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

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

argha halder (author)  Josehf Murchison2 years ago

Where should I add the resistor and tuning capacitor?

try this patteren

Answer 99.png
rickharris2 years ago


There are 100's of FET oscillator circuits at this link - I can not see a single one that does not use a capacitor to provide the tuned circuit.

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.

There isn't anything there to make it oscillate?

rickharris2 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.

argha halder (author)  rickharris2 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!

It depends:

1. On the frequency your wanting

2. on the inductance of the transformer


is a calc to help sort this out.

Experiment with different values. BIG capacitor slower oscillations.

seandogue2 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