modified square wave driven coil

[rickb]

Within the past month or so, I saw a post about driving a coil with a modified square wave via an H-bridge. I think it was in the Geotech forum, but not 100% sure. Anyone recall it?



Rick

[Skippy]

This kind of drive is sometimes used in power inverters (eg 24V DC to 110V AC) probably because it's more efficient, but it will also result in a cleaner sine-wave output, too.
As for MD coil driving, it's not necessary for single freq operation, but if you were contemplating using a non-tuned coil, and wanted to change frequencies, or change to a dual-freq mode, then it makes some sense.

[Qiaozhi]

This kind of drive is sometimes used in power inverters (eg 24V DC to 110V AC) probably because it's more efficient, but it will also result in a cleaner sine-wave output, too.

Modified sine-wave is only used for small off-grid inverters, and cannot be used to drive certain devices. The only reason for using modified sine-wave is to simplify the conversion from DC to AC, and to reduce the level of unwanted harmonics. Otherwise you have to use PWM and some heavy filtering to generate a pure sine-wave.

As for MD coil driving, it's not necessary for single freq operation, but if you were contemplating using a non-tuned coil, and wanted to change frequencies, or change to a dual-freq mode, then it makes some sense.

That's also the only treason I can think of, but I suspect the harmonics would be a problem with an untuned coil.

[rickb]

This is just an experiment to see how narrow the pulses can be and still have a low distortion sine wave in a high Q tuned coil. The idea is to add just barely enough energy each cycle to keep it going.

[Skippy]

An interesting experiment. Commercial machines typically put less than 20 mW into the coil, and have 10+ times that for the remaining electronics, so improving coil-drive efficiency is probably not that important. But the idea of reducing distortion and higher harmonics sounds like a more worthwhile use for the technique.
I had a quick look online to see what the optimal duty cycle was for minimum distortion, but couldn't see a definite answer, something like 75% on, 25% off seems near, (eg. 38% +ve, 12% Off, 38% -ve, 12% off).
Some background reading here:
http://mathscinotes.com/2013/02/ac-b...ted-sine-wave/

[Davor]

In reality the harmonics are more of a problem in tuned than in untuned systems, because in untuned ones there is no sharp phase shift to make harmonics lively, and there are no even order harmonics to begin with. The rate of change of harmonics in tuned systems is proportional to the harmonic's order, hence 5th is more lively than the 2nd etc.
Now, the reason why it is more of a problem for tuned tanks is not intuitive, but it has all to do with tank loading due to the proximity of conductive object and soil. Because the harmonics, although attenuated, are very lively, they turn a rough soil surface into a series of false positives. Full wave Rx configuration discards even harmonics, hence it is less prone to the effect.

[rickb]

That's also the only treason I can think of, but I suspect the harmonics would be a problem with an untuned coil.

Some dare call it treason

[Qiaozhi]

Some dare call it treason

That was an FFT problem ... "Fat Finger Trouble".