BIPOLAR ALTERNATIVE TO H BRIDGE

[moodz]

Good job, Moodz! I've been playing with your circuit in LTSpice and I'm eager to try it in real. I understand that only 2 high voltage mosfets are required since the rest don't see the flyback voltage. It's amazing how well it works with such a low power supply voltage.

Thanks Lucifer ... I have not tested the lowest effective voltage operation but 1.24 volts was very good IMHO.

[green]

Ordered some parts to make the circuit, including some polypropylene and NPO ceramics to compare across the coil. Reply #25 suggests constant current slope should be around .2A/ sec or less. Haven't been able to with spice, wondering how to measure slope in real circuit. Maybe a test to verify slope isn't causing a problem?

[Carl-NC]

An easy way to measure the current slope is with an RX coil. Current slope will induce a +/- offset in the RX coil.

[green]

Hi moodz,
Thanks for sharing your circuit! Can you please explain how it is performing better than the standard H-bridge circuit with diodes instead of your isolation switches? Is there any benefit in using the MOSFET body diodes instead of normal diodes? In terms of efficiency your additional switches add a bit more resistance in the current path when turned on, though that would be negligible if low on-resistance MOSFET is used.
Is a single gate driver capable of driving all 3 the transistors at once?
[ATTACH]55539[/ATTACH]

I have a similar bipolar Tx circuit on the bench. Trying moodz circuit to compare. [With the same change in Tx current at switching] target signal should be the same. Moodz circuit should subtract less so Rx signal should be higher for some targets. Moodz circuit would have higher average current but maybe less power supply watts. Moodz circuit looks better to me, getting constant current slope low enough is one thing that concerns me.

[green]

An easy way to measure the current slope is with an RX coil. Current slope will induce a +/- offset in the RX coil.

Thanks, I can control ramp rate(20A/sec to 12500A/sec) or hold current constant at .5A with my target tester. Connected it with separate Tx and Rx coils(Rx amplifier gain=500) to see how low a ramp rate I could detect. Current is in a control loop, looks good when scoping current feedback resistor, but Rx signal to noisy when Tx is on. Current not in a control loop with moodz circuit so it should work. Reply back when I get the circuit working.

Maybe moodz could try with his circuit?

[green]

Thanks, I can control ramp rate(20A/sec to 12500A/sec) or hold current constant at .5A with my target tester. Connected it with separate Tx and Rx coils(Rx amplifier gain=500) to see how low a ramp rate I could detect. Current is in a control loop, looks good when scoping current feedback resistor, but Rx signal to noisy when Tx is on. Current not in a control loop with moodz circuit so it should work. Reply back when I get the circuit working.

Maybe moodz could try with his circuit?

Thinking I should have tried without the amplifier. Both Tx and Rx were 8inch diameter around 300uH coils. Rx was shielded(thicker) and was taped to Tx. (E=IL/T) 12500*.0003=3.75V, 500*.0003=.15V. (Coupling)1.7/3.75=.453. 0.15*.453=.068 close to measured. 1A/sec calculates an offset of 136uV at Rx(68mV change at amplifier out, gain=500).

[green]

Tried again with the amplifier. Used 133mm fig8 Rx. Looks like I'm getting closer to be able to see a 1A/sec change. Still learning, anyone have any suggestions how to improve the resolution.

[green]

Still thinking about measuring steady state current(A/sec)after polarity change, X signal. Still wondering if .2, 1 or 5A/sec is to much. Thinking 250us on time each polarity, .5 to .6 A peak(1 to 1.2A change). I can sample at 4us with the bipolar circuit I'm testing now. Looks like I could sample sooner with Moodz CCPI circuit in spice. Think I might need the X signal gone around 3us after polarity changes. Coil current change is probably between 1 and 2 million A/sec at switching polarity. Still wonder if I can detect a 1A/sec change 3us after a 1.5millionA/sec polarity switch. Wondering if there might another method to detect X signal I tried looking at amplifier out with some different targets with my unipolar circuit. The nickel went negative(X signal)before going positive(R signal). Ground appears to have X and R signal. Ferrite appears to have X signal an maybe small amount of R, R signal maybe do to my circuit. Wondering if there is a ferrite target that has X signal only. Overdamped coil shows more X signal.

[green]

Remembered reading in another forum about using a ferrite core to ground balance GPZ7000(CCPI). Didn't make sense because ferrite core has little or no R signal. Wondering if ferrite core is used to adjust circuit to eliminate X signal?

[Carl-NC]

Remembered reading in another forum about using a ferrite core to ground balance GPZ7000(CCPI). Didn't make sense because ferrite core has little or no R signal. Wondering if ferrite core is used to adjust circuit to eliminate X signal?

The ferrite is needed to GB the GPZ because of the DOD coil. The DOD is subtractive so it has first-order cancelation of a broad ground signal. So a point-source ground is used instead.

[green]

The ferrite is needed to GB the GPZ because of the DOD coil. The DOD is subtractive so it has first-order cancelation of a broad ground signal. So a point-source ground is used instead.

I use a figure8 Rx coil. I tilt the coil on he ground, one end higher than the other when I ground balance. Seems to work, does the DOD with 7000 act different? I've read the GPZ7000 coil is wired subtractive in some replies and additive in other replies. Are you certain is subtractive?

[Carl-NC]

I've also read that it depends on the mode. I don't know for sure. But, yes, if you tilt the coil it will help with GB.

[6666]

I've also read that it depends on the mode. I don't know for sure. But, yes, if you tilt the coil it will help with GB.

I have not heard of tilting the coil before how does this help with GB ? thanks.

[green]

I have not heard of tilting the coil before how does this help with GB ? thanks.

I use a figure8 Rx coil. Figure8 can cancel ground signal, tilting the coil(one end closer to ground)causes a ground signal. Need a ground signal before you can adjust something to cancel. Reason I asked about using ferrite with the GPZ7000. The ferrite I have doesn't have a R signal to cancel. Maybe their ferrite has the same R signal as ground?

[6666]

I use a figure8 Rx coil. Figure8 can cancel ground signal, tilting the coil(one end closer to ground)causes a ground signal. Need a ground signal before you can adjust something to cancel. Reason I asked about using ferrite with the GPZ7000. The ferrite I have doesn't have a R signal to cancel. Maybe their ferrite has the same R signal as ground?

Ah thanks, so tilting CREATES the ground signal, thats interesting .