BIPOLAR ALTERNATIVE TO H BRIDGE

[lucifer]

The key is NOT the supply. A constant current source has a high source impedance and cant be used in this application. The detector coil ( and associated magnetic field ) is waving around in the air anyway so the motion ( acceleration / deacceleration ) is more of an issue.

moodz.

Some refer to this as a current source inverter. For example see attached extract from an old book on power electronics. Another similar concept with some improvements is presented in the second PDF paper attached.

[moodz]

The key in those circuits is the supply. It has to be a constant current source. Then both the standard H-bridge and moodz circuits work. I've been playing for a while with a standard H-bridge powered by a constant current constant voltage module. Power consumption was OK. However the current pulses were not completely flat-topped but I was using a high inductance coil and didn't measure the A/sec rate. Seems that a lower inductance coil is better. Still have the circuit laying somewhere round here.

The key is NOT the supply. A constant current source has a high source impedance and cant be used in this application. The detector coil ( and associated magnetic field ) is waving around in the air anyway so the motion ( acceleration / deacceleration ) is more of an issue.

moodz.

[Riss]

So they don't gnaw me dark thoughts anymore (that there is no way to get it clean DC ) , I took 6 pieces IRF630 and the soldering iron to see how the circuit works on the oscilloscope ? The key phrase in this thread is What's missing? 06-03-2021, 09:47 AM #4 , Carl -NC . My coil is about 1.5 millihenry / 4,5 ohm , i added 4.7 nanofarads cap. in parallel . Frequency - about 1 kHz . Power supply - 2.5 volts . I really liked how it worked - there was little difference between the starting current and its magnitude before the reversal . Of course something is missing - this is not PERPETUUM MOBILE - a simple compensation circuit must be added to compensate active losses in L and C ( when the current through the coil reverses ) . Current generator ideas are unreasonable -heating energy became more expensive .

[lucifer]

Tried some MOSFETS with lower Coss. Was thinking it might help. Not. Circuit works but can't get Tx constant current(A/sec)after reversal as low as I would like. Still not sure how low it should be. Maybe my coil is causing some of the problem. Back to playing with unipolar and standard bipolar.

The key in those circuits is the supply. It has to be a constant current source. Then both the standard H-bridge and moodz circuits work. I've been playing for a while with a standard H-bridge powered by a constant current constant voltage module. Power consumption was OK. However the current pulses were not completely flat-topped but I was using a high inductance coil and didn't measure the A/sec rate. Seems that a lower inductance coil is better. Still have the circuit laying somewhere round here.

[green]

Back to playing with the unipolar circuit. Added a 7V Tx supply so Tx signal would stay the same as battery discharged. Interested in what others see when using ferrite as the target with their detectors. I have been using a toroid core about 25mm diameter. I have tried other cores similar size and bags(around 10 cores)with smaller cores with similar results. Thinking ferrite target should be X signal with very little if any R signal.

Tx_328uH 4.2 ohms, Rx_133mm fig8

Haven't given up on bipolar yet but so far unipolar is winning for me.

[moodz]

The transmit coil in the GPZ7000 is very low resistance ...

I think the designers of the GPZ would have had lots of trouble eliminating your issue which partly is due to coil resistance.

[green]

Tried some MOSFETS with lower Coss. Was thinking it might help. Not. Circuit works but can't get Tx constant current(A/sec)after reversal as low as I would like. Still not sure how low it should be. Maybe my coil is causing some of the problem. Back to playing with unipolar and standard bipolar.

[green]

Playing with moodz CC circuit on the bench again. Changed R2(polar2g)to 3.33 ohms. M1 and M2 not the same as simulation. Looks like it corrects CC like the simulation. Want to try some different MOSFETS for M1 and M2. Increase Tx coil current and frequency. Any other suggestions to try? Wondering why amplifier doesn't come out of saturation sooner(appears Tx pulse is about 1.5us).

[moodz]

One of my goals lately has been to detect #9 lead shot with a PI. Appears I need to start sample 3.5 to 4us after Tx off or polarity switch(CCPI). Including some scope recordings with unipolar, bipolar and CC Tx that I have been playing with. The ferrite core has the least effect with the unipolar Tx. I have moodz CCPI circuit working with spice. I get more lose with my real circuit when polarity switches so my spice circuit won't work. I like moodz circuit, should be able to increase Tx pps 4times faster than unipolar(1000pps to 4000ps). Haven't given up on moodz circuit yet, need another approach to keeping Tx current constant during target and ground sample time or reducing lose when Tx polarity switches. Any suggestions appreciated other than reducing Tx resistance(would like to use my existing coils if possible)The unipolar Tx seems to work but I think moodz CCPI would be better if I could get it working.

I have a GPZ7000 detector that uses a similiar transmit waveform and the way it does it is not simple .... but I do find alot of lead shot with it.

My Magpi project can detect very small shot and samples in the range you want .... https://www.geotech1.com/forums/show...462#post289462

The CCPI circuit is only a few weeks old ... I have not done enough bench time yet to meet your requirements.

moodz

[green]

One of my goals lately has been to detect #9 lead shot with a PI. Appears I need to start sample 3.5 to 4us after Tx off or polarity switch(CCPI). Including some scope recordings with unipolar, bipolar and CC Tx that I have been playing with. The ferrite core has the least effect with the unipolar Tx. I have moodz CCPI circuit working with spice. I get more lose with my real circuit when polarity switches so my spice circuit won't work. I like moodz circuit, should be able to increase Tx pps 4times faster than unipolar(1000pps to 4000ps). Haven't given up on moodz circuit yet, need another approach to keeping Tx current constant during target and ground sample time or reducing lose when Tx polarity switches. Any suggestions appreciated other than reducing Tx resistance(would like to use my existing coils if possible)The unipolar Tx seems to work but I think moodz CCPI would be better if I could get it working.

[moodz]

Are these pictures taken with the same circuit from reply#2? (https://www.geotech1.com/forums/atta...622687902)from reply#2

YES except ...

The same circuit but NOT the same coil .... since I am a PI man ( not Homer LOL ) and mostly mess with mono coils .. I had to find a coil more like the one I think you were using.


The coil in the picture has one winding that is 400 microHenry and another that is 3.4 milliHenry. I was going to connect the TX to the 400 coil but it would not work ( looked like there were diodes across it shorting the flyback ) ...
so I thought what the heck I will connect it across the 3.4 mH coil and thats what I measured at the 400 coil ... better result than I get with PI coils LOL.

So the peak current is not so high the flyback is lower and takes 6 microseconds ( prob because its 3.4 mH ) not 500 nanoseconds ( used 300 uH Pi coil ) as per the link above.

Its a commercial coil .. dunno whats inside ??

... looked everywhere and cant find my linear hall sensors .. ( if you dont need something you will trip over it ... if you do need something it will not be there )

[green]

Hmmm ... I am not treating it as a pulse induction style waveform for RX.
I did use a PI to generate the gate drive though.
I did not use an amp either ... CRO is connected directly to coil RX and TX.

The TX coil impedance is 3.4 mH and several ohms.

I am going to dig out a linear hall effect sensor to measure the mag field .... post later :-)


[ATTACH]55697[/ATTACH]
[ATTACH]55698[/ATTACH]
[ATTACH]55699[/ATTACH]

moodz

Are these pictures taken with the same circuit from reply#2? (https://www.geotech1.com/forums/atta...622687902)from reply#2

[moodz]

Hmmm ... I am not treating it as a pulse induction style waveform for RX.
I did use a PI to generate the gate drive though.
I did not use an amp either ... CRO is connected directly to coil RX and TX.

The TX coil impedance is 3.4 mH and several ohms.

I am going to dig out a linear hall effect sensor to measure the mag field .... post later :-)






moodz

[eclipse]

Lower resistance coil <1 Ohm, lower resistance Ron mosfets perhaps?

[green]

Finally got moodz CCPI circuit working after zapping a couple resistors and a few IC's, wiring errors. Loosing more energy than the spice simulation when switching polarity. Tried both polypropylene and NPO ceramics across the coil, didn't see a difference. Have some different series high voltage MOSFETS to try. Was hoping to use my existing coils, Tx resistance 3 to 4 ohms. Seems to work alright in spice. Any other suggestions what I might try?