I have a step down transformer that converts from 220 volt to 15 volt. I want to connect the secondary coil of this transformer to the output of 555 IC that produces ( 5 volt / 100 KHz ).

Is there any increase in voltage at the primary coil ? Is it square wave ?

How can I calculate the voltage and current at the primary coil ?

How much the frequency of 555 affect the output voltage ?

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    \$\begingroup\$ A 50/60Hz mains transformer will not work very well at all at 100KHz. \$\endgroup\$ Commented Nov 5, 2014 at 10:08

1 Answer 1


Yes, a transformer will worth the opposite when connected backwards.

You have a roughly 16:1 transformer - for every 16 volts you put in you get 1 volt out.

In a perfect world, for every 1 volt you then put in the secondary you'd get 16 volts out of the primary. So 5V p-p would ideally give 80v p-p out.

However, it's not quite that simple.

Transformers are designed to work most efficiently at certain frequencies. To get the most out of your transformer you should run at 50Hz, not 100kHz.

Also you ask if it will be a square wave out. No, it won't. It'll be a wave at the same frequency as the input, but it will be far from square. It would look more like this:

enter image description here

You get big peaks at each edge as the magnetic flux changes, but while it's stationary (during the flat bits of the square wave) the induced current in the transformer quickly decays. To get a faithful representation of your input waveform you would have to limit yourself to a constantly changing wave, like a sine wave, or triangle tooth wave. A saw tooth wave would work, but the rapid flux change at the vertical edges of the teeth would probably get a bit spiky.

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    \$\begingroup\$ +1 A useful answer to the question asked :-). (That may just maybe be a very very gentle dig but, hey, it's also a +1 :-) ). ie you o lots of positive useful stuff - it's a shame when some input seems somewhat cynical to a beginner's detriment. But perhaps I'm just seeing it wrong (happens. alas :-) ). \$\endgroup\$
    – Russell McMahon
    Commented Nov 5, 2014 at 12:16

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