Under which conditions does NPN astable multivibrator start oscillating?

[rob77]

1nF caps are too small , if you need a reliable higher frequency oscillator, then choose a different circuit.
why 1nF might be way too small is obvious when you re-draw the circuit as attached - the astable multivibrator is in fact a 2 stage AC coupled amplifier with 100% feedback. (1nF might be too low to provide enough feedback to start oscillations - depends on the transistors, biasing...etc... of course)

[king.oslo]

Looking back I noticed this comment:-

Neither of you used the same resistor and capacitor values as I. With certain values, it will oscillate, but not with most values I tried yesterday.

Was this in the simulation,or with the real thing?

If it is the real circuit,you already know it is wired correctly.
It is possible,but unlikely,that the transistors are running out of gain at 300kHz & a supply of 5v.

Are you sure the 5v stays at that figure-----if the "turned on" transistor draws enough current to make it dip substantially,oscillation may not proceed.

Breadboard i think

[Paul Price]

Don't blame your astable problems on LT Spice or your breadboard or layout.

The problem with the OP original circuit is that his circuit is completely balanced and oscillation has a chance to be suppressed by this perfect balance, current flow is just too symmetrical at turn on.  In common use 50-yrs ago, the OP circuit shown is slightly incomplete because the emitters are not connected together to ground through a small valued resistor.

Connecting both emitters of the 2n2222's together and adding a resistor that is approx 1/10 of the collector resistors ensures a highly regenerative action even with the tinniest imbalance in collector current at powering on.

[Audioguru]

I think you zapped the 2N2222 transistors because in the first circuit the supply was too high at 12V. The reverse-biased emitter base was trying to go to -11.3V but the maximum allowed voltage is only -5V on its datasheet. The emitter base junctions probably had avalanche breakdown at about 6V which overheats the tiny junction that has no cooling. Then the transistors are ruined.

If you use a supply voltage higher than 5V with that simple circuit then you must add protection diodes.

[Paul Price]

I saw that the OP's original circuit was using a 5V(not powered with a 12Vsupply which would be a problem) with a tiny amount of AC ripple added in desperation to get it to work.

[vk6zgo]

I think you zapped the 2N2222 transistors because in the first circuit the supply was too high at 12V. The reverse-biased emitter base was trying to go to -11.3V but the maximum allowed voltage is only -5V on its datasheet. The emitter base junctions probably had avalanche breakdown at about 6V which overheats the tiny junction that has no cooling. Then the transistors are ruined.

If you use a supply voltage higher than 5V with that simple circuit then you must add protection diodes.

Pray tell me where the -11.3 V comes from?
The circuit runs from a +12V supply.
The emitters are at zero volts, the bases cannot go negative w.r.t them,hence no reverse bias.

Wait!--I see where you're coming from!
When the previously cut off transistor conducts,the capacitor is effectively inverted,presenting apptox -12v a the other transistor's base.
(It's many years since I had to analyse the operation of a simple two-transistor M/V)
I have seen many such multivibrators operate happily with +12V supplies in the days before +5V became so common,so there is maybe more to it.

[vk6zgo]

I'm thinking that the transistors we used had a higher base-emitter reverse voltage rating than the 2N2222,so we got away with it.

I learnt about astable M/Vs back in the tube days,& musing about that made me realise you were correct.

If it didn't happen the way you said,tube astables wouldn't work.
Tubes need to be biased hard negative to cut off,& of course,the voltage rating grid w.r.t. cathode,(negative or positive) is only limited by internal breakdown of the vacuum or the mounting insulation,so there is no problem.

[IconicPCB]

To guarantee start up, consider adding a couple of diodes, Google will help with circuit schematic.

[nwvlab]

FWIW... I have set up the circuit using BC337-25 transistors and the OP's original R and C values (1k, 3.3k, 1nF) and it works fine at 5V, nice and symmetrical.

I also tested with the only 2n2222s that I could find in my stash, a mismatched pair, one from Motorola the other from ST. Both metal cans. These sort-of worked, but touchy and not nice and symmetrical. (Touching the metal can with a finger killed the oscillation and required power-cycling to restart it.) The frequency was higher with these transistors, around 338 kHz.

MPSA18s did not work.

The scopeshot below is with the BC337-25 pair.

wow! What kind of caps are these? They seem to me glass diodes