Joule Thief: Supplement: Photo Transistor NTE3037 Response Linearity Test
In order to determine the light output curves for JTs, one needs some kind of reliable and consistent detector system to measure the brightness level, and thus the light power output, of the JT’s output LED.
I’ve just begun evaluating the NPN phototransistor NTE3037 for this purpose. It’s in a TO-18 case and has a little lens and is pretty narrow-angle view.
Vcbo is 50 V, dark current at Vce=30 is 200 nA and the response time is 2 microseconds typical.
I have it wired like this: collector to regulated, current limited PS at +26V. Base not used. Emitter to 330R to PS ground. Signal is across the 330R.
In this video I show its response to the light output of a bright white LED that is being driven by the F43 Function Generator using a triangle wave and a sine wave, at around 3 kHz. The phototransistor’s response is quite acceptably linear. It will take a bit more fiddling about to get to a real light intensity measurement system but at least the phototransistor seems up to the task.