LEDs in Parallel; Which Transistor, and do I Need Resistors?

Question

perhaps you can help out.

So, I'm making a prop replica and light is a big part of it. To this end, I picked up three cheap LED torches, which have arrays of 9 LEDs in each. Cheaper than I could buy the LEDs separately!

Firstly, to power them I was thinking I'd use a transistor to switch them on and off, as the microcontroller couldn't handle the load if it was to switch them directly. Can you recommend a transistor that'll happily switch enough current for the 27 white LEDs when I give it a low-level signal from the microcontroller?

Secondly, as I dismantle the torches I notice they don't have any resistors in series with the LEDs. The LEDs themselves are connected in parallel, and run from three batteries (providing 4.5V) and I was thinking I'd connect the three arrays also in parallel -- so 27 LEDs in parallel, effectively.

I'm using a 5V power supply for all this, and I'm thinking the extra half volt shouldn't worry the LEDs too much, right? But do I need to add any series resistors in here, given that there weren't any in the torches?

Thanks in advance for all your advice!

Answer 1

The torches are apparently relying on the internal resistance of the LEDs and the battery to limit the current. Typical voltage drop across a single LED junction itself is usually less than 1.5V, but I've seen "single" LEDs that have a number of individual diodes inside to make up a given voltage drop, which may be what is going on.

Without measuring, it is difficult to guess the current drawn when the LEDs will be connected to your 5V supply, Typically I run them with at least 10-20 mA to light them up to a decent brightness.

As for a transistor, any mosfet with a "logic level" gate drive that can handle enough current would be suitable. One example is AOT460, which turns on when the voltage applied from gate to source is +2.2V or more. Cost $1.63 At Digikey, I would buy a few just in case.

Or search Digikey for "FETs - Single" then restrict the results based on Vgs. Make sure the "drain to source voltage" is sufficient, as well as . The power dissipated in the mosfet will be (Rdson)*I^2, but I expect any of the ones that have a few mOhm for Rdson will not heat up too much. When you search make sure to click the "in stock" check box.

Answer 2

The cheap LED torches that have an array of LEDs in parallel rely on the fact that the forward voltage drop of the LEDs is not too much different from the battery voltage and that the batteries used are small (typically AAA type) with a fairly high internal resistance. Both these factors lead to a limit of how much current can be shoved through the LEDs.

The behavior of this type of torch is less than ideal as the battery voltage decays as they wear down. At some point the battery voltage begins to get down to the forward voltage drop of some of the LESs and they will dim in comparison to the others. Further decay of the battery voltage will show some LEDs failing to light at all and some may flicker as they sit right on the edge of being on or off.

The construction and design of this type of LED torch has almost nothing to do with the quality, lifetime or reliability of the product. It as everything to do with cost to manufacture!! So just because these LED arrays are designed as they are is no justification to extend the concept to other usage scenarios.

You would not want to take multiple of the LED arrays and simply parallel them and connect to a higher voltage supply. Especially a supply that had quite different output impedance characteristics than a set of 3-AAA batteries. As a bare minimum you should put a suitable small resistor in series with each module array to limit the current to that module. But at the same time be prepared for the array of LEDs to not all show uniform brightness.