Tests many different types of transistors such as NPN, PNP, FETs, MOSFETs, Thyristors, SCRs, Triacs and many types of diodes. I've also added a photo of the same circuit, but on a proto-board (this one is "Capital US-1008", available at digikey), and with a 0.47-ohm R3.ĮSR Meter / Capacitance / Inductance / Transistor Tester KitĮSR Meter kit is an amazing multimeter that measures ESR values, capacitance (100pF - 20,000uF), inductance, resistance (0.1 Ohm - 20 MOhm), it doen't look like much, but it works really well and is cheap and easy.Īlso, tying the two wires together helps reduce strain on the wires also. just glob on the silicone, and make an effort to get rid of any air bubbles. this will make it mechanically strong and waterproof. an easy way is to put a large blob of silicone glue all over the circuit. assuming it works, we just need to make it durable. You're done! we'll make it less flimsy in the next step. Now connect the negative wire from the power source to pin 1 of Q1. connect the other lead of R3 to pin 1 of Q1 glue R3 to the side of Q2 so it stays in place. it probably would have been easier to do that first actually. attach the positive wire from the battery or power source to the LED-plus wire. solder the other leg of R1 to pin 1 of Q2. solder resistor one leg of resistor R1 to that dangling LED-plus wire this has the added benefit that if Q2 gets very hot, it will cause Q1 to reduce the current limit - a safety feature! So: start by connecting the wire from the LED-negative to pin 2 of Q2įirst, glue Q1 upside-down to the front of Q2 so that it is easier to work with. laying the parts in front of you with the labels up and the pins down, pin 1 is on the left and pin 3 is on the right. i'll just connect the leads of the parts in mid-air! but you can use a small proto-board if you want (see photos at the end for an example).įirst, identify the pins on Q1 and Q2. This circuit is so simple, i'm going to build it without a circuit board. So we've created a "feedback loop", which continuously tracks the current and keeps it exactly at the set point at all times. Turning off Q2 reduces the current through the LED's and R3.
When too much current flows through R3, Q1 will start to turn on, which starts turning off Q2. The main current flow is through the LED's, through Q2, and through R3. 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. 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. The only real limit to the current source is Q2, and the power source used. Output voltage: up to 0.5V less than the input voltage (0.5V dropout) Here i'll explain how the circuit works, and what the maximum limits are, you can skip this if you want. R3 power is 0.1 watt, so a standard 1/4 watt resistor is fine. I set the LED current to 225mA by using R3 of 2.2 ohms. R3 power: the power dissipated by the resistor is approximately: 0.25 / R3 LED current is set by R3, it is approximately equal to: 0.5 / 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. Power source: I used an old "wall wart" transformer, or you could use batteries.
LED: power LED (Luxeon 1-watt white star LXHL-MWEC) Q2: large N-channel FET (Fairchild FQP50N06L) Q1: small NPN transistor (Fairchild 2N5088BU) R1: approximately 100k-ohm resistor (Yageo CFR-25JB series) Circuit parts (refer to the schematic diagram)