Need help on 2N3055 12v DC to 220v AC Inverter Circuit

I am about to build a 12V DC to 220V AC inverter using power transistor 2N3055. I am following this circuit diagram.

The transformer is 220V to 12V step down (12-0-12) 4 Amp transformer. And the battery is 12V 7 Ah Lead Acid battery.

First of all - the circuit doesn't work on Simulator. However, I found several blogs claiming that the circuit is valid and works. Could you please let me know why it doesn't work on simulators? And secondly, are the values of R1 - R4 correct in order to allow 2N3055 to oscillate? I am planning to use all the 4 resistances 10Watt rated.

• I'd guess that your simulator is simulating an ideal transformer and probably ignoring its inductance. – brhans Mar 6 '16 at 15:02
• You have no grounds - add those. Note that the output will also need as path to ground - I usually use a 1 Meg resistor for isolated designs. SPICE style simulators require a ground to work properly. – Peter Smith Mar 6 '16 at 15:04
• @PeterSmith What's the purpose of a 1M resistor? There won't be any current flow. Just short circuit to establish ground reference. – PDuarte Mar 6 '16 at 15:58
• @PDuarte: The purpose is to provide the simulator with a DC return path as it cannot otherwise find the DC operating point. – Peter Smith Mar 6 '16 at 15:59
• @PeterSmith Agreed, so just short circuit. No need to insert a resistor. – PDuarte Mar 6 '16 at 16:02

This kind of oscillator depends on the transformer reaching saturation. Most simulators only have "ideal" transformers, and will not show the behavior required for oscillation.

If it weren't for saturation, one transistor would switch on and the current through it would increase indefinitely. In a real transformer, the core reaches saturation and this causes the base drive for that transistor to disappear, triggering the circuit to switch to the opposite state.

jp314 is right. When simulating symetric circuits like this one, the simulator has no reason to imbalance one side vs the other. It's much like an astabile multivibrator circuit. That does not simulate as well for the same reason. And might even have start problems in real life.

have a push button switch in your simulation to fire it up on one of the transistor bases only. After that is should run.

The component where frequency depends on in this case is the transformer. It will need to have inductance in the simulation.

I would suggest connecting transistors as Darlingtons or Sziklai pairs. After doing that, you'll be able to use lower power, cheaper resistors.

Also, in a SPICE simulator the components as you have drawn them are perfectly matched, so (even with saturating transformers) SPICE will find a (non-practical) perfect balance point with all transistors nearly fully on.

Add some asymmetry in the circuit (make one of the 75 ohm equal to 76 for instance), and ramp the DC supply from 0 to 12; don't just start it at 12 V.