robwhizz photoblog

I’ve been really busy just lately and haven’t done any building of the ringlight. I have tested the light spread (above) and drawn up some scale blueprints (below).

With a light spread of only 20deg, I’m not totally happy with the angle that the LEDs need to be. The norm is around 30deg so I’m going to test that again, just to make sure.
The bulk of the LED has a smaller diameter than it’s base, about 5mm. I’m going to be running 2 rows of LEDs. With the tilt of the LED I shouldn’t have any problem getting 99 LEDs in two rows. 50 on the outside and 49 inside.
I need to have as high a voltage as I can get. I wanted to build a high efficiency step up transformer, so I could use 1.2V AA rechargeables. Unfortunately the cost of the components was too high for this budget build. So I plumped for using 9V batteries.
4 x 9V wouldn’t be too bulky and would give me 36V to play with. I don’t want to use an external battery pack that plugs in as it’s just something else to carry around and connect up. I’d need an unfeasible amount of AAs to get anything like 36V.
The reason to keep the voltage high is one of heat and efficiency. My LEDs have a rated forward voltage of 3.0 – 3.2V. In testing the variation between different LEDs was very small, and 3.1V was the average.
To power 1 LED is a very simple affair. You use a voltage higher than the forward voltage of the LED and calculate the resistor needed to control the current you want to pass through it.

R = (Vs – Vf) / I

Vs is the voltage from the power source.
Vf is the forward voltage of the LED.
I is the current you want running through the LED.
R is resistance.

Powering lots of LEDs becomes very problematic in terms of wiring and design. The simplest way on paper is to hook each up the LEDs in parallel. That means each LED connects directly to the power source. That’s 2 wires and a resistor for each LED. That adds a lot of complexity and design issues, space for the wiring – a lot of wiring – and space for over 100 resistors. It’s also very wasteful in terms of power.
What is needed is to run several series parallel arrays. Wiring in series is useful because you can drive as many LEDs as you like as long as their cumulative forward voltage is lower than the source voltage, and you only need one resistor for the whole array. This cuts down on wiring and makes the circuit more efficient with less resistors needed.

So, with 36V, I can run 11 LEDs in series:
R = (Vs – Vf) / I
Vf = 3.1 x 11 = 34.1V
36V – 34.1V = 1.9V
I want to push 20mA through the LEDs. So 1.9 / 0.02 = 95 Ohms.
A 100 Ohm resistor will be perfect.

I can then build an array of these series groups running in parallel. I can pretty much run run as many LEDs as I like in multiples of 11 as long as my power source can cope with the current draw.

I’m looking to run 99 LEDs with a current draw of 180mA. That’s a high draw for a 9V but I think I’ll just be OK. I’ll just have to suck it and see. If I can’t get a decent run out of the batteries, then I’ll consider using 3 x 12V packs that can cope with high draw and use a belt clip or something similar.
I’ve pushed a single 9V to 160mA with no problems what so ever. I think I’ll be OK with the draw, but I doubt they will last for long!

Only thing left to do now is build the bloody thing …

… although, I’m already thinking about running 3 rings of LEDs …