will this voltage correction work

[maicael]

the ic shown makes the fets conduct alternately so i want to control voltage at the gate of the fets using common emitter follower configuration by using the variable resisitor to control voltage which enters the base of the pnp transistor and then when connected as shown and power from the output of the ic flows through the 1k resistos this causes the voltage that flows into the gates to be the same as the voltage applied at the base of the transistors thereby controlling the conduction of the fets and ultimately controlling output voltage.
please am a novice so feel free to educate me more if am wrong.thanks

 

[throbscottle]

I'm not too familiar with FET's and MOSFET's, but...

So you are wanting to adjust the bias voltage on the FET's. This will affect the point at which they switch on or off, not their gain - I hope this is what you wanted to achieve? Depends on the waveform coming out of the IC as to what effect this will have on the voltage from your transformer.

Small but important point - you don't appear to have any power source going into your output circuit. I'm assuming the transformer primary is supposed to have a centre tap and you just forgot to draw it in.

You might want to put a resistor on each emitter of maybe couple hundred ohms. This will give a bit of negative feedback to the transistors so the control is a bit smoother.

This sort of thing I'd just build it and see what happens, others on here will be able to create accurate predictions of what will happen

 

[maicael]

i want to control the voltage that turns on the fets which in turn will control the output AC voltage of the transformer and yeah the transformer is a centre tap and the whole system including the ic is powered with a 12V source.the emmiters will have the 1k resistor coming out from the output of the IC connected to them and when voltage coming from the ic flows into the emitter coupled with the voltage going into the base of the transistor this causes the configuration to act as an emitter follower where voltgae into the base will be voltage out from emitter going into the gate of the FETs

 

[ronv]

FETs don't really like to work like that. If you check out a data sheet for threshold voltage (the voltage where they start to turn on) you will see they can easily vary by 2 volts or more. So one could be on real good and the other not on at all. The threshold also varies quite a bit as the FET heats up.

 

[maicael]

ok so after reaching the threshold voltage and the FETS turn on they will start to conduct right? if we continue to increase the voltage at the gates wont this affect the conduction of the FETS

 

[ronv]

Yes, Kind of. What are you trying to do?

 

[maicael]

an inverter actually although its experimental as i have to make a presentation as to my experience in electronics so far.

 

[alec_t]

For an inverter you really need the FETs to be fully turned on or else fully turned off. If they're only partly turned on (as set by your control voltage) they will get very hot and will probably fail.

 

[maicael]

actually when setting the control voltage,the FET will be turning on and off at the set voltage.the FEts switch on and off but when on that set voltage is what turns it on

 

[alec_t]

the FET will be turning on and off at the set voltage

No, it won't. It will turn only partly on. Unless the FET is specified as a 'logic level' type it will need a gate voltage of ~ 10V to turn it fully on.

 

[ronv]

If what you want is to keep the gate voltage from going over say 10 volts it will work but if what you want is to control the voltage going into the transformer it won't work. The turns ratio on the transformer and the input voltage controls the output voltage of the transformer and what you hook up to the output of it determines the current.

 

[maicael]

are you saying that if say 5v goes into the gates then wont 5v come out from the drain which is connected to upper and lower side centre tapped trasformer which will then saturate the transformer giving proportional ac output could you explain why it wont work.pardon me am still learning.

 

[maicael]

No, it won't. It will turn only partly on. Unless the FET is specified as a 'logic level' type it will need a gate voltage of ~ 10V to turn it fully on.

ok yeah partly on then so this means that they will conduct proportionally to this voltage and increasing the voltage will increase conduction.

 

[alec_t]

increasing the voltage will increase conduction

....and the increased current will cause a lot of heat to be dissipated by the FET because the drain-to-source voltage will not be negligible if the FET is only partly turned on.

 

[audioguru]

Why are you making a simple square-wave inverter? it is good for driving incandescent light bulbs or an electric fry pan.
It will poorly drive an electronic speed control on an electric motor and will not drive many electronic products that rely on the higher peak voltage of a sine-wave.

A pure sine-wave inverter does not use a linear amplifier, instead it uses a complicated pulse-width-modulation circuit that switches the Mosfets on and off at a high frequency. The lower frequency sine-wave is produced by varying the on and off times. The Mosfets switch fully on and fully off so they do not get too hot.

 

[maicael]

Why are you making a simple square-wave inverter? it is good for driving incandescent light bulbs or an electric fry pan.
It will poorly drive an electronic speed control on an electric motor and will not drive many electronic products that rely on the higher peak voltage of a sine-wave.

A pure sine-wave inverter does not use a linear amplifier, instead it uses a complicated pulse-width-modulation circuit that switches the Mosfets on and off at a high frequency. The lower frequency sine-wave is produced by varying the on and off times. The Mosfets switch fully on and fully off so they do not get too hot.

i know that a square wave inverter wont power most equipments but i want to have something to show that i have learnt alot about electronics in my school besides a sine wave inverter is complicated as you said so a novice like me attempting it might be a daunting task unless you have a sine wave circuit i have no choice but to attempt this.am grateful for the help as i have learnt a few things.thanks so much

 

[throbscottle]

I don't think this is a good choice of project to show what you have learnt, (now excuse me this could be the whiskey talking) I think it does more to show what you have yet to learn, obviously not what you want.

 

[audioguru]

I agree that a square-wave inverter circuit is very, very simple and shows that almost nothing was learnt.
There are many of them on the internet to be copied but only a few of them work properly.
If you simply copy a circuit then you are only learning how to solder wires together.