555 mosfet transformer dc to ac confusion

Thread Starter

Insignificantinvisiblespo

Joined Nov 7, 2021
30
Hi. If a 555 circuit switches on and off a mosfet at 60 hz and the output of the mosfet is hooked up to the input of a transformer, does the output of the transformer put out a 60 hz sinewave or what kind of wave if not a sinewave? Im guessing its a sinewave because the transformer input should be synonymous with bringing a magnet close and then backing the magnet away aka im guessing turning off and on the supply to the tranformer input would create a rising and collapsing field, which should create a smooth rising and collapsing sinewave. But all i have is ideas/guesses and no degree, and so i humbly ask for your help understanding this. Thanks.
 

Papabravo

Joined Feb 24, 2006
21,159
At low frequencies, like the powerline frequency, a square wave in will produce a square wave out. Your reasoning about how circuits behave is faulty. Time to crack some books, or maybe youtube videos if books are not as available to you as they used to be. See the following simulation of a transformer driven by a unipolar square wave. I don't see anything that looks remotely like a sine wave.

1636330933364.png
 

Thread Starter

Insignificantinvisiblespo

Joined Nov 7, 2021
30
At low frequencies, like the powerline frequency, a square wave in will produce a square wave out. Your reasoning about how circuits behave is faulty. Time to crack some books, or maybe youtube videos if books are not as available to you as they used to be. See the following simulation of a transformer driven by a unipolar square wave. I don't see anything that looks remotely like a sine wave.

View attachment 252115
Good idea. I have been looking into this to learn more than i do. What happens at higher frequencies? You make it sound like something different occurs.
 

MrChips

Joined Oct 2, 2009
30,707
A square wave has all kinds of additional frequencies. It is all associated with the frequency response of the system.

Imagine a square wave that has extremely fast rising and falling edges. In theory, if the rise times and fall times approach zero seconds, i.e. infinitely fast switching times, then the system must have a bandwidth that extends to infinity. No such system exists in real life.

1636333346969.png

1636333371775.png

How much a square wave deviates from the ideal shape depends on the frequency response of the system. At some point a square wave will start looking like a triangular wave because the voltages in the circuit can change only so fast, limited by something known as the slew rate.

1636333576104.png

A pure sine wave represents a single frequency. In order for a square wave to become a sine wave you have to remove all other frequencies except the one frequency of interest, which is usually the fundamental frequency. You can achieve this using a band pass filter or a frequency resonant circuit, i.e. the circuit is tuned to oscillate or resonate at a single frequency, much like a flute or tuned pipe.

Every transformer has resistance, inductance and capacitance. When you couple all of this together you create a system with a given frequency response. How it distorts the wave will depend on this inherent frequency response.
 

Audioguru again

Joined Oct 21, 2019
6,672
A 555 produces a squarewave or rectangular output and the Mosfet produces the same. A good transformer also produces the same output.

A 60Hz squarewave produces the 60Hz fundamental plus many odd numbered harmonics of 180Hz, 300Hz, 420Hz, 540Hz, 660Hz, 780hz etc.
 

Thread Starter

Insignificantinvisiblespo

Joined Nov 7, 2021
30
I think we need to take a step back. I was under the impression a 555 outputs a DC pulse. But im getting the impression a 555 outputs a square shaped wave that goes positive one half and negative the other half. Does a 555 put out an AC wave?
 

MrChips

Joined Oct 2, 2009
30,707
In this context, DC vs AC is relative, based on what you call 0V.

A square wave can go from V1 to V2. You can make V1 and V2 any voltage you please.
For example
V1 = 0V, V2 = +10V
or
V1 = -5V, V2 = +5V

A 555-timer circuit can put out any voltage (within limits) depending on where in the circuit you label 0V.
 

Thread Starter

Insignificantinvisiblespo

Joined Nov 7, 2021
30
In this context, DC vs AC is relative, based on what you call 0V.

A square wave can go from V1 to V2. You can make V1 and V2 any voltage you please.
For example
V1 = 0V, V2 = +10V
or
V1 = -5V, V2 = +5V

A 555-timer circuit can put out any voltage (within limits) depending on where in the circuit you label 0V.
Is there a specific name for a circuit that uses a 555 or a different solid state component to output an AC waveform? From your response it sounds like a 555 can output an AC waveform.
 

MrChips

Joined Oct 2, 2009
30,707
There are two usages of the terms DC & AC.

1. In the traditional way, DC means Direct Current, where the current never reverses direction. AC means Alternating Current where the current reverses direction.

2. From a frequency perspective, every signal may have both DC and AC components. DC refers to the frequency component at 0Hz. AC refers to everything else above 0Hz. DC also refers to the time average value of the signal. If you remove this value you end up with AC. In other words, a high pass filter removes DC and leaves you with AC, for example, as with an inter-stage coupling capacitor.

A 0-10V square wave signal would be DC with no AC in the traditional sense (1).
In sense #2 it could also be considered as a -5V to +5V AC signal with an added +5V DC signal.

If you take the output of a 555-timer oscillator and feed it into a transformer, regardless of the Vmin and Vmax voltages of the signal, you will get an AC signal (in the traditional sense). This is because the DC component cannot traverse the windings of the transformer.

Series capacitors and transformers are similar in this respect. They both block DC.
 

crutschow

Joined Mar 14, 2008
34,281
If you take the output of a 555-timer oscillator and feed it into a transformer, regardless of the Vmin and Vmax voltages of the signal, you will get an AC signal (in the traditional sense). This is because the DC component cannot traverse the windings of the transformer.
But note that the unipolar signal from a 555 (powered from a single supply) will have an average DC value which will tend to saturate the transformer core.
Coupling the output through a capacitor will block the DC component and avoid any transformer saturation.
 
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