12 V to 3 V Question

[xenobie]

Hi all,

Sorry for a simple question but I need to be sure. I have a 12V 2A power supply (thus making it 24W). I need to power some LEDs which take 3V 1A (thus making each led 3W).

Can I use a 7803 with heatsink for each LED with this power supply (upto a max of 8 LEDS? 3 x 8 = 24W)

Thanks

 

[Hero999]

I've never seen an LM7803 before. I assume you're talking about a linear regulator, like the LM7812 but set to 3V.

This will not work for a couple of reasons:

LEDs are designed to work from a constant current source, not constant voltage. You need a current regulator, rather than a voltage regulator.

A linear regulator is very wasteful, the input current is the same as the output current so connecting 8 linear regulators, each drawing 1A, will draw 8A in total: a 3V linear regulator wastes 75% of the power supplied to it.

You ideally need a switched mode constant current regulator to power the LEDs.

You need a more powerful power supply. Although a switched mode power supply is typically 80% to 95% efficient so most of the input power is delivered to the load, you still need to make up for the 5% to 20% power loss so you need a power supply capable of between 25.2W to 28.8W.

I assume you're powering this from the mains? If so your best bet is to buy a proper mains powered LED driver.

Failing that, a crude solution is to connect two strings of three LEDs in series, each with a 3 Ohm resistor to your 12V supply. This is only 75% efficient but it's better than using 3V linear regulators.

 

[xenobie]

Thanks for your reply. Yes I know resistors are current driven but these mammoths are different. When you give them 3v, they draw 1A, but when you give them 3.5, they simply draw too much and burn (obviously because they need a resistor). According to the datasheet, these LED take 3V continuous.

Yes I assumed a 7803 would exist. So a PWM PSU should do the trick than. In that case, I may be able to convert a computer power supply...

 

[Hero999]

Thanks for your reply. Yes I know resistors are current driven but these mammoths are different. When you give them 3v, they draw 1A, but when you give them 3.5, they simply draw too much and burn (obviously because they need a resistor). According to the datasheet, these LED take 3V continuous.

No, all LEDs have pretty similar voltage/current characteristics and drive requirements, unless you count the type with build in resistors or a regulator. The only difference between your LEDs and the standard type is they're much, much bigger and will most probably have a metal back designed to be fitted to a heat sink.

You'll also find that the forward voltage differs widely from one LED to another and is temperature dependant, as the temperature rises the forward voltage will drop. This is why it's undesirable to drive them from a constant voltage supply. Suppose you set the voltage so the current draw is exactly 1A say the voltage is exactly 3V, the LED's temperature will start to rise so the forward voltage will fall, causing the power supply to increase the current, the current will keep rising until the regulator limits it or the LED burns out.

A constant current regulator, keeps the current the same, regardless of the LED voltage so the thermal runaway situation won't occur.

Do you have the data sheet for the LEDs?