IB Rx coil at Tx off

[Carl-NC]

I would start out on the high side. You can remove turns easily and take data as you do it.

[green]

Trying to decide on turns for figure8 coil. https://www.geotech1.com/forums/atta...2&d=1590926403
Tx: 15 turns gives 4A peak with 300us Tx. 15turns instead of 20turns for the DD. Maybe less signal?
Rx:20 turns for each figure8 coil should give highest signal? Inductance adds so Rx coil inductance=1178uh. Would 16, 17 or 18turns be better?

[green]

Think I'll do the 30inch DD first. Entered coil data from reply #18 into spice reply #17. Coil energy=I*I*L/2, 4A*4A*570uH/2=4.6J, 4.6J*480pulses/sec=2.2W. 2.2W heat the MOSFET and Rd at Tx off. Wondering how close to predicted the actual circuit will come(except for coil coupling, effected by target distance). Does the coil selection make sense(20turns Tx and Rx)? Any other thoughts before I make the coil? Goal to detect a US quarter at 24inches.

[Carl-NC]

400V@1A is 4 watts and if that is at a 1% duty cycle then you are at 40mW. That's low, but feel the MOSFET and see how hot it's getting. ?

Boy, I really screwed that one up... 400V@1A is 400W and at 1% DC you get 4W. Not sure what I was thinking. Anyway, at 4A you've got some serious heat issues. A higher breakdown FET moves the heat problem to the damping R and also the clamp R. In a total-loss PI system there is no way around the heat problem, the coil has been energized and that energy has to go somewhere. An alternative to total-loss is to capture that energy on a capacitor and re-use it in a kickstart circuit. That's been discussed, I think in a thread initiated by Deemon.

[green]

Constantly avalanching a MOSFET is an abuse of a silicon junction, but it's done all the time in PI and seems to do no long-term damage. What you have to watch out for is heat. 400V@1A is 4 watts and if that is at a 1% duty cycle then you are at 40mW. That's low, but feel the MOSFET and see how hot it's getting. That 4W peak creates heat that doesn't dissipate back down to ambient in one cycle, it builds up. If you avalanche 400V@4A then 16W is gonna make thing really hot... you'll either need a heat sink or a higher voltage MOSFET. There's still no Free Lunch, because now the damping resistor has to suck up all that energy, and it's gonna get hot. You may need a TO220 resistor with a heat sink.

400V@1A is 4 watts and if that is at a 1% duty cycle then you are at 40mW. That's low, but feel the MOSFET and see how hot it's getting. ?

[6666]

Thanks, easy to charge.

[Carl-NC]

14.4V

[6666]

Yes.

Thats an amazing result to get a coin at almost twice the coil diameter .
I read up on a couple of reviews on the AQ and they said that an external battery will be available some time ?, if its not a secret can I ask what voltage the external battery is, just wondering if it could be charged by solar panel in the field, thanks

[Carl-NC]

Constantly avalanching a MOSFET is an abuse of a silicon junction, but it's done all the time in PI and seems to do no long-term damage. What you have to watch out for is heat. 400V@1A is 4 watts and if that is at a 1% duty cycle then you are at 40mW. That's low, but feel the MOSFET and see how hot it's getting. That 4W peak creates heat that doesn't dissipate back down to ambient in one cycle, it builds up. If you avalanche 400V@4A then 16W is gonna make thing really hot... you'll either need a heat sink or a higher voltage MOSFET. There's still no Free Lunch, because now the damping resistor has to suck up all that energy, and it's gonna get hot. You may need a TO220 resistor with a heat sink.

[green]

Peak flyback is proportional to peak current so if 1A avalanches, say, a 400V MOSFET then 4A will require >1600V. Probably need to look at SiC FETs.

My coils are spiral so they are fast. They all avalanche at 1A, 1A avalanche hasn't damaged the MOSFET. Not sure how fast the bigger coils will be. Don't think the MOSFET can handle 4A avalanche continous? Tried spice simulation reply #17 with the 15turn figure8 Tx coil with avalanche battery disconnected. Close to 3000V. Been looking at MOV's or something similar. Looks like they might work but don't know if they can handle continuous? Other options? Would like to keep the speed, though it probably isn't needed with the quarter.

[Carl-NC]

Peak flyback is proportional to peak current so if 1A avalanches, say, a 400V MOSFET then 4A will require >1600V. Probably need to look at SiC FETs.

[green]

Calculated some coils. Should be able to get 4A peak Tx at 300us or 450us by selecting the right coil. Have been letting the Mosfet avalanche with 1A peak. With 4A peak, probably need to prevent avalanche. Any suggestions?

[Carl-NC]

Is that with the 12" coil ?

Yes.

[green]

Haven't tried calculating coil turns yet. A spice simulation to check calculations for 4A peak.

[green]

Thanks for the replies. More thoughts, any additions or suggestions?
Rx_AWG28 magnet wire
figure8_24inch diameter, turns___
DD_30inch diameter(24inch diameter for calculation), turns___

Tx_AWG19 magnet wire
figure8_24inch oval(40inch diameter for calculation), turns___
DD_30inch diameter(24inch diameter for calculation), turns___

Tx rate_480Hz adjustable+-, Tx on_300us min maybe 450us max, about 4A peak

power supply_12V