MPP with DD coil 580 uH and short delay

[Teleno]

Today I hit gold for the first time with this machine. About 5 cm deep in wet salty sand.

[6666]

Great find.

[Teleno]

The MOSFET (IRF740) is avalanching at 400V for a few usecs. Not good. I've replaced it with a 900V MOSFET in an attempt to increase the transient peak voltage and shorten its decay.

The new peak is 550V with no avalanche. Less than expected. The reason for this low peak is the high capacitance of the coil (estimated 750pF).

No improvement with the transient's decay, instead I got a slightly longer saturation period at the preamp. Minimum sample time increased from at 5,5us to 6us.

I'm expecting a new DD coil (for ML GPX) to arrive next week. It should have less L and lees C.

The coil I'm using now is this:

it probably has some capacitor inside because it's meant for VLF detectors. No the best choice for a PI.

[eclipse]

I've tested my Delta Pulse with high voltage Mosfet (950V), results where overheating of both power resistors and 50% reduced depth (pulse width 100us-400us)
Mosfet was high-end, latest generation (low Coss, low Rds< 0.1R, low Qg and so on).

[Teleno]

I've tested my Delta Pulse with high voltage Mosfet (950V), results where overheating of both power resistors and 50% reduced depth (pulse width 100us-400us)
Mosfet was high-end, latest generation (low Coss, low Rds< 0.1R, low Qg and so on).

Did you check how high the peak voltage was after the replacement? How about the output of the preamp?

My guess regarding depth is that the gain of the preamp has to be reduced because the signal is now stronger, which causes saturation at early times.

Regarding overheating... how does the Rds_on of the new MOSFET compare to IRF750? If it's lower then the current at the coil increases and you should either reduce the pulse width or add an appropriate resistor in series.

[eclipse]

The Mosfet was STW40N95K5

N-channel 950 V, 0.110 Ω typ., 38 A MDmesh K5

Coil was 1.6R @ 550uH, added resistors in series (4 x 2.5R, 2 Watts each) - those showed signs of high heat too (!)

Have no scope at this time so no measures were taken, just swapped and tested.

Depth was significantly lowered no matter the pulse width (100-400us).

Mosfet had almost no heat, could handle all this without a heatsink.

AAA!.. some more info:

I used diode in series RHRP15120_F102
Also had DC/DC converter installed and tested different voltages to power the mosfet from 8-15V.
Results were... at 8-10V the depth wasn't that bad, heat wasn't too much. At 10-12V things started to heat up, more than that you could feel the heat on the resistors (high quality 3W resistors by IRC / TT Electronics
Depth was going DOWN as voltage went up. At 12V+ loss was 50%

[green]

The MOSFET (IRF740) is avalanching at 400V for a few usecs. Not good. I've replaced it with a 900V MOSFET in an attempt to increase the transient peak voltage and shorten its decay.

The new peak is 550V with no avalanche. Less than expected. The reason for this low peak is the high capacitance of the coil (estimated 750pF).

No improvement with the transient's decay, instead I got a slightly longer saturation period at the preamp. Minimum sample time increased from at 5,5us to 6us.

I'm expecting a new DD coil (for ML GPX) to arrive next week. It should have less L and lees C.

The coil I'm using now is this:

it probably has some capacitor inside because it's meant for VLF detectors. No the best choice for a PI.

[I can tell you what my case is:

Circuit: MPP rev-D,
DD balanced coil.
Tx pulse: 100us,
Tx Current: 1.1A,
Tx coil: 580uH - 1 Ohm, damping: 700 1Ohm,
Rx coil: 500uH - 5.4 Ohm, damping: 1000 1Ohm,

The MOSFET (IRF740) is avalanching at 400V for a few usecs. Not good. I'll replace it with a 900V MOSFET and see how it improves the decay]from another thread

Don't know why it should take a few usec. Avalanche until coil current drops to 400/700(.57 amps). Decay rate at 400v=400/580(.69amps/usec). 1.1amps-.57amps=.53amps. .53/.69=.77usec at avalanche.

[Teleno]

Apart from the mods in posts #1 and #3, I've also replaced some IC's with improved equivalents as follows:

- replaced U5 (preamp) with an LM4562 (Dual, Low noise, 55MHz bandwidth, 20V/us slew rate, 100uV input offset, 120dB PSRR)
- replaced U8 (integrator) with a TLC082 (Dual JFET, 10MHz bandwidth, 16V/us slew rate, 60uV input offset).

Do not mistake the TLC082 with the good old TL082, the TLC is a modern development with huge improvements in all parameters.

This set-up (#1, #3 and #23) can detect a 0.3 g gold nugget at about 3 cm distance in air. A gold chain at about 5 cm.

[Teleno]

Detection of the 0.3g gold nugget.

[6666]

replaced U8 (integrator) with a TLC082 (Dual JFET, 10MHz bandwidth, 16V/us slew rate, 60uV input offset).

Thanks this is interesting information, did you measure the actual offset with the opamp in circuit ?

[Polymer]

I have shorted C12 in my MPP Rev-D following the advice of Eric Foster in this thread:

http://www.geotech1.com/forums/showt...381#post219381








The detector seems to work just the same (indoor tests). Recovery after a strong target signal seems faster.

I'll check whether the EF signals I had been noticing will go away.

I have tried different capacitors and short circuit as to C1 (MPP E) mentioned here, measuring voltage along the way at R19 on the shown schematic.
These are the results:

short: - 130mV
470nF: -100mV
150nF: 0mV
100nF: +100mV
33nF: +920mV

I can't seem to discern any major difference in detection, but have yet to try what I just read - Teleno mentions maybe faster recovery time.

Not sure what to make of my results or how to interpret them.

Any pointers?

Thanks!

[sinclairuser]

Hi guys! I'm back from my vacation in NW Spain where I had lots of detecting and gold prospecting fun. I must say the MPP passed the beach tests with fying colors, it's able to find tiny coins like "perra chica" (1870) at great depths in a mixture of wet sand and rubble.



Met people who are detecting nuggets in old riverbeds but I hadn't the time to gove it a try. Panning did show up some flakes, though:



I'll be back next summer and would be glad to meet you guys over there to share the fun. Detecting is legal, no permits required. The same for gold prospecting and there's plenty of it. Who would like to join?

hi teleno, i thought detecting was not legal in spain?.

[Teleno]

hi teleno, i thought detecting was not legal in spain?.

Spain has become a conglomerate of kingdom, the autonomic regions, and each one has different regulations. A permit is requires for using a metal detector in all Andalucia, in Asturias where I've been you're free to detect on puclic places aincluding beaches, as long as you stay 2 Km away from an archeological site. Gol prospecting is sllowed as long as you only use manual means, no machinery.

[sinclairuser]

Spain has become a conglomerate of kingdom, the autonomic regions, and each one has different regulations. A permit is requires for using a metal detector in all Andalucia, in Asturias where I've been you're free to detect on puclic places aincluding beaches, as long as you stay 2 Km away from an archeological site. Gol prospecting is sllowed as long as you only use manual means, no machinery.

thanks teleno good to know!.

[goldfinger1]

Hello, can you post your images in a different format? the system does not recognize them, can you tell me how you checked the coil induction, and how to work out the damping resistors?