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Why is my oscillator circuit providing electricity only when electricity is removed? Answered

So I built an oscillator using JFET transistor J310, an audio transformer 1:100 ratio, and this lights up a 3 volt led at 1.5 volts to even .3 Volts!!  But the problem is that, it lights up the led for a second only when the battery is removed. Is there a way to fix it? I really like how the circuit can use power from small power sources, only if I can get it to work while the battery is connected. It would be great if you could help me fix this circuit!     

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Josehf Murchison
Josehf Murchison

5 years ago

Turn your battery around and I would add current limiting resistors and a tuning capacitor.

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argha halder
argha halder

Answer 5 years ago

Where should I add the resistor and tuning capacitor?

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Downunder35m
Downunder35m

5 years ago

If build a joule thief you should do it right ;)

The diode goes between ground and Jfet, where the Jfet connects to the coil.
But for this any audio transistor should do the trick.

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rickharris
rickharris

Answer 5 years ago

There isn't anything there to make it oscillate?

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rickharris
rickharris

5 years ago

It isn't oscillating!

When the fet initially conducts the magnetic field builds in the primary transformer. As this cuts through the secondary it generates electricity (but this is the wrong polarity for the LED so it doesn't light)

As the FEt cuts off the field collapses again it cuts the secondary but in the other direction so this time it generated a pulse of electricity and lights the LED.

This is a good demo of the back EMF the is generated by relays and motors when turned off which needs to be caught so it doesn't kill your other electronics

To oscillate the FET needs to have positive feedback. Really the circuit needs to be tuned by the addition of a capacitor to make it oscillate at a set frequency.

This circuit is a typical Hartley oscillator.

hartley.jpg
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argha halder
argha halder

Answer 5 years ago

So to fix this circuit, I need to add a capacitor, and reverse the polarity of L.E.D? If so, what should be the type of capacitor and the capacitance of the capacitor?? Thanks a lot!

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seandogue
seandogue

5 years ago

A transformer only transforms time-variant energy. No transition, no transfer. You light lights because that's when your circuit is seeing a transional voltage