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#090

In this video we take on the LM334z and turn it into a constant current power source for LEDs. But because it can only pump 10mA at most, we need to make it drive a transistor to throw more juice at our LEDs. So we have it control a BC558b, which can do 100mA all day long. One more serious concern with the LM334z is addressed: Temperature Dependency. Since the LM134/334 is essentially a temperature sensor, it is no good in the field where the temperature can go all over the place. So we apply a circuit from the datasheet to make it "Zero Temperature Coefficient". What is nice about the approach I took: 1. LEDs can be added in series with no fuss. 2. The Vcc that supplies the LEDs and the LM334 can vary wildly with no impact on either. 3. Replacing the BC558 (hFE=334) with a MOSFET or more powerful transistor would be just as easy! 4. I used a 1N4148, but any generic diode would do just fine here. 5. Be mindful of the pinouts of your LM334 and the transistor you choose to gate (before you turn on the bench PS!). 6. This will set the current to the LED(s) constant, so don't mix LEDs of different current needs. I'm careful to show the important math necessary at the beginning without it getting too dry. I try to not repeat calculations available elsewhere. Certain things of note: 1. To start, you will need to know: your transistor's hFE (gain ratio). 2. Your LED's current that you wish to drive it at. 3. I(CTRL) on the base of the transistor = mA (LED)/hFE 4. 80 deg F = 300,000 deg Kelvin 5. W = 227uV x 300,000 6. R1 = W/I(CTRL) --- the current at the transistor base & also through the LM334 half of the circuit. 7. R2 ~= 10 x R1 8. Using [trim] pots for R1 & R2 once you're close enough on paper is essential. Beware of sites that have videos/drawings of doing this. Instead of trusting everyone, I did all the behind the scenes math myself, and found that these sites have severe errors that could burn out your parts if you follow them blindly. I recommend you do the math (start at the LM334 datasheet) yourself first.