troubleshooting joule thief - Page 1
Forum section: electronics / beginners

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 12:10:07 am
content: I'm trying to get this circuit going but it's got me perplexed.Basic joule thief oscillator but with a PNP instead of NPN.Pinout is collector, base, emitter.I don't have an oscilloscope. Multimeter works.Tried to model it in Qucs Spice but sim gives a syntax error.

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 01:23:14 am
content: I flipped the diode but it doesn't light.The emitter just goes to a leg on the transformer. Does it matter which one? I wound it symmetrically.

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 01:30:20 am
content: Here's the actual setup.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2016, 01:32:15 am
content: To use all-PNP in place of all-NPN circuit:Substitute all transistors with opposite type.Flip all diodes.Flip all electrolytic capacitors, anything else polarized.Flip supply polarity ("+9V" becomes "-9V", GND remains GND).It's just that simple!Tim

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 01:57:37 am
content: It works!I had every component backwards.I mostly was confused that the battery needs to attach to the emitter.The spice simulation still doesn't work. Any obvious errors from my screencap?

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 02:02:09 am
content: Follow up question; Can i drive a voltage multiplier from this output?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 25, 2016, 02:13:55 am
content: The dot convention on your transformer is wrong I think. The toroid or other core should be wound all in the same sense, and the center tap makes the "bottom" of the secondary connected to the "top" of the primary, if you want to look at it that way. Try reversing the connections of _one_ of the coils in your Spice simulation.

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 03:34:58 am
content: You lost me, care to post a picture? Visually, the schematic looks just like my windings. I tried rotating it 1/4tau but that looks really wrong. Still getting that same error.Code:line 4: syntax error, unexpected Identifier, expecting '"'Edit: Turns out it didn't like my resistor unit KOhm...

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 03:54:17 am
content: So the transient simulation shows no oscillations...that can't be right.Two steps forward, one step back. O0

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 25, 2016, 04:39:43 am
content: I'm telling you, I think you have the dot convention on your coils wrong. The dot indicates the "start" of the winding. Since you are trying for a plain centertapped coil set, the center tap should join the END of one winding to the START of the other winding. The way you have it, you have the center tap connecting the STARTs of both windings.Look at my LTSpice simulation below. The _only_ difference between the two schematics is the way I have the coils oriented wrt the dot convention. In the working version (Righthand panes) the center tap connects the dotted end of one coil with the non-dotted end of the other coil. One way works, the other way shown in the LH panes (the way you have it) does not.So... reverse the connections of one of your coils and see if it starts working. This is something that all JT builders know: If it doesn't work at first, reverse one of the coil connections.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 25, 2016, 04:52:20 am
content: What I mean is.... try it this way.

author: moonquasar
author_rank: Contributor
timestamp: August 25, 2016, 05:11:11 am
content: Thank you for the detailed response!I tried it but my graph doesn't change.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 25, 2016, 06:12:31 am
content: Hmmm..... I don't use Qucs, I'm running LTSpice under WINE on my Linux box and it works fine.Maybe the problem has to do with how the transformer is specified in Qucs. As you can see I've specified a 0.9 coupling constant K, and 10 mH inductances for both coils. Try using the "Mutual Inductors" component instead of the "Transformer" in Qucs. This allows you to set the inductances and coupling constant.

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2016, 08:00:15 am
content: Quote from: moonquasar on August 25, 2016, 05:11:11 amIt still looks like the emitter isn't properly connected. Delete all connections to the transistor's emitter and redraw them with the lines parallel to the emitter, as you've done with the collector.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 25, 2016, 09:38:22 am
content: OK... you forced me to learn Qucs.notes:1. Ground must be located as shown.2. I couldn't get it to work with the "Transformer", like I said, I had to use the "Mutual Inductors" component.3. Use a blue LED from the Components Library. Ditto for the transistor, pick a suitable part number from the list.4. As Hero999 and Blueskull said, in your diagram the Emitter doesn't look like it's actually connected. Be sure to connect to the end terminals of the component.