Cute little oscillator, but theory of operation? - Page 1
Forum section: electronics / general

author: eti
author_rank: Super Contributor
timestamp: December 24, 2021, 10:01:57 am
content: Found this nice little oscillator (boost conv?) online, and it works a treat; I’ve got it running TWO white 2.5v LEDs in series from one “AA” battery… but I’m at a loss as to the complete theory of operation. Clearly the field collapse across L is causing the voltage boost, but I can’t work out why it remains in oscillation.If anyone could de-muddy the waters, I’d be indebted to them. Many thanks.

author: eti
author_rank: Super Contributor
timestamp: December 24, 2021, 07:55:05 pm
content: Oddly (or NOT so oddly), it doesn't oscillate in LTSpice OR in "Every Circuit"; surprise surprise...

author: KE5FX
author_rank: Super Contributor
timestamp: December 24, 2021, 09:51:54 pm
content: Quote from: eti on December 24, 2021, 07:55:05 pmYou may not have let it run long enough. Try adding a .startup directive.

author: Bud
author_rank: Super Contributor
timestamp: December 24, 2021, 10:08:38 pm
content: Don't assume simulators always work. They do not. Years ago i was simulating a _simple_ RF mixer. Out of 4 simulators only one produced proper result. I recall it was Agilent ADS.

author: amyk
author_rank: Super Contributor
timestamp: December 25, 2021, 01:13:18 am
content: It's a simple blocking oscillator. When power is first turned on, the two transistors start conducting and the current through the coil increases, while this is happening the top is positive and through the feedback capacitor keeps the transistors on. Once it saturates, the potential difference across it is no longer enough to keep them on, and that results in the transistors turning off, causing the field in the inductor to collapse and pull current through the LED (it is now in series with the inductor and the power source, so you get a boost in voltage). Since its top is now negative, it keeps the transistors off. The transistors start turning on again once the energy has dissipated, and the cycle repeats.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 25, 2021, 01:20:23 am
content: Seems to run fine in LTSpice if and only if I add that optional cap (I arbitrability picked 1nF, edit: but as little as 0.1pF seems to work) where the author has it drawn, then it fires up fine with no tricks.

author: rfclown
author_rank: Frequent Contributor
timestamp: December 25, 2021, 02:19:11 am
content: A single AA battery is less than 3 diode drops so both transistors don't initially conduct. With a 1.5V supply the PNP is almost always conducting, but the NPN cycles on and off. Circuit doesn't work well with two cells.

author: eti
author_rank: Super Contributor
timestamp: December 25, 2021, 02:38:36 am
content: Quote from: KE5FX on December 24, 2021, 09:51:54 pmAh righty ho, now it works! Thanks ever so much and Happy Christmas 🎄

author: eti
author_rank: Super Contributor
timestamp: December 25, 2021, 02:39:52 am
content: Quote from: amyk on December 25, 2021, 01:13:18 amThank you, and thank you to everyone else. God bless you Happy 🎄 Christmas

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 25, 2021, 02:55:23 am
content: Quote from: rfclown on December 25, 2021, 02:19:11 amThe "duty cycle" depends a lot on the supply voltage. Beyond a certain voltage, it won't work properly indeed. And then below a certain voltage, it won't work either. It would probably not work right for the whole life of a typical AA alkaline battery, and would just hardly start with a NiCd battery, I think.

author: eti
author_rank: Super Contributor
timestamp: December 25, 2021, 03:03:43 am
content: Quote from: SiliconWizard on December 25, 2021, 02:55:23 amStrangely someone had it working at 0.2-0.3v!!!

author: ledtester
author_rank: Super Contributor
timestamp: December 25, 2021, 03:48:25 am
content: Another well-known blocking oscillator design is the "Joule Thief".This guy has been running one continuously for over five years:Blinking Joule Thief: 5 Year Update -- Eman2000https://youtu.be/AWDX3L07qyc

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 25, 2021, 07:12:13 pm
content: Quote from: eti on December 25, 2021, 03:03:43 amI'd be curious to see that, and even more curious to get an explanation. (Oh and if you haven't actually witnessed it yourself, don't believe everything people say...)Unless maybe this was done with hand-selected germanium transistors.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2021, 07:59:28 pm
content: Quote from: rfclown on December 25, 2021, 02:19:11 amWith 1.5 volts from an AA cell divided three ways to place 0.5 volts across the emitters, they do conduct a little bit which is enough to start operation.