Appropriate power supply for 3 LEDs

[Fman1234]

Hello!

I'm trying to decide on how to power just three 3.2V 20mA LEDs. I'd like to use a standard AC/DC adaptor that I can plug in to a mains socket at home. I've been reading quite a bit online (https://www.ledsupply.com/) to try to understand exactly what items I'll need. As far as I can tell I can either set up my configuration with:

1) an AC/DC power supply - LED driver - my LEDs

or

2) an AC/DC power supply - a resistor - my LEDs

I'd rather go with the second option as it seems as though it would be considerably cheaper but I'm having such a hard time finding an AC/DC power supply with an appropriately low output current & wattage that I suspect I may be misunderstanding something. Could any kind person help point me in the right direction?

 

[(*steve*)]

If you use a 5V adapter (which are really common) you can connect a 100Ω resistor in series with each LED and then connect each of these to the 5V.

The voltage is low enough that even if you connect the LEDs up backward (they won't light up) they won't be damaged.

 

[Fman1234]

Ah thank you so much Steve! So am I correct to understand that I should use one 100Ω resistor and connect the circuit (in series) as such:

5V AC/DC power supply - 100Ω resistor - LEDs - back to power supply

Should I use a power supply with any particular output current or will the LEDs just draw whatever current they require regardless?

Also from my understanding the voltage required for the circuit should be the sum of all three LEDs [(3.2V)(3)] which would be 9.6V total, could you help me to understand why I would not need to supply the circuit with 9.6V instead of the 5V you suggest? Thanks again for the response!

 

[dave9]

^ No that would be 3 parallel strands with one LED and 100 ohm resistor on each.

They will only draw the amount of current set by the drop across the resistors. You merely need a PSU capable of a little margin over [edit] 60mA (3 * 20mA) [/edit] which in this case is any/all of them.

Since I have a few 5V cellphone chargers, I might do it like that, running off 5V, but it would be more efficient if you used a 12V regulated (switching) AC-DC adapter then put the 3 LEDs in series with one 120 ohm resistor. Resistors are cheaper than PSU so I'd base it on which PSU you have to spare as a few mW efficiency usually isn't worth thinking about when it's coming from AC mains power instead of batteries.

Also note that if you want long life out of them and they are generic LEDs, you might want to run them lower than 20mA, or make sure the LEDs' cathode is short and connected to a large amount of copper to 'sink heat away.

 

[Fman1234]

Dave thank you so much, I feel I have a better grasp on it now. Ye I think I'll probably dig out a 12V adapter and put the LEDs in series instead! Much appreciated!)

 

[Audioguru]

An LED is not an incandescent light bulb with a fixed voltage rating. Instead it is a semiconductor and a "3.2V" LED could be 2.8V to 3.6V. If you want it to operate at 20mA then you must calculate its resistor value properly at its minimum and maximum voltage. If the supply is 5.0V and the LED is actually 2.8V then a 100 ohm resistor will cause a current of (5V - 2.8V)/100 ohms= 22mA which will be fine since the maximum allowed current in the little LED is probably 30mA. If the LED voltage is actually 3.6V then its current with a 5V supply and 100 ohm resistor will be (5V - 3.6V)/100 ohms= 14mA which might appear to be dimmed a little.

Three 2.8V LEDs in series with a 12V supply and a 120 ohm resistor will have a current that is (12V - 8.4V)/120 ohms= 30mA which will make them very hot. If the LEDs are 3.6V then their current will be only 10mA. The supply voltage is too low for three of these LEDs in series. If the supply is 15V then three 2.8V LEDs and a 300 ohm resistor will have a current that is 22mA and if the LEDs are 3.6V the current will be 14mA.

 

[KJ6EAD]

There is a resource here on the site to help as well which includes illustrations and schematics.

https://www.electronicspoint.com/resources/got-a-question-about-driving-leds.5/

 

[dave9]

It seems like the bottom line is that you need to be active in setting this up to measure current before finishing it in place, and if the current is too high or low, adjust the resistor value accordingly.

I would go with a driver if it were 1W or higher LEDs, and might even abandon the idea of using lesser than 1W LEDs with 1W being so cheap these days, unless the idea is spread out low level lighting... or I was just tinkering around with some LEDs I already had. Sometimes it's just a learning experience rather than a time effective lighting solution. eBay has dirt cheap solutions but you won't learn much using them unless you run into problems.

 

[Fman1234]

Hi guys! Thanks for all the help and advice, I read through the resources page suggested and have a good grip on things now too. Got the 3 LEDs working with a 12V power supply and a 120Ω resistor in the end. Very happy!