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

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Brumby:
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.

Brumby:

--- Quote from: bob21 on February 09, 2020, 10:07:16 pm ---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?

--- End quote ---
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.

Zero999:
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!

bob21:
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:



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!

Brumby:

--- Quote from: Zero999 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!

--- End quote ---
Yeah ... that would be undesirable.


--- Quote from: bob21 on February 10, 2020, 08:24:10 pm ---It seems to suggest the voltage can go up to 250V, but I am guessing the important part here is the mA of 250?
--- End quote ---
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?

--- End quote ---
Yes.

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