![]() ![]() Q1 is used as an over-current sensing switch, and R3 is the "sense resistor" or "set resistor" that triggers Q1 when too much current is flowing. Circuit function: - Q2 is used as a variable resistor. With no heat sinks at all, Q2 can only dissipate about 1/2 watt before getting really hot - that's enough for a 200mA current with up to 3-volt difference between power supply and LED. If you want more, look at my Instructable on LED circuits to see how the circuit needs to change. The Q2 transistor specified will work up to about 18V power supply. with a large heatsink, this circuit can handle a LOT of power. so Q2 will need a heatsink if there is a high LED current or if the power source voltage is a lot higher than the LED string voltage. Q2 acts as a variable resistor, stepping down the voltage from the power supply to match the need of the LED's. Specifications: input voltage: 2V to 18V output voltage: up to 0.5V less than the input voltage (0.5V dropout) current: 20 amps + with a large heatsink Maximum limits: the only real limit to the current source is Q2, and the power source used. Here i'll explain how the circuit works, and what the maximum limits are, you can skip this if you want. the LED's are from Future electronics, their pricing ($3 per LED) is far better than anyone else currently. where to get the parts: all the parts except the LED's are available from, you can search for the part numbers given. R3 power is 0.1 watt, so a standard 1/4 watt resistor is fine. Calculations: - LED current is set by R3, it is approximately equal to: 0.5 / R3 - R3 power: the power dissipated by the resistor is approximately: 0.25 / R3 I set the LED current to 225mA by using R3 of 2.2 ohms. selecting R3: The circuit is a constant-current source, the value of R3 sets the current. prototyping-boards: i didn't use a proto-board initially, but i built a second one after on a proto-board, there's some photos of that at the end if you want to use a proto-board. for more current you need to put the LED and Q2 on a heatsink (see my notes in other power-led instructables i've done). that limits us to about 200mA LED current. heat sinks: here i'm building a simple light with no heatsink at all. that's why this circuit is convenient! you can use a wide variety of power sources and it will always light up exactly the same. to power a single LED anything between 4 and 6 volts with enough current will be fine. ![]() Circuit parts (refer to the schematic diagram) R1: approximately 100k-ohm resistor (such as: Yageo CFR-25JB series) R3: current set resistor - see below Q1: small NPN transistor (such as: Fairchild 2N5088BU) Q2: large N-channel FET (such as: Fairchild FQP50N06L) LED: power LED (such as: Luxeon 1-watt white star LXHL-MWEC) Other parts: power source: I used an old "wall wart" transformer, or you could use batteries. ![]()
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