p-channel mosfet replacement

[ionut_mtb]

I see most of the pi detectors are using n-channel mosfet to drive the coil. Why is it better?

I would like to find a better mosfet for surf/bara, as pushing the detector for more depth irf become hot, even with heatsink. Did someone experiment other mosfets? What do you recommend?

Thankyou

[6666]

there is simply more choice for n-channel as they are cheaper and easier to make for higher voltages.

[kt315]

a problem that you do not see. or do not want to see.
if you take mosfet with more SD voltage it will be with more Rds too. so you get again more hot.
its rule. mosfet in surf and bara was choiced optimal under fixed 50us PULSE WIDTH and by the way low cost.

[ionut_mtb]

a problem that you do not see. or do not want to see.
if you take mosfet with more SD voltage it will be with more Rds too. so you get again more hot.
its rule. mosfet in surf and bara was choiced optimal under fixed 50us PULSE WIDTH and by the way low cost.

Surf pi irf 9640 has VDS -200V, RDS(on) 0.50ohm. i had modified all the timings. my flyback voltage of mosfet is about 220V, power consumption don't know as i do not have all equipment with me where i stay now. i do only testing, i keep detector on for maybe 10 minutes and get's hot with heatsink, maybe burn if keep hunting for hours. Mosfet's Rds is not varying to much 0.5-1ohm.

My level of knowledge in theoretical electronics is down to 0. I do only practice. My brain does not helping me with theory. Thanks for understanding.

[kt315]

set 1ohm resistor in source, with capacitor >2200uf from source on gnd. it has to decrease hot.

[Sean_Goddard]

It became hot due to it going into Avalanche mode. This is probably because your flyback pulse exceeds the Vds max of the FET you are using. You need to look at the peak flyback and pick a FET with the appropriate rating. Start with anything above 450V.

[kt315]

on >200v with ordinary probe of your o-scope you can burn last one easily. you have to use especial HV probe.

[ionut_mtb]

on >200v with ordinary probe of your o-scope you can burn last one easily. you have to use especial HV probe.

Thank you for answers guys.
I use X10 switch on the oscilloscope probe when i measure across the coil leads.

[ulix]

N Mosfets are faster, because of their doping!

[kt315]

-----------------10ohmcoil----
Beachscan

Posted by: Eric Foster(---.ipt.aol.com)
Date: April 6, 2004:35PM

This is an example of a PI detector that breaks with traditional principles to some degree. Instead of pulsing a high current through a low resistance and low inductance coil it pulses only 0.2A peak through a 10 ohm coil that has an additional 39 ohms inseries. The coil also has an inductance of over three times the usual amount but we are still able to sample at the magic 15uS for use on a wet beach. The MosFet can be a small one with a relatively low voltage rating and the mean dccurrent taken from the batteries is only 65mA giving good life from 8AA cells.
Eric.

[ionut_mtb]

"I have designed PI detectors that run coils of 1mH and above but they have lots of series resistance to improve the time constant and run at at much higher repetition rate i.e 5 - 10kp/s.
Eric"

Many designs for PI detectors. What detection depth capability had Beachscan?

[kt315]

i think Beachscan is perfect for high mineralized ground and was done for that conditions.