Hello! I'm new to the forum and pretty new to electronics as well!

So I heard about joule thieves and I am totally fascinated by them! I've been playing around with some simulators and I got them to work in the falstad simulator and also the LTSpice program.

Now I'm trying to change the circuit a bit so that the voltage spikes from the inductor run into a capacitor and it charges up, and then I want to add a zener to control the voltage charged in the capacitor. Normally, the voltage reaches a max of around 200V. (For some reason I can't get this to change, even with different inductor sizes...but in falstad I can easily)

Here is my circuit diagram (w/o zener):

http://www.flickr.com/photos/73128199@N03/8522723460/


And here is the simulation result (w/o zener):

http://www.flickr.com/photos/73128199@N03/8522724358/


Now adding the zener in parallel, reversed with the capacitor should let me control its max V right? So I add in a 100V zener, yet the Vcap doesn't reach 100V for some reason, it won't go past about 15V. This is where I am confused.

http://www.flickr.com/photos/73128199@N03/8522737530/



Now for some reason, this works perfectly fine in the falstad simulator. Here is the same circuit, just with different size inductors and the bottom diode is removed because it isn't needed.

http://www.flickr.com/photos/73128199@N03/8524560566/


And here, the same circuit in falstad with a zener set to 30V...you can see the voltage ramp stops close to 30 as expected. No clue why these things don't work in LTSpice

http://www.flickr.com/photos/73128199@N03/8523448061/

 

Not sure why I can't edit my post...but flickr sucks so you might not be able to see those images.

Diagram:

LTSpice Sim Result (w/o zener)

LTSpice Sim Result (w zener)

Falstad (w/o zener)

Falstad (w zener)

 

Youre LTSpice circuit has two series rectifier diodes back-to-back. Only leakage current will flow in that circuit.

 

Youre LTSpice circuit has two series rectifier diodes back-to-back. Only leakage current will flow in that circuit.

Sorry, I realized the bottom one was useless. But that is besides the point because the capacitor is there just to charge up to a certain level until another circuit is connected across the capacitor.

My real confusion is why the zener in LTSpice does not work as expected, and also why changing the inductor size doesn't change the level off voltage on the capacitor. You can see in the pictures from falstad that I removed the useless diode, and it seems to work fine. But in LTSpice even with that diode removed, it does not work as expected.

 

Actually,it's not besides the point. The diode is more than useless, it prevents the circuit from operating correctly.

Post your LTSpice results with the circuit correctly specified.

 

Well now I'm really confused. With the bottom diode removed, it doesn't even oscillate as before.

 

The diode sould not conduct until the transistor turns off. Try lowering the supply voltage or string enough diodes together so that their combined threshold voltage is > 3V.

 

Hey thanks for the help! I think I got it working perfectly. I was using the wrong type of diode, that MUR460 I threw in because it had a high reverse breakdown, it must be some huge diode, and for some reason LTSpice doesn't list the diode's fwd voltages but it must've been pretty high, enough to not turn on, or I don't really know why. I replaced it with an LED, and now it oscillates fine and the zener regulation also works perfectly.

 

LED's have a higher forward voltage, and so it could be the one you used corrected the flaw in the circuit. I've had problems getting diodes to simulate correctly in LTSpice before. Glad you got it working.