Chuck D. Bones
Circuit Wizard
I was running some breadboard tests with a soft clipper using green LEDs. The circuit was basically the gain stage from a Tube Screamer. With 5mm super bright green LEDs installed, the opamp (TL071) was saturating under high-drive conditions. I removed the LEDs and tested them on my CCTT (Cheap Chinese Transistor Tester). The Vf (measured at around 5mA) was 2.74V, very high for a green LED. Those were from EG, unknown mfgr. Then I tried some 5mm super bright green LEDs from Tayda. They measured 2.44V, also high for green. After that I tried a 3mm and a 5mm "regular brightness" LED. They measured 1.97V and 2.00V. Those measurements are what I would expect for a green LED. So why were the super brights higher Vf? Turns out that making high brightness green LEDs has been challenging, so the LED manufacturers resort to a technique called phosphor down-converting. They make a blue LED and coat the die with phosphors that absorb the blue light and re-emit green light. This same technique is used to make white, aqua and pink LEDs. Different phosphors emit different colors. That explains the higher Vf; the diode part of the super bright green LED is blue, not green. I replaced the super bright LEDs with 3mm diffuse green LEDs and everything was good. To get a little more headroom, I replaced the TL071 with a CA3130.