Welcome to a new Voltlog, in this video I’m gonna walk you through the design process of this small but important piece of circuit which is a T10 LED based lamp. We’ll go through why I need a T10 LED lamp, why I made the various design choices and in the end we will test the resulting PCB to see if it functions according to our specs.
So I started thinking about designing my own T10 LED bulb. First I picked some high quality Cree LEDs that would fit the purpose. These have a forward voltage of 3V, they have a high CRI of over 95 and I can pick from a variety of output colors, after a bit of experimenting, I ended up going with 4000K which is this nice neutral white.
Next I had to figure out how to drive these at constant current and really here I needed to decide between a switch mode drive and a linear driver. A switch mode driver would be more efficient, generating less heat but it will need more parts, which would increase the cost and complexity so in the end I went with a linear driver, which is the AL5809, this is a nice two terminal device, it comes in a variety of current presets and you just place it in series with your LEDs.
There is one constraint though, it needs 2.5V over the LED string voltage to operate correctly. That means that with our 3V rated LEDs, we can place two of those in series, plus the driver, which raises the min voltage to 8.5V. It might be possible to add a third led on board to bring it up to 11.5V and it would still work in the car but installing that 3rd led would be difficult so I decided to stick to just 2 LEDs.