Simple oscillator not working

Thread Starter

user151281

Joined Jun 12, 2017
5
Hi all

I am new to electronics and I am new to this forum too, I hope this is the right section.

I tried to make an oscillator, but slightly different than usual, in the tank circuit instead of placing an inductor put a connection to a transformer. Thus I just added an input source, but no extra output sources. Trouble is that there is no current entering the transformer, the circuit does not oscillate. It slowly (albeit very slowly) drains the power, so it seems something is happening.
The circuit design is in the attached image.

For the design I started from the Colpitts oscillator C1 and C2 are constant film capacitors and C3 is an electrolytic capacitor.
The main differences are:
1) I didn't use the fourth capacitor that I saw in many designs online.
2) I did not add any earth connection.
3) R2 is connected before R3 and the transistor (this was actually a soldering mistake).

What could be the issue? Is it a bad idea to use directly the tank circuit for the output oscillation or could it be one of the other points?

Thanks in advance for any help you can provide.
 

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Alec_t

Joined Sep 17, 2013
14,280
Welcome to AAC!
It would help if you add component values to the schematic; also if you post the 'usual' oscillator circuit you based yours on.
 

ian field

Joined Oct 27, 2012
6,536
Hi all

I am new to electronics and I am new to this forum too, I hope this is the right section.

I tried to make an oscillator, but slightly different than usual, in the tank circuit instead of placing an inductor put a connection to a transformer. Thus I just added an input source, but no extra output sources. Trouble is that there is no current entering the transformer, the circuit does not oscillate. It slowly (albeit very slowly) drains the power, so it seems something is happening.
The circuit design is in the attached image.

For the design I started from the Colpitts oscillator C1 and C2 are constant film capacitors and C3 is an electrolytic capacitor.
The main differences are:
1) I didn't use the fourth capacitor that I saw in many designs online.
2) I did not add any earth connection.
3) R2 is connected before R3 and the transistor (this was actually a soldering mistake).

What could be the issue? Is it a bad idea to use directly the tank circuit for the output oscillation or could it be one of the other points?

Thanks in advance for any help you can provide.
Can only guess with what you've given - if its an iron cored transformer, it might be too lossy for that kind of circuit. A lot of turns on the unused winding will have inter-winding capacitance that could be damping it out.
 

Thread Starter

user151281

Joined Jun 12, 2017
5
Thank you ian field, you gave me the right hint. I replaced the transformer with a small inductor and I could see some current flowing through.

Could bigger capacitors help? C1 and C2 I am using now are 0.68 µF.
 

SLK001

Joined Nov 29, 2011
1,549
There is no such thing as a "simple oscillator". Many things have to correctly line up for a wanted, controlled oscillation to occur.
 

OBW0549

Joined Mar 2, 2015
3,566
There is no such thing as a "simple oscillator". Many things have to correctly line up for a wanted, controlled oscillation to occur.
That's only partly true, as dictated by Murphy's Law. On the one hand, if you're trying to make an oscillator it will most likely just sit there like a lump the first time you turn it on, perhaps amplifying a bit in some half-baked fashion. On the other hand, if you're trying to make an amplifier, chances are it will oscillate like a politician at first, until you take the necessary steps to stabilize it.

Circuits know what we want them to do, and they conspire with Murphy to confound us by doing something else.
 

ian field

Joined Oct 27, 2012
6,536
There is no such thing as a "simple oscillator". Many things have to correctly line up for a wanted, controlled oscillation to occur.
Its the bane of the design engineer - most things you want to oscillate don't and most things you don't want to oscillate do.

Getting things to oscillate isn't difficult - lining up all the important parameters can be.
 

BR-549

Joined Sep 22, 2013
4,928
user151281.......if I were you.....I wouldn't start off with transistor/analog oscillators.

Start with op-amp oscillators. These are easy to build and adjust.
 

ian field

Joined Oct 27, 2012
6,536
Thank you.
Fixed.
That looks like it probably doesn't need that many uF - probably not very difficult to find a metalised foil type big enough.

It would probably work better than an electrolytic - but if its working and you're happy with it, why bother fixing something that aint broke.

MLCCs in big values are easy, but get lossy if you go for much in the way of frequency.
 
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