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I have bought some of the following LEDs (going to try either for brightness/size):

I wish to arrange them in three groups of five as shown in the attached block diagram. The idea is there would be three separate strands going off in three directions, each with five LEDs on.

What would be the best arrangement to connect in this way (series or parallel) and also what is a suitable value for the current-limiting resistor for these LEDs, how many and where should they be placed? Also what would be a suitable battery to use - should I go for a PP3 or will a couple of AA's be suitable?

I would appreciate a circuit diagram if someone can help please!

Many thanks

enter image description here

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  • \$\begingroup\$ What voltage is your battery? \$\endgroup\$ – WhatRoughBeast May 11 '16 at 23:06
  • \$\begingroup\$ @WhatRoughBeast He doesn't know. He's inquiring about the battery too. \$\endgroup\$ – Nick Alexeev May 11 '16 at 23:11
  • \$\begingroup\$ As above, not chosen battery - will take advice. I have got some 28SWG enamelled copper wire to connect the LEDs too \$\endgroup\$ – Ian May 11 '16 at 23:18
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Well, first, having just one big resistor will work the same as several small resistors. Next, parallel leds need more current (bigger batteries and lower value resistors) while series strings need more voltage (higher voltage batteries and higher value resistors) so you're going to have to decide there as you could go either way. You'll need to know what voltages your leds need and get a battery slightly higher than that (the closer you can get the led - or led string - voltage to your battery voltage, the less energy gets burned off as heat in the resistor). E.g. a bunch of parallel 2.3V leds could be run of a 3V battery with only a small resistor, while a series string of 3x2.3V leds would need a battery of 7.5V or more (5x1.5V) but may still only need a small resistor as the voltage of the led string is still only a little bit lower than the battery voltage. just be aware, not all leds are created equal, some red leds only need 1.5V or so to run, greens can be 2-3V, blues and whites can be 3-4V and some uv leds need over 5V, so just keep in mind that what works with one led might not work with another (3x1.5V leds need 4.5V but so might a 1.5V led in series with a 3V led). Thankfully there are lots of led resistor calculators on the internet to help you out.

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  • \$\begingroup\$ Hi Tom, many thanks for your comprehensive answer. Regarding the resistor calculators I have used them before but the calculation is complicated slightly as I have three 'branches'. Should I treat them as the separate circuits when choosing a current limiting resistor? \$\endgroup\$ – Ian May 14 '16 at 6:54
  • \$\begingroup\$ You can either treat them separate and give each branch it's own resistor or you can run three strings in parallel and use a resistor with 1/3 the resistance, but using one resistor per string will give you a more even brightness, leds all vary a bit (batch to batch, temperature etc) so they don't always share when put directly in parallel so it'd still work, just maybe not as well as having a seperate resistor for each branch (those 100W leds you see on eBay are 10 strings of ten leds in parallel with no resistors and at low currents, different leds glow at different brightnesses) \$\endgroup\$ – Sam May 14 '16 at 23:00

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