Misc Electronic Questions

[audioguru]

Some little FETS like a 2N5457 or 2N5484 have a maximum current of only 5mA.
Many power Mosfets like an IRF3711Z have a max continuous current rating of 92A or 380A in brief pulses. It is also a FET.
The datasheet for ALL Mosfets shows their maximum on-resistance, then you can simply calculate their voltage drop.

 

[Billy Mayo]

So mosfets and transistors have a voltage drop across them?

Do they use transistors for switching because on the collector there is more current than a FET's output?

Is a transistor or FET , faster at switching? which one is faster?

A FET , isolates the input and output, the gate and drain are isolated
A transistors base and collector is not isolated

Wouldn't a FET be better for switching since it isolates the input current/voltage and the output current/voltage?

 

[Billy Mayo]

I'm not sure why the transistor switch doesn't go to Zero voltage because of the 2 diodes on the emitter

Any reason why the designer didn't want the Turn OFF position of the transistor to not go to zero voltage but to 1.4 volts?

 

[Billy Mayo]

What reasons would it be for a transistor switch's OFF state to be at 1.4 volts instead of zero volts?

any examples please

 

[KeepItSimpleStupid]

This **broken link removed** should cover post #202.

--

Can't tell. What is the transistor driving?

 

[Billy Mayo]

Thanks that was good

My work using a triangle waveform and a sine waveform to create a PWM modulation output

Take a look:



What I don't understand is
1.) How does a triangle waveform and a sine waveform , the difference between the two will create a pulse width modulation waveform?

2.) I can't tell on my oscilloscope the difference between a square waveform VS a pulse width modulation waveform, they both look the same to me, so how do I tell the difference between the two?

 

[Billy Mayo]

This is how they do level shifting

The transistors Switches are in series

Take a look:

 

[KeepItSimpleStupid]

I think your UPS is basically a Class D amplifier for a single frequency. This **broken link removed** should help.

It's not the difference, but rather a comparison of a sine wave and a triangle wave that creates the pulse width modulation.

 

[kubeek]

How is that in series? That is clearly a buffer with open collector output.
Wonder how and how did that schematic, without points denoting junctions it is really confusing and ambiguous.

 

[kubeek]

2.) I can't tell on my oscilloscope the difference between a square waveform VS a pulse width modulation waveform, they both look the same to me, so how do I tell the difference between the two?

A square wave has 50% duty cycle, pwm has variable duty cycle.
The triangle is much faster than the sine wave, so the output is high whenever the triangle is lower then the sine. So for a voltage on the sine input just a bit lower then the peak of the triangle you get a narrow pulse on the output, and you get a wide pulse when the sine is just a bit above the bottom of the triangle.

 

[Billy Mayo]

It's not the difference, but rather a comparison of a sine wave and a triangle wave that creates the pulse width modulation.

What you mean a comparison? how so , a sine wave and triangle wave are different wave shapes

I still don't understand how it take a sine wave and triangle wave and creates a PWM waveform

Whats the difference between a square waveform and PWM? I can't tell the difference on the oscilloscope

 

[Billy Mayo]

A square wave has 50% duty cycle, pwm has variable duty cycle.

I thought a square wave can have a duty cycle between 0%-99%

The Frequency of the triangle wave will change what on the duty cycle or PWM waveform?

 

[Billy Mayo]

That is clearly a buffer with open collector output.

So the first transistor is a buffer and the second transistor is the open collect output?

Why are they using an open collect output? any reason

 

[kubeek]

square wave is called square because the high and low is the same length. What you mean is correctly called a rectangular wave.
The frequency of the trianlge will not change anything on the pwm duty cycle, but only as long as it is significantly faster then the modulating signal. Otherwise if the modulating signal is changing too fast the output will not make a lot of sense and the duty cycle will not be proportional to the modulating input.

 

[KeepItSimpleStupid]

Look carefully at the eetimes link and the pics in the article.

Maybe, this is a better article: **broken link removed**

 

[Billy Mayo]

oh ok thanks for correcting me there sir

So any squarewave that has a duty not of 50% is called a rectangular wave?


What I don't get is
1.) I don't see the modulation on the oscilloscope, all i see is a square waveform
What does the modulation look like?
How it it a modulating signal when I looks to me like a normal square waveform, where is the modulation at?

 

[kubeek]

This should hopefully give you a better idea how is the pwm achieved.

 

[Billy Mayo]

Yes you're right it does look like a Class D amp.

 

[Billy Mayo]

Thanks for that kubeek

I'm not understand the PWM's ON TIME and OFF TIME

PWM's ON TIME is when the triangle wave and sine wave are? matched positive cycles?

PWM's OFF TIME Is when the triangle wave and sine wave are ? matches negative cycles?

 

[kubeek]

No, the pwm output is on when the triangle is higher than the sine, and vice versa.
The output has a bit slow rise and fall times, so the transition doesn´t look exact, but they occur at the point where the sine and triangle cross.