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Topic: Phase change through transformer? (Read 3 times) previous topic - next topic

Socco

Hi there

I am wanting to to detect 240V mains zero crossings for a TRIAC based heater controller.  I have a 240/12 Volt transformer whose output is rectified, regulated to 7.6Vdc which powers the arduino.

My question is: Will the transformer's output be phase shifted with respect to its input?   I am planning to put the 12Vac lines into a comparator (op-amp) and have arduino check the comparator's output to sense the Mains zero crossing.  Anti-phase is of course no problem.  Unfortunately I have no oscilloscope to check directly. 

Will this work, or will the transformer induce a phase-shift?

thanx,
Socco

retrolefty

Quote
Will this work, or will the transformer induce a phase-shift?


Either 0 or 180 degrees phase shift, depending on which terminal of the secondary winding is used for circuit common. Either way zero crossing should have no phase difference.

Lefty



jackrae

Can't recall the reason why but I believe there is a 30 degree phase shift between input and output.  I experienced this in reality when trying to connect a pair of power circuits in parallel many years ago.

The 0 or 180 degrees mentioned previously refers to start and finish end of the secondary winding.  However this does not consider the shift from primary to secondary.

If you don't believe this then there is a very simple experiment that can be done to illustrate that there is indeed a phase shift.

Set up two transformer circuits

Circuit "a" contains only one transformer , say 240 to 12 volts

Circuit "b" contains 2 transformers  the first is 240 to 110  and the second is 110 to 12

If there was no phase shift through the transformers then the two 12 volts outlets would be in-phase

However you will find they are not in phase, in fact they will be out by 30 degrees

Each transformer has a 30 degree phase shift so circuit "a" is lagging the input by 30 degrees and circuit "b" lags the input by 60 degrees (due to having 2 transformers)


pluggy

Doesn't a triac only switch on or off at zero crossing anyway ?  I've had to put snubber circuits on triacs so they'd turn off at all with inductive loads.

MarkT

Firstly triacs only switch _off_ on zero crossings, they can switch on at any time.

Secondly a perfect transformer doesn't introduce a phase shift (and its windings have infinite inductance)!  In reality a good transformer at full load will act pretty close to a perfect transformer, but if its at low load the inductance of the primary will start to matter.  So expect a small phase shift.

However this is all moot as the triac switches off at zero _current_ and the transformer will sense the zero _voltage_ crossings - if your AC load is not purely resistive the load itself is introducing a phase shift.   Fortunately heaters are usually 100% resistive at mains frequencies.

[ I won't respond to messages, use the forum please ]

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