what type of multivibrator is this?

[Miracletech]

Hi. Please what type of astable multivibrator is this?

https://i0.wp.com/inverter-circuit.com/wp-content/uploads/2011/09/DC-to-AC-Inverter.jpg?fit=809%2C390&resize=1280%2C720

Thank you.

 

[audioguru]

It is a standard old circuit like this:

 

[Diver300]

It's a standard astable multivibrator, as described here:- https://www.build-electronic-circuits.com/astable-multivibrator/

There are two differences. Firstly, the circuit that you posted uses PNP transistors, while the classic circuit uses NPN, so the whole circuit is the opposite polarity, but otherwise works the same.

Secondly, in the circuit that you posted the multivibrator drives two power transistors that drive a transformer. The circuit is a rather crude way of making 240 V ac from 12 V dc.

NPN transistors always have a somewhat better gain than their PNP equivalents. That is why the standard multivibrator circuit uses NPN transistors. In the circuit that you posted, the transistors that drive the transformer are NPN, and they are the large transistors, so it is more important they there are the ones the work the best, so they are NPN. That means the multivibrator part has to have the transistors switching the +ve, so PNPs are used for that.

The circuit is really old. The 2N3638A is transistor in an old ceramic package that is obsolete. The 2N3055, while a classic that many have heard of, is also obsolete and there are many better transistors available nowadays. It was first made in 1967. The output transformer is repurposed from driving the filaments of valves. If you need as small inverter to give you 240 Vac from 12 V, buy a modern one, it will be much cheaper.

 

[Miracletech]

How can I calculate the switching frequency? I just need only the oscilllator.

 

[Miracletech]

What should be r1 and r2 for 50-55hz?

 

[Diver300]

If you multiply the resistance of R1 by the capacitance of C2, you should get about 0.01 to give you 50 Hz.

https://en.wikipedia.org/wiki/RC_time_constant

You may need larger capacitors than your circuit shows. If the resistance is too large, or the capacitance is too small, the transistors won't be turned on for long enough.

 

[Miracletech]

So this is a 50hz oscillator sir?

 

[Miracletech]

What formuula can I use to calculate the resistor values?

 

[Diver300]

It's quite difficult to know the frequency accurately, as a lot of things will affect it a bit.

However, the time for each half-cycle will be about R * C.

For this oscillator R is 15000 and C is 0.000000047 so R * C is about 0.0007 so each half-cycle will be about 0.7 ms and the frequency will be about 700 Hz.

You would need to increase C to about 0.68 uF to get near 50 Hz. As I said in post #6, just increasing the resistor may not be a good idea.

 

[audioguru]

Why do you want that very old inverter circuit that produces a squarewave output? Don't you want a sinewave output?
The obsolete PNP transistors will soon be destroyed by the reverse emitter base voltage pulses that are much higher than allowed because two protection diodes are missing.

 

[Miracletech]

I know sir, but its the least I could try for now, I just need to get a 50-55 hz frequency and that's all. The sine waving will be done by me.

 

[Miracletech]

If My Proposed circuit is an astable multivibrator, that means it should work like the other astable multivibrator circuits. That means, I can still get my desired frequency by editing the RC Network to get the time constant. So, I think I should use. A 180k Resistor and A 0.1 uf capacitor, or, a 2.2uf capacitor and 12k Resistor like many other astable multivibrators do. Shll I go ahead?

 

[schmitt trigger]

Breadboard the circuit, without the output transformer first.
Then tweak the values to get the frequency you require.

 

[Miracletech]

I don't have an oscilloscope, but in some common sense of logic, it would still act as the other astable multivibrators work right?

 

[Miracletech]

Wow .Looks like I have gotten the point. Its just a pnp astable multivibrator used to drive NPN transistors, but would still act the same way. In audioguru's picture, I found out that for the NPN, the LED's are connected Differently from the PNP's. Its kind of like an inverted solution for another problem, or to eliminate R1 and R4 in other Multivibrator Circuits. This is what happens when you don't research

 

[audioguru]

You never said your supply voltage that might destroy the transistors without adding protection diodes.
You also never said the load of your oscillator which determines the output resistor value of the oscillator which also determines the timing resistor value.
Now you say you will add a filter to try converting the squarewave into a sinewave, then why not make a sinewave oscillator in the first place?
Here is a squarewave multivibrator with protection diodes:

 

[Miracletech]

Thank you Sir. I just needed an astable multivibrator

 

[Nigel Goodwin]

Thank you Sir. I just needed an astable multivibrator

EXACTLY what are you trying to do? - you say you want a sine wave, in which case you don't want a multivibrator.

 

[Miracletech]

I am trying to test some circuits to see if I will be able to acheive a sine wave. I would add an RC on the output to try my luck. Its a PROTOTYPE. I'm just doing what I call "Testing the Microphone". If I need a sine wave, I would use An MCU instead of a multivibrator. And by the way, I'm gonna shape the output square wave from it using a single RC circuit. Hope it works

 

[Nigel Goodwin]

I am trying to test some circuits to see if I will be able to acheive a sine wave. I would add an RC on the output to try my luck. Its a PROTOTYPE. I'm just doing what I call "Testing the Microphone". If I need a sine wave, I would use An MCU instead of a multivibrator. And by the way, I'm gonna shape the output square wave from it using a single RC circuit. Hope it works

A single RC is a REALLY crappy filter - so you won't get much like a sine wave after it.

Why not just build a sine wave oscillator?