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

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bob21:
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.

bob21:
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 :)

bob21:
Anyone?

Safe? Unsafe? Ok? Not ok?

Brumby:
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.

bob21:
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?

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