My opinion on MOSFETs

[Tec]

Reposted graphs

Thanks for the replies, I've posted the graphs in nice PNG format.

These are the actual coil spikes, on the coil (50V per division)

Notice one has a slight ring right at the beginning, this I believe is what makes it very sensitive to wet sand, and makes a huge difference..
This ring will occur regardless of coil damping,..increase or decrease the damping, the ring will still be there..
THe different mosfets required different damping resistance..the damping resistance seems more to do with mosfet than coil.. The fast mosfet needed more damping (250 ohm), the slower mosfet needed 330 ohm.

damping is selected based on what is happening after the amplifier..which shows up any slight ringing. Again, these graphs are of the coil spike at the coil.

[Rumcajs]

Hi,

as far as I know, reducing the switch-off time can be an driver/isolation transistor between the driver ICs and MOS-FET (such as Barracuda, Delta Pulse, Hammerhead).

From the ,,555 Data book,, the driver IC provides enough energy to turn on the MOS-FET, BUT the corresponding off-speed performance of the IC is no longer sufficient, beacuse charging the reelative high capacity of the MOS-FET.

The result is definitely better than using an low capacity MOS-FET, while the voltage affects the search depth.
In the sandy beach of the targets 90% of 5-9cm from the top is in, so this is probably enough depth to be 75V.
To lowest voltage may be use an smaller puffer capacitor or/and lower suppy voltage.
Result: longer battery lifetime.

The speed increase would be good to explicitly dedicated RF-transmitter MOS-FET also try to availability and price is good.

It is very important for the equivalent damping resistance is set, otherwise do not get a satisfactory result. The resistance value of the setting, see this pdf.: Carl Moreland: Hammerhead.

Ok, from the pictures:

What are the two oscillogram photo?
Which IC, which have pin?

If your figure is from 5534 preamp IC, the your coil is strongly UNDERDAMPED.

The after spike must be an certain time period of negative phase, an it followed by a smaller peak and the finally an exponential section.

Good hunting

Rumcajs

[Davor]

Can you please attach images in some graph-friendly format, such as gif or png? These are small and not scalable, and I have only vague idea about what's going on there.

[Tec]

Hi,

WIth regard to PI detectors,

There is the general belief that higher voltage break down, means faster discharge, and therefore sensitive to smaller targets.

I tried a MOSFET with much 'better' specs : lower resistance, faster turn on and turn off time, higher voltage breakdown (600V).

And it made the detector sensitive to wet sand, this is because an oscillation occurred (to shield, or ground or something) (see attached) I think.

With the standard mosfet: IRF2405, the detector is sooo quite and stable, even in wet sand, and in the water.

However, simply changing the mosfet, can turn it into a noisy piece of garbage.

Observations:
The high breakdown voltage, does not really improve the response to smaller targets, it simply means the discharge will be faster initially at the very start., but as soon as the threshold voltage is reached, i.e for most of the discharge, there will be a good rapid magnetic field change.

Even if the maximum flyback voltage is limited to 75 volts (by a diode), the detector can still be very sensitive to small targets.. intially the discharge is slow, and the voltage is greater than 75V, however when this point is reached, the change in magnetic field, is almost the same as with the higher voltage ...the delta-magfield has only been delayed.

The most important aspect of target sensitivity and fast discharge is COIL CAPACITANCE..

If anyone has any ideas what going on with the above curve, to make the detector sensitive to wet sand, let me know.

Note, with the standard mosfet, I can dig a hole in the sand under the water with the search coil, and it wont beep. (unless there is a target).