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

ledparallelswitchingtransistors

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!

Best Answer

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.