Joule Thief: Supplement: More on AC/DC Coupling, Scoposcopy, 2n2222a JT
Here I use the standard Joule Thief circuit, with a 2n2222a metal-can transistor and an inductor with 1:1 winding ratio from an old TV set, to illustrate the use of the AC coupling function of the oscilloscope to visualise and measure the small oscillation sitting on top of a much larger DC voltage. NOTE: The base resistor is 1K, not 600R. Sorry about the miscue! I’m sure 600R would work too. Interestingly…. setting up for this experiment, I found that the AC coupling function on Channel A of my scope was inoperative… there was no change in trace position when switching from AC to DC and back. A moment’s thought produced the likely culprit: a shorted coupling capacitor inside the scope. So I pulled the vertical amplifier — a 30 second job — and took off some shielding plates and got to the switch, unsoldered the 0.1 uF 600 V poly film coupling cap and tested it: sure enough, shorted through. A quick search for a suitable replacement part in my stash, a couple of minutes of soldering and screwdrivering, and we are once again good to go. The entire repair took less than 20 minutes, thanks to the great HP design philosophy. Try that on your favorite DSO when it malfs !!