# Transformer question

Discussion in 'General Electronics Discussion' started by RobSmith, Jan 6, 2012.

1. ### RobSmith

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0
Dec 16, 2011
I am just trying to figue out a question I have about transformers.

It might sound a daft question but how does a transformer work with a square wave input....
There are lots of simple inverter circuits on the internet that do this but I always thought the voltage had to be contantly changing to have a changing magnetic field to induce the secondary winding to create an output.

Looking at a time/voltage graph of a square wave ac then the voltage is +ve and constant for a bit and then -ve and constant for a bit. The change from +ve to -ve taking no time.

Does the magnetic field in the core take its time in reversing and hence create an output? .... and is it square wave or has got smoothed out a bit by the magnetism taking its time?
Edit: Thinking about it... does the output become +ve / -ve / +ve / -ve spikes with dead sections between?

Are there any transformer gurus here that can shed light on this?

Rob

Last edited: Jan 6, 2012
2. ### davelectronic

1,087
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Dec 13, 2010
I am not an expert, but the square wave is the the moddifed version of a sine wave aftifical sine wave, there is no + / - volts to it to the best of my knowlege it at a higher frequency usually but made by switching semiconductors of one type or another.

It still acts like a sine wave setting up a magnetic circuit in the primary relation between copper and iron, mutal inductance coupled to the secondary winding.

3. ### BobK

7,682
1,686
Jan 5, 2010
A transformer does act as a low-pass filter since there is a lot of inductance. So, when fed a square wave it is naturally going to smooth it out somewhat.

Bob.

4. ### Resqueline

2,848
2
Jul 31, 2009
Yes, a square wave is just a sync'ed assembly of sine waves (being 1, 3, 5, 7, 9, etc. times the square wave frequency).
If you apply a too "long" square wave then the flat tops will droop towards zero. Eventually it becomes sawtooth shaped, and ultimately just spikes.
The transformer will most likely draw a heavy primary current by that time btw..

5. ### RobSmith

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Dec 16, 2011
Am I right in saying that if I applied a an increasingly large resistive load to the output the secondary windings would drain the core of magnetism more quickly and the output shape would become less square, or less saw tooth shaped and more and more spikey?

Just trying to get my head around what goes on inside a transformer.

I just read a bit on the internet about back emf which explains why mains transformers don't go bang. It is quite interesting.

Rob

Last edited: Jan 6, 2012
6. ### ElectronWorks

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Aug 20, 2009
if you put a square wave on the primary of a transformer, the current ramps linearly with time. When you collapse the voltage across the primary, the voltage across the secondary rises up until something conducts. then the secondary current ramps down, thus discharging the transformer core. There is a very good website explaining this - with models in LTspice. Have a look at:
http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

and the associated flyback transformer tutorials and this should show you the waveforms that are happening