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LED circuits - Help and advice.


Davidrebolton

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4 hours ago, Cheshiretaurus said:

The 450mA supply I mentioned was an example, What you actually need would depend on your application. Whats the current of the LEDs you will use have you got a link to them?

Hey,

 

Ah sorry. This is taken from the data sheet. I presume this is what you ment?

 

Forward Voltage / Current: 3.2V-3.4V

 

Thanks matey 👍

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That's not relevant. The datasheet will give you the peak value for the current they can support. An LED will draw as much current as you feed it through the resistor. Exceeding the supported current will cause the LED to "burn".

 

The math you need to do will be solely based on how bright you want them to be, which is why I suggested you play around with resistor values and grouping the LEDs. When you're happy with the brightness you can do the math and figure out how big the power supply needs to be.

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57 minutes ago, Cheshiretaurus said:

Thanks, does it say how many mA they draw?

Yes it says 20mA

 

Thank you 

38 minutes ago, bmwh548 said:

That's not relevant. The datasheet will give you the peak value for the current they can support. An LED will draw as much current as you feed it through the resistor. Exceeding the supported current will cause the LED to "burn".

 

The math you need to do will be solely based on how bright you want them to be, which is why I suggested you play around with resistor values and grouping the LEDs. When you're happy with the brightness you can do the math and figure out how big the power supply needs to be.

So there is quite a bit of leway so long as its not obviously to stressed or underpowered? And when adding another current of another circuit, the resistors will again need to be played with for the brightness required- including subsequent Led's or circuits? 

 

Cheers 

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I would just get a bog standard 5V phone charger that can "supply" 1A. It should far exceed your needs. Years ago I used a 350mA charger to power up the LEDs in my model cars for a show. The thing stayed on for 3 days in a row and it wasn't even warm.

The resistors don't need to be played with again unless:

1- you change the power supply and the new one has a different voltage

2- you add/remove LEDs from a group that was in series with the resistor. A group of LEDs with it's own resistor won't be affected by you adding another group (with it's own resistor) in parallel. Like the schematic one of our colleagues drew for you on the first page.

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15 hours ago, bmwh548 said:

I would just get a bog standard 5V phone charger that can "supply" 1A. It should far exceed your needs.

 

This would be perfect for standard LEDs however its 5v so wont supply the 9v or more required for your disco LED but still good as a starting point for simple circuits

 

Ive found an online LED resistor calculator that you might find useful. https://ohmslawcalculator.com/led-resistor-calculator

 

Did you say that your orange LEDs are in groups of 7 (7x20mA)? if so with 3.3v x 140mA = 0.462watts, in LED terms thats going to be super bright, so probably no need to go anywhere near full power with them so that will help reduce the power requirement.

 

 

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On 3/11/2021 at 2:52 PM, Davidrebolton said:

I see. I had found this one https://www.ebay.co.uk/itm/164630867449

But going from what you say I see its only rated at 300mA. Would you be able to point me in the direction of which I require with a link 🤔 - if you don't mind. 


You need to consider how many and the configuration of the LEDs you are driving.  The current of the PSU must exceed the total on current of all the LEDs.  For example if your circuit has 30 LEDs on at a time then you will need at least 300mA so you should choose a supply that meets that plus a little headroom.

 

But voltage is important too.  If your LEDs are all configured as singles in parallel then you only need a 5V supply.  Anything more will be dropped across the current limiting resistors and wasted as heat - uf you are dropping a lot of voltage across the resistors then you might even need to consider their power rating.

 

If using higher voltage supplies you want to consider identifying LEDs that are always lit at the same time and wiring those in series.  Consider this example

 

You have a 12v supply and five LEDs.  
 

If you wire them singly in parallel then the current through the resistor is 50mA and the voltage across  the resistor is 10V.  You would need a (10/0.05) 200 ohm resistor.  The power wasted across the resistor is (10 x 0.05) 0.5W which is too much for the miniature resistors which tend to be 0.25W or 0.125W - so you will need a bigger resistor and it will get hot.

 

But if I wire those 5 LEDs in series then the total drop across them is 10V.  This leaves just (12-10) 2V across the current limiting resistor.  And the current is just 10mA - it is the same 10mA that flows through each of the LEDs.  So the resistor needed is (2/0.01) 200 ohms but the power in that resistor is just (2 x 0.01) 0.02W that is 25 times less than the first configuration.  You can use a miniature resistor and it will stay nice and cool.

 

And that is how those strips works.  That light transformer you listed - I suspect that is intended for running strip lights under kitchen cabinets.  They will be white LEDs so have a higher operating voltage then the red, orange, green types (more like 3.5V compared with 2V) so will be designed to drive sets of three LEDs configured in series.  Of course there will be more LEDs than that in the strip, so what they do is to configure them in a hybrid series/parallel arrangement.  Let’s say there are 24 LEDs, they might be configured as 8 sets of 3 in parallel.  You win’t be obvious because they will be in a strip, but that is what the wiring will be like inside the strip.

 

Cheers,

 

Nigel

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Keep in mind that in the series arrangement if a single LED fails, none will light up. I would stick to the parallel arrangement but I would do the resistors in parallel too. Say you need a 200 ohms resistor, you hook up two 400 ohms resistors in parallel (can't remember the standard closest value you'll find but I think it's like 460 ohms). That means the current will be shared equally on the resistors so there's less heat. If you have space in the base you can hide the resistors there so you can check on them and swap them for bigger ones if needed (1W resistors will still be reasonably small).

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