Jewel Thief Help

The coil doesn't oscillate, the entire circuit oscillates.
If you use the proper ferrite core then there aren't any eddy currents to waste power.

The resistor makes the transistor conduct. The increasing transistor collector current in the coil causes the transformer action to also increase the transistor's base current.
The transistor conducts as much current as it can then the increase in current stops, and also the transformer action stops which causes the transistor's base current to reduce and then the transistor's collector current in the coil also reduces and transformer action futher reduces the base current until the transistor stops conductiong and the transformer action also stops. Then the resistor starts the transistor conducting again.

we are doing things and we don't know how they work! isn't that great Audioguru?

whiz115 said:

we are doing things and we don't know how they work! isn't that great Audioguru?

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Not really, it's just a crude blocking oscillator - but damn clever!.

whiz115 said:

we are doing things and we don't know how they work! isn't that great Audioguru?

Click to expand...

Not really, it's pretty pointless to do things you don't understand.

Do you know why it wouldn't work when the coil as connected in the wrong direction?

Nigel, audioguru, jcox and anyone else which a lot of experiance, please don't respond to this question, let them figure it out for themselves.

Hero999 said:

Do you know why it wouldn't work when the coil as connected in the wrong direction?

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I think that has to do with the biasing of the transistor... it doesn't conduct if the secondary of the transformer is not reversed although i'm not sure...

I only now that when two coils are placed very close and we apply AC the magnetic field for the one coil induces ac current to the other...

audioguru said:

The coil doesn't oscillate, the entire circuit oscillates.
If you use the proper ferrite core then there aren't any eddy currents to waste power.

The resistor makes the transistor conduct. The increasing transistor collector current in the coil causes the transformer action to also increase the transistor's base current.
The transistor conducts as much current as it can then the increase in current stops, and also the transformer action stops which causes the transistor's base current to reduce and then the transistor's collector current in the coil also reduces and transformer action futher reduces the base current until the transistor stops conductiong and the transformer action also stops. Then the resistor starts the transistor conducting again.

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That is what I meant.

whiz115 said:

I think that has to do with the biasing of the transistor... it doesn't conduct if the secondary of the transformer is not reversed although i'm not sure...

I only now that when two coils are placed very close and we apply AC the magnetic field for the one coil induces ac current to the other...

Click to expand...

i answered but i didn't got any responce yet...

The quote sounds right. Good job. When one coil is induced with a flux, the other coil has a increased flux on it.

This is so simple I can't believe you don't understand it?.

An oscillator requires positive feedback, the transformer is wired the correct way round to give positive feedback, so it oscillates. If you wire it the wrong way then it gives negative feedback, so there's no way it can ever oscillate.

Nigel Goodwin said:

This is so simple I can't believe you don't understand it?

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Simple circuits can be baffling:
1) I copied the voltage stepup circuit from my solar garden light and it works for about 1 minute then stops. Its frequency gets lower and lower just before it stops. I don't know why. Maybe the 1nF capacitor is bad.

2) I copied a flasher circuit from Bill Bowser's site and it doesn't oscillate. Both transistors turn on and stay on. I don't know why. Maybe it needs a 1k load.

There was a PNP transistor used as a gate with a 100k series base resistor between the circuits that I removed.

As illustrated in your circuit, the flasher circuit doesn't actually do anything so it's no surprise that the LED just stays on.

Hero999 said:

As illustrated in your circuit, the flasher circuit doesn't actually do anything so it's no surprise that the LED just stays on.

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It is the very simple flasher oscillator that doesn't work.
Both transistors stay turned on.

The resistor at the base of the NPN transistor begins to turn it on.
The NPN transistor then turns on the PNP transistor a little.
Positive feedback through the capacitor from the PNP transistor turns on both transistors very hard.
When the capacitor is fully charged then there is no more strong positive feedback so the NPN transistor turns off slightly which turns off the PNP transistor a little and positive feedback through the capacitor quiickly turns off both transistors completely.
Then the capacitor charges through the base resistor until the NPN begins to turn on again.

The current gain of the transistors is so high that I think my circuit needs to have a 1k load resistor instead of the 10k so that the transistors can turn off a little when the capacitor is charged.
I'll try it in a few minutes.

I fixed my Flasher Oscillator.
I suspected that its load resistance was too high at 10k so I tried 1k and it oscillated, but not with a steady frequency. Also its duty-cycle changed.

So I connected my DVM to the base of the 1st transistor and it worked fine. I soldered a 22M resistor to ground there and it works perfectly.