the "+" connector of line 1, 41 mA come out of line 1's "-" connector and flow into line 2's "+" connector, and again 41 mA come out from line 2's "-" connector and flow back to the PSU.

So C's "total draw" is still 41mA.

But the voltage drops of the components add up. If one string has a voltage drop of 2.86V, then two strings in seris will have a voltage drop of 2 * 2.86V = 5.72V.

This leads to the following calculation for your sub-circuit C:

Vpsu = 6 V (measured)
Vled = 2.86 V (measured)
Iled = 0.041 A (measured)
Vresistor = Vpsu - Vled - Vled = 0.28V
R = Vresistor / Iled = 0.28 / 0.041 = 6.8293 Ohm ==> 6.8 Ohm
Presistor = Vresistor * Iled = 0.011480 W

The problematic thing here is that after subtracting 2 * Vled from Vpsu there are only 0.28V are left for the resistor (which leads to the low resistor value of only 6.8 Ohm).
Under these conditions the resistor is not supposed to ensure a very stable working point for the circuit.

Note, however, the above calculation applies only if the PSU output is 6V clean DC!
For "pulsating DC" (i.e. full-wave rectified sine-wave) we will get different numbers!
In this case 6.8 Ohm will be ways too low and would lead to a significantly higher (average) current than 41mA!
I don't know details about your PSU, but I have the feeling that it does not deliver clean DC.
« Last Edit: February 05, 2020, 06:11:53 pm by gf »
 

Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #56 on: February 05, 2020, 06:38:05 pm »
I think you still didn't get it. If you put two components in series, then the currents don't add up.

You are correct, I didn't get it, but I do now --> Current does not add up when in series. Thanks for being patient with me.  :-+

But the voltage drops of the components add up. If one string has a voltage drop of 2.86V, then two strings in seris will have a voltage drop of 2 * 2.86V = 5.72V.

Just to clarify, this is a drop to 2.86V from 2.93V so a loss/drop of 0.07V. Or have I somehow measured this incorrectly? If I have measured this incorrectly, and I need a PSU with more voltage and current, I have a whole box of them here. I am looking at one here now that is 12V 1.5. Would that be more suitable?

Note, however, the above calculation applies only if the PSU output is 6V clean DC!
For "pulsating DC" (i.e. full-wave rectified sine-wave) we will get different numbers!
In this case 6.8 Ohm will be ways too low and would lead to a significantly higher (average) current than 41mA!
I don't know details about your PSU, but I have the feeling that it does not deliver clean DC.

How do I test if it provides clean DC? You said the LEDs would flicker if unclean, but they do not......
 

Online gf

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #57 on: February 05, 2020, 06:50:47 pm »
Note, however, the above calculation applies only if the PSU output is 6V clean DC!
For "pulsating DC" (i.e. full-wave rectified sine-wave) we will get different numbers!
In this case 6.8 Ohm will be ways too low and would lead to a significantly higher (average) current than 41mA!
I don't know details about your PSU, but I have the feeling that it does not deliver clean DC.

Now the same calculation for unfiltered, pulsating DC with an average voltage of 6V:
(The calculation below is Matlab/Octave code. I'm doing the integration in a very simple way numerically.)

Code: [Select]
t = [0:99] * 0.01 * pi;       % 100 steps 0 <= x < pi for numerical integration
Vpsuavg =  6                  % measured average voltage
Vpsupeak = Vpsuavg / 2 * pi   % peak voltage of the rectified sine wave
Vpsu = sin(t) * Vpsupeak;     % PSU voltage as function of time
Vled = 2.86
Iledavg = 0.041
Vresistor = Vpsu - Vled - Vled;
Vresistor(Vresistor<0) = 0;                     % set to 0 in the off-time where Vpsu < 2 * Vled
Vresistoravg = mean(Vresistor)                  % avarage voltage across resistor
Vresistorrms = sqrt(mean(Vresistor.*Vresistor)) % RMS voltage across resistor
R = Vresistoravg / Iledavg
Presistor = Vresistorrms * Vresistorrms / R

This gives the following results:
Code: [Select]
Vresistoravg =  1.4235 V
Vresistorrms =  2.0477 V
R =  34.720 Ohm
Presistor =  0.12077 Watt

So we would now require 33 Ohm (while we had only 6.8 with clean DC).
 

Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #58 on: February 05, 2020, 07:07:42 pm »
Ok, but see my post above. Did you think the voltage drop was by 2.86V or to 2.86V? Because it was a drop to the values listed, so:

Line 1: loss = 0.07
Line 2: loss = 0.08
Line 3: loss = 0.06
Line 4: loss = 0.07

I appreciate your time explaining the differences in ohms when the DC supply is dirty, but all of this is going way over my head now. If the PSU is not suitable, I would rather:

1. Test my existing PSU to see if it really is / is not clean - you specifically mentioned a few posts back that unfiltered DC would cause flickering - and I replied that my PSU does not cause even slight flickering
2. Try another PSU (I have tons here), most are from gadgets like phones, power banks, speakers etc.. surely they can't all be dirty?
3. Buy a PSU that is clean - is this an example? https://tinyurl.com/uutxd9v

I just want the simplest easiest way to power these 4 strings off a wall outlet that won't catch fire to my house.

If I need to buy something else I will. I've just bought a new DMM and a ton of resistors. What else do I need?

I am an electronics n00b. I knew nothing about any of this 4 days ago and my brain hurts. I am trying my best to take all this in as quickly as possible. I've even tried calculating values myself and not just expecting other people to do it all for me. Yes, some have been wrong, but I am posting in the beginners section, after all.
« Last Edit: February 05, 2020, 07:24:50 pm by bob21 »
 

Online gf

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #59 on: February 05, 2020, 08:21:45 pm »
Quote
I appreciate your time explaining the differences in ohms when the DC supply is dirty, but all of this is going way over my head now.
[...]
I am an electronics n00b. I knew nothing about any of this 4 days ago and my brain hurts. I am trying my best to take all this in as quickly as possible. I've even tried calculating values myself and not just expecting other people to do it all for me. Yes, some have been wrong, but I am posting in the beginners section, after all.

OK, I see, you primary aim are obviously quick results.
Here's one of several possible approaches:
  • Don't connect strings in series, but 4 all strings in parallel to the PSU.
  • Use a separate current limiting resistor in series with each string.
Determine resistor values experimentally, for one string at a time:
  • For string #1 start with 82 Ohm (>= 1/4 W), connect to the PSU and measure the current
  • If it is lower than 41 mA reduce to 75 Ohm and measure the current again
  • Use the same resistor value for string #4
  • For string #2 start with 100 Ohm (>= 1/4 W) and measure the current
  • If it is lower than 36 mA reduce to 91 Ohm and measure again
  • If it is still lower than 36 mA reduce to 82 Ohm and measure again
  • Use the same resistor value for string #3

EDIT:
Note, if you renounce the desire to connect two stings each in series, there is significantly more voltage headroom for the resistor to establish a stable working point. Additionally, clean vs. "dirty" DC does no longer make such a huge difference, so that resistor values in the range 82...75 Ohm should fit with the 41mA strings, and 100...82 Ohm should fit with the 36mA strings. When you determine the value experimentally, then start with larger resistor values, measure the current, and decrease R until you hit the string's current target (+/- 5% or even +/- 10% is perfectly OK).
« Last Edit: February 05, 2020, 08:44:55 pm by gf »
 
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Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #60 on: February 05, 2020, 09:39:04 pm »
Thank you.  :-+

This is exactly the information I was after. I am more than happy to connect each in parallel and forget series if this is overall a better approach.

My resistors have been dispatched, I am hoping they will arrive tomorrow.

I will post again with an update when I know more and have had to chance to experiment.

Ps. I want to say again that I appreciate the time you have taken to explain a lot of this stuff to me, but being from a non-electronics background, I need some additional digestion time before the next project. Maybe then we can go into deeper stuff like dirty DC.
 

Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #61 on: February 07, 2020, 08:07:24 pm »
Done!

I used some male and female jack connectors in an ABS box that are just connected to the PSU, so each port can provide the 6V 300mA that the PSU is designed to provide.

I then did as you suggested, and have soldered the resistors in series to the positive side of the cable going to each line. I heat shrinked the whole lot.

I considered putting the resistors in the ABS box, but then if a line were plugged into the 'wrong' port, it would receive the incorrect current/voltage. So this way, any line can be plugged into any port. I figured human error should not be ignored. I did a quick google, and it seemed to suggest that heat shrinking 0.5W resistors would be perfectly safe as in this particular application they aren't drawing anywhere near their max load.

Here is a pic of the box (Yes, I know the wiring isn't the prettiest, but it's definitely secure and soldered well):



The final values were:

Line 1:

R39 + R33 (in series) both 0.5W
Resultant Current: 45mA
Resultant Voltage: 2.85V

Sufficient wattage resistors? Yes: I x I x R = 0.145

Line 2:

R82 + R10 (in series) both 0.5W
Resultant Current: 35mA
Resultant Voltage: 2.84V

Sufficient wattage resistors? Yes: I x I x R = 0.112

Line 3:

R82 + R10 (in series) both 0.5W
Resultant Current: 35mA
Resultant Voltage: 2.86V

Sufficient wattage resistors? Yes: I x I x R = 0.112

Line 4:

R39 + R33 (in series) both 0.5W
Resultant Current: 38mA
Resultant Voltage: 2.67V

Sufficient wattage resistors? Yes: I x I x R = 0.103

Note: Line 4 is the longest wire (and 15 LEDs). I assume this is why the mA and V readings are lower. I suppose I could reduce the resistance on that at some point maybe try a single 68, but they are sufficiently bright for now.

So what's the verdict? Did I do ok? Is it safe to heat shrink the resistors? Do the values look ok?

Edit: I am considering doing something similar for my sons bedroom (he has a secret den with a lot of battery powered LED lights and it would be great to have them hooked up to one plug that is voice controlled) so any input on the safety and/or appropriateness of this would be greatly appreciated :)
« Last Edit: February 09, 2020, 11:07:35 pm by bob21 »
 

Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #62 on: February 08, 2020, 07:48:55 am »
Anyone?

Safe? Unsafe? Ok? Not ok?
 

Offline Brumby

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #63 on: February 09, 2020, 01:00:22 pm »
First comment - use I for current.  It might seem strange, but that's the normal notation.  Blame the French guy - Ampère - whose name is used for the unit, which is most often abbreviated to amp.  He used I for current.

Next:  I haven't checked you math, but if things are working OK, then you are halfway there.  As for heat-shrinking resistors, this is completely ok, subject to thermal considerations.  Heatshrinking reduces the cooling effectiveness, but if you can comfortably hold a resistor between your fingers for 10 seconds, then they should be fine.
« Last Edit: February 09, 2020, 01:02:33 pm by Brumby »
 
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Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #64 on: February 09, 2020, 10:07:16 pm »
I'm not opposed to such peculiarities. I'm sure there was a reason :) 'I' for current. Right you are. Edited previous post to reflect correct usage (I x I x R)

And no, absolutely no hotness at all. If I were being totally picky, I would say perhaps the 'stone coldness' (to use a very nontechnical term) had gone in favour of 'not stone coldness' (that really is the best way to describe it). I'd hardly call it luke warm, body temp, warm, and definitely not too hot after 10 seconds. It's just no longer stone cold.

A bit off topic now, but what is the best way to determine the wattage of these resistors? If they weren't written on the bag, I wouldn't know. I used a colour calc to confirm tolerance and R rating, but how to reliably know what wattage? Let's say they all get mixed up. I can use the coloured striped to determine R value and tolerance, but some 0.5W look the same as the 1W... seems odd to me?
« Last Edit: February 09, 2020, 11:12:15 pm by bob21 »
 

Offline Brumby

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #65 on: February 09, 2020, 11:50:55 pm »
Sounds good.

Having four separate parallel circuits will have one negative aspect: each resistor will be dropping more voltage than with your series/parallel idea which means more power wasted in heat.  BUT for the levels of current involved here, that is not a real concern unless you wanted to max out the number of LEDs you want to hang off this one supply.

There are, however, a number of positives!
 1. There will be no need to match the current draw of pairs of LED strings
 2. If one string fails, then only that string will stop working and you will know which string to check out.  In series connections, an open circuit failure will result in all LEDs in that arm will stop working and you will have to go hunting.  (Brings back memories of chasing a blown globe in a string of mains powered Christmas lights in my teen years.)
 3. If the LEDs get a short circuit, then the resistor (being a larger value) will not get as hot as one would for the series arrangement.  It might still get hot and it might fail if this happens, but it has a better chance of surviving.  Less heat in a failure situation = better safety.
 4. You can add, remove or change LED strings (up to the capacity of the supply) without having to re-work any of the other strings.

As far as other safety considerations, there are two I would suggest:
 1. If the supply has a particular capacity, then set yourself a lower limit for your circuit to use.  For example, with your 300mA capable supply, keep it under 200mA.  Things that aren't pushed to their maximum all the time will last longer - this applies to most things, from power supplies to motor vehicles.
 2. Add a fuse.  The rated current of the fuse should be:
    (a) No more than the rated current of the supply
    (b) Subject to (a), sufficiently high enough to avoid nuisance blowing.  For your setup, I might suggest a 250mA rating.
   Also, for other characteristics:
    (i) Fast blow is fine for LEDs
    (ii) Any fuse type will be suitable for the voltages and currents involved here.  No need for ceramic or anything fancy - a basic glass M205 would be absolutely fine.
 
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Offline Brumby

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #66 on: February 10, 2020, 12:04:56 am »
A bit off topic now, but what is the best way to determine the wattage of these resistors? If they weren't written on the bag, I wouldn't know. I used a colour calc to confirm tolerance and R rating, but how to reliably know what wattage? Let's say they all get mixed up. I can use the coloured striped to determine R value and tolerance, but some 0.5W look the same as the 1W... seems odd to me?
That is often a challenge even for those of us who've been playing with them for years.  If you have been working with some particular types a lot, you might be able to identify their power rating purely from experience.  Most of us aren't that well versed.

Physical size is just one factor.  The materials from which they are made is another.  It is a well known fact that you can have a half watt resistor that is smaller than a quarter watt resistor.  If you do the math, you will find out that the smaller half watt resistor will, at the limit of its rating, produce twice the heat of a quarter watt resistor - and when this happens in a smaller package, the temperature goes up significantly higher.  Here's where the materials matter - where the smaller resistor is made from materials than can withstand this temperature.
 
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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #67 on: February 10, 2020, 05:07:28 pm »
Another limiting factor to the power dissipation of a resistor is the surrounding materials, such as the PCB, solder and any nearby components. A high temperature ceramic resistor might happily be able to sit at 300oC, but it's no good if it melts the solder, falls off the board and burns a hole in the plastic case!
 
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Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #68 on: February 10, 2020, 08:24:10 pm »
Ok, understood. Thanks again both for the information. I feel like I am starting to understand more and more of this.

So the resistors seem fine as they are, but I will bear heat dissipation in mind for any future projects.

I will also do as you suggest Brumby and add an M205 glass fuse to the PSU wire (pre-ABS Box) as an additional safety component. Just to be clear, do you mean this:

925568-0

It seems to suggest the voltage can go up to 250V, but I am guessing the important part here is the mA of 250? One of these in a basic fuse holder solder in series to the positive wire will do the job?

That’s an au site, but CPC have this, same thing? https://cpc.farnell.com/multicomp/mc000893/fuse-5x20mm-glass-quick-blow-250ma/dp/FF03065?st=250ma%20fuse

Thanks to all for the time spent helping this n00b out!
 

Offline Brumby

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #69 on: February 10, 2020, 10:01:12 pm »
Another limiting factor to the power dissipation of a resistor is the surrounding materials, such as the PCB, solder and any nearby components. A high temperature ceramic resistor might happily be able to sit at 300oC, but it's no good if it melts the solder, falls off the board and burns a hole in the plastic case!
Yeah ... that would be undesirable.

It seems to suggest the voltage can go up to 250V, but I am guessing the important part here is the mA of 250?
Absolutely correct.  The voltage is a maximum rating.  The figure of 250V means it can be used as a mains fuse pretty much anywhere in the world ... or at any voltages lower.

A common glass fuse is fine for the power levels involved here - all the way up to domestic mains power situations for a lot of equipment.  Ceramic and high rupture capacity (HRC) fuses are complete overkill here, but if that's all you had access to, then one of those would be OK.

Quote
One of these in a basic fuse holder solder in series to the positive wire will do the job?
Yes.
 
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Offline Brumby

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #70 on: February 10, 2020, 10:24:44 pm »
To be fair - so that you don't get too complacent with your knowledge of fuses - there is a whole industry that is built around fuses.  It does become quite involved, especially when you get into high current and high energy circuits (yes, they are different things).  Do a Google search on "types of fuse" or something like that when you have a couple of days to get totally lost.  I would also suggest you check out the challenges breaking DC circuits (particularly high voltage) as against AC ones.

Fortunately, for the vast majority of situations you will find yourself in as a hobbyist, this is one example of a table that covers the basics:


It is possible you could encounter ceramic and HRC fuses (in a multimeter, for example), so being aware of those is useful.

Just one more word of caution ... if you come across any fuse that is thicker than your average cheapie ball-point pen, don't mess around with it unless you are absolutely certain of what you are dealing with.  I'm not just talking about the equipment being protected, but also, the capability of the power source, for example: how far a spark could jump and how big a spanner bus bar* it could vaporise.

* Edit: Change automotive reference to a power systems one.
« Last Edit: February 10, 2020, 10:59:50 pm by Brumby »
 
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Offline bob21Topic starter

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Re: Hooking up 4 x 3v LED string lights to a 6v PSU
« Reply #71 on: February 12, 2020, 06:45:27 pm »
Understood. Thank you for the additional information. I have some fuses on order. I've never seen a fuse larger than a standard UK plug, I wouldn't be comfortable playing with anything that large anyway. Sounds to me like high voltage AC stuff, which isn't my cup of tea. :) I will keep that image for reference though.

Thanks guys :)
 


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