Build low power (10W) LED lamps for general illumination.
Step 1: Parts and Tools
2200uF >200v electrolytic capacitor
108 BT30WCS LEDs
Insulated copper wire
2x 5VAC transformer or similar to achieve 1:1 ratio
2x 10 ohm resistor
1x 2.2 kilo ohm resistor
1x 100 kilo ohm resistor
1x 2n2222 NPN BJT
1x TIP48 NPN BJT
1x DF04 bridge rectifier
5x perfboards 3 x 10cm (4 x 1.5 inch)
Step 2: LED Arrays
There are four arrays connected in parallel, each with 27 BT30WCS LEDs connected in series. Each LED has a voltage drop of about 3.2v therefore the whole LED array uses 27 x 3.2v = 87v with a forward current of 30mA.
The four LED arrays in parallel draw 30mA x 4 = 120mA.
Step 3: Circuit. HIGH VOLTAGE!!
CAUTION! HIGH VOLTAGE!
We need about 60VAC to make this circuit work. A custom transformer can be made or, if you are lucky, maybe you find one with this rating. This circuit uses two transformers, first a 120 to 5v (24:1 ratio) transformer followed by a 120 to 9v (13.3:1 ratio) inverted to get a total 120 to 66v (1.8:1 ratio).
So with a 120VAC input we get 5VAC aprox, after the second transformer the voltage is 66VAC. After the rectifier and filter stage the DC voltage is in theory 66 * 1.41 = 93VCD.
The constant current thru transistor T1 will be Ic = 0.7v/R where R = R1||R2. The 0.7v are the theoretical Base-Emitter PN silicon junction voltage drop of the BJT transistor T2. For a current of Ic = 120mA the resistor value is R = 0.7v/120mA = 5.8ohms.
In this case, the transistor B-E junction has a voltage drop of 0.62v so R = 0.62/120mA = 5.16homs, this resistor value is approximated with two 10ohm resistors connected in parallel.
In the schematic, each LED (D1-D4) represents one LED array of previous step.