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Voltage regulator or resistors to manage LEDs? Answered

I want to run a load of LEDs mounted onto a PCB. This is simple enough.
I'd like to know what's best, using LEDs with resistors (with a little help from http://led.linear1.org/led.wiz)
Or using a voltage converter (with a little help from http://www.reuk.co.uk/LM317-Voltage-Calculator.htm), and doing away with resistors

Source voltage will be either 4.8 (4 NiMH AAs) or 12V (old wall transformers), LEDs will be white ones (3.3v)



Best Answer 8 years ago

I don't understand the question. You say "a load" - how many? And what LEDs, in what arrangement etc..

Can you tell us what you've got V & Qty, and what arrangement you're after?
Custom board?


On the 4.8v circuit, if I use resistors, it'll be 3.3v 20ma LEDs, each wired in parallel, each with their own 82 ohm resistor. There may be around 30 LEDs, the whole circuit should draw 600mah.

On the 12v circuit, using the same LEDs, (10 groups of) 3 LEDs in series with a 120mah resistor in each 3 LED block,

Right, 30 off a 4.8V supply. If you can get one power-resistor, it'd be better than having 30. All-parallel.
The 12V sounds quite right,
Do you want a custom PCB?

I don't have access to any PCB-making equipment, so it's veroboard/stripboard for now (row upon row).

I'm intrigued at the sound of a power-resistor though, and would like to hear more

What I meant by "power resistor" is essentially a big one, that can take getting a bit hot. E.g. if you've got 3x 3.3 V powered from 12V, you'll have ~2V across the resistors. At 120mA that's 0.24 Watt. But if you had just 1 instead of 10, handling 1.2A it's 2.4 Watt - gets "warm".
There are usually a few in old TVs.



8 years ago

Personally I would go for resistors with the 4.8V or a regulator (like the common LM3805) + resistors in case of the 12V.

Even with a Low Dropout regulator, it would be hard to drive the leds and you wouldn,t be able to use the full potential of your AAs. And in any case: it is allways 'good practice' to use resistors with leds when you are not 100% sure that the won't burn out.

If you use 4.8 volts you'll have to use resistors as I believe there is too much voltage drop in a regulator to still drive the led's.

Using 12 v. I would use  a regulator and resistors or a variable reg.