Henri Abrahams astable multivibrator

[ratstar]

I thought I got the two phases right until I noticed the caps are the wrong way around!
Have I got this wrong? (im pretty sure I do... I need some help, but ill work it out later, im getting tired.)

The two circle arrows are the discharge paths for the capacitors, and the rest of the red is the conduction from the battery.

 

[bertus]

Hello,

Have a look at the following sites:
https://www.electronics-tutorials.ws/waveforms/astable.html
https://electrosome.com/astable-multivibrator-transistors/
https://www.build-electronic-circuits.com/astable-multivibrator/

Bertus

 

[ratstar]

Oh I see!!! I didnt quite work that out, that when its charging the right cap, its discharging the left cap down the same transistor. I missed it...

Hang on a second, theres alot of versions of this oscillator that are different... I think I shouldnt have put the LEDS at the top, they are supposed to go on the bottom...

 

[Harald Kapp]

they are supposed to go on the bottom...

No, "top" which is the collector side in electronics terms is good.

 

[Martaine2005]

Have a watch of this video. It’s quite well explained.

Martin

 

[Audioguru]

Where does the guy in the video get 0.95V? My transistors reach a Vbe of 0.65V in that circuit.
Where does he get -8V? My circuit has his 8V reduced to -6.3V to -4.3V by the LEDs.
Note that the transistors probably have a maximum allowed reverse emitter-base voltage of 5V so a battery voltage higher than about 10V will slowly damage the transistors unless protection diodes are added.

 

[Martaine2005]

The reason I liked the video was not to be too concerned about numbers. He did say “usually about .6V, but these are .9V. Purely saying nothing is perfect and that’s why the circuit works.
I haven’t bread boarded the circuit so can’t say. But tolerances in the resistors have a role in this circuit working.
It was to show ratstar how the circuit works from a visual explanation rather than reading about it.

Martin

 

[Audioguru]

I think that Ratstar does not know that the maximum forward-biased base-emitter voltage is only 0.6V to 0.7V then the capacitors do not charge "backwards".

 

[ratstar]

I think that Ratstar does not know that the maximum forward-biased base-emitter voltage is only 0.6V to 0.7V then the capacitors do not charge "backwards".

They actually do, I drew it wrong. If you have the leds on the bottom, when your charging the right one forwards, it charges the left one backwards.

It says discharging, but im pretty sure its charging it backwards with the battery/source/vcc or whatever.


But im just being cheeky, I know what you mean.

[edit] nah im wrong... r1 has too much resistance so it wouldn't happen?? [/edit]

 

[Audioguru]

I looked in Google for the transistor multivibrator with the LEDs at the emitters and found only the one linked above.
It might not work especially when the battery voltage is low.

I also found a circuit with two serious errors.

 

[ratstar]

are the transistors the wrong polarity in the second one?

 

[Audioguru]

are the transistors the wrong polarity in the second one?

The transistors have the correct polarity but they are the wrong type of transistor. The circuit uses NPN type of transistors but the wrong type shown is PNP.
Also the LEDs do not have current-limiting resistors. The base resistor values are so high that transistors with low hFE (110) will barely light the LEDs with the low supply voltage and transistors with high hFE (800) will burn out the LEDs.

 

[Martaine2005]

are the transistors the wrong polarity in the second one?

Yes, they are PNP transistors. The emitter is positive. Not like the schematic.
But both have issues with supply voltage. Take away all the known voltage drops e.g LED and Vbe. Along with resistance, can the circuits work with 0.2 or 0.3V?.
Do the calculations. V=IxR.
I will bread board this circuit next week when I’m home.

 

[ratstar]

Oscillators are beautiful, and teach us alot about electricity.

 

[Martaine2005]

Oscillators are beautiful, and teach us alot about electricity.

Oscillators are ‘on and off’ devices. Or ‘left and right’ devices. They teach us nothing except ‘hi or low’. Be it monostable, bistable or astable. A pure clock pulse keeps ‘almost’ perfect timing. Astable is erratic by definition of being unstable.
Your heart rate is astable. What is beautiful about it?. I prefer shift registers or Johnson counters.

Martin

 

[ratstar]

I think if I finally get this oscillator done, ill know enough to make my computer, the oscillator is the hardest bit in my mind.

If I finally get my one finished, ill be so proud of it, itll get me right out of the dumps!!!

 

[Martaine2005]

Your computer?
What will it do?
Out of the dumps? What dump are you in?
Need a solicitor?
Are you sure a ‘hi, low’ will help you?.

Martin

 

[ratstar]

Making an oscillator is a really successful thing to me, if you dont appreciate it, its up to you... I dont know why you wouldnt be so happy...