confirmation of theory (LED) - true beginner

[daealc]

Hello all,

I am new to the world of electronics but am planning to make my own LED lighting for my desk.

I believe I have the calculations and math right I just wanted a quick check.

the LED's I am using have a forward drop of 3.4V (3.3-3.5 permitted) they are here:
https://proto-pic.co.uk/led-super-bright-blue-100-pack/

They have a load of 20mA (peak 30mA) recommended: 16-18mA

I was planning on using this power supply: https://proto-pic.co.uk/breadboard-power-supply-usb-5v-3-3v/
Supplied via: https://proto-pic.co.uk/usb-power-supply-adaptor-uk-plug-1-2a/

For the connection between the two I was going to use:
https://proto-pic.co.uk/breadboard-power-supply-usb-5v-3-3v/

am I correct in thinking that as the power supply can supply 500mA and I will be using 18mA per LED (ill use 20 in the calculation as to leave some room) I will be able to sodler the above components together an it should be able to run 25 LED's?

 

[tusemo]

Use 300 ohm resistance for each led, than yes you are right your power supply can drive 25 leds

 

[daealc]

Use 300 ohm resistance for each led, than yes you are right your power supply can drive 25 leds

Just shopping for the parts when I stumbled over something slightly strange... there appears to be no 300 Ohm resistors or atl east from where I am purchasing: https://proto-pic.co.uk/categories/Components/Resistors/

The closest I can find is 330: https://proto-pic.co.uk/33-ohm-1-4-watt-pth/

Or should I use two of these in serial before each led: https://proto-pic.co.uk/150-ohm-1-4-watt-pth/

 

[betwixt]

I would urge caution! The LEDs Vf is 3.3V to 3.5V but the power source is only 3.3V. Some LEDs may not operate at full brightness.

It would make more sense to forget the 3.3V regulator and run the LEDs stright from the 5V output of the power adapter. The resistors, assuming 20mA current should be (R = V/I) (5 - 3.4V)/0.02 = 80 Ohms. I took 3.4V as being the average Vf. The nearest standard value resistor is 82 Ohms which gives a range of currents between 18.3mA and 20.7mA.

Brian.

 

[daealc]

Yes after some thought about the design and use of the led arrays on ##electronics (freenode) its been pointed out that I should just use one of my old computers mini-itx (ATX) power units and use te 5V rails from that and use larger modular lighting arrays.

The reason I want 'rectangles' of leds is for my desk, but I wanted it so that they could be hinged so for instance with 3 arrays I could make a triangle, however I also ahve a friend who is a DJ would like to rig upto 10 in a row like a semi circle for around his decks. and or over his decks (the physical support for his plans are not my problem! ) but it be nice to be able to make them fully modular; so I can make each 'array' all yellow or all blue and he can pick and choose what he wants.

Initially this would have meant a USB cable running to each array which could be a pain, how would you suggest I made it so that each 'rectangle' had a power in and power out to allow them to be chainable, what other considerations should I start thinking about?

-- Thank you for the help
-- paul

 

[betwixt]

Easy peasy...

Stay with 5V, wherever it comes from. Be careful with computer supplies though as they sometimes have MINIMUM load ratings and wont work unless sufficient current is drawn from them. You would also have to link the auxlliary 5V and start-up wires together as these power units are normally turned on or off by the motherboard.

To make them modular, you 'daisy chain' them. That is, you have two power connectors which are linked directly together so what goes in on one comes out on the other, this lets you carry the power from one array to the next by linking them with a power cable. You tap into the power at each module with a resistor and LED for each light you want. Don't be tempted to use one resistor and several LEDS, it works but isn't reliable, instead use a resistor in series with each of the LEDs. A typical ATX supply should be able to run about 1,000 LEDs but obviously it varies from one model to another. I would suggest you polarize the connectors as reversing the voltage on an LED can kill it instantly. Alternately, you can add a series diode in each module to block reverse polarity connection. It doesn't make it reversible but it does prevent damage if connected backwards. Take note of the total current, remember that as you extend the chain, the first in line (nearest the PSU) is carrying all the current of downstream modules as well as it's own. Small power connectors may not be able to carry high currents.

Brian.