IRF9640 GOOD SNUBBER SCHEMATIC NEEDED

[daverave]

anyone please got a good working snubber schematic for a IRF9640 as used in surf pi.

[dantech]

IRF9640 works very well in the 2nd ChancePI (described on this forum) . I do not know what you need in fact but there you can see how IRF should be used properly in the PI

[daverave]

i tried pushing the IRF9640 by using a 70 uS tx and 15 volt supply and used a fair sized heatsink but after about 20 mins running time i found the detector loosing depth so i then switched off and allowed to cool but the mosfet didnt recover and i lost several inches in air depth.

[dantech]

There are only 2 possible reasons of your problem:

- too wide pulse
- too high frequency of pulses

I did a lot of test and I find that there is no sense to use frequency higher than 400Hz and pulse witdh wider than 140us.
In my opinion for coils with resistance 1 ohm (eg. basket coil wound by 0,8 mm wire) pulse width 120us is enough to saturate them. Frequency should be 100, 200 or 300Hz for slow sweeping, normal and fast. The same parameters I use in 2nd ChancePI project.
Low resistance coils are able to saturate faster.
Maximum power consumption for fast sweeping and 120us together with detector logic is 110mA and MOSFET is little bit warmer than hand without any heatsink. Power source is 12V gel battery - 13,8V when fully charged)

In my opinion in your case 70us pulse width won't saturate your coil properly and you use frequency much higher than 400Hz. Frequency is the reason your IRF is overloaded and one is hot.

People excessively want to overdrive their PI by pulse width and frequency but it is wrong way. The coil can store some limited energy and nothing more.
After coil saturation the rest of energy is converted into a heat in the weakest point - it means IRF
After that people say that PIs consumes too much energy

Is it clear now for you?

[kt315]

put a resistor in fet source 1-2 watt 1-2 ohm. it must take some heat energy from fet on itself.

[dantech]

There is no sense. It is necessary to remove the source of problem. Serial resistor is not good solution.
Keep in mind that every excessive heat drains your battery in fact.

[daverave]

There are only 2 possible reasons of your problem:

- too wide pulse
- too high frequency of pulses

I did a lot of test and I find that there is no sense to use frequency higher than 400Hz and pulse witdh wider than 140us.
In my opinion for coils with resistance 1 ohm (eg. basket coil wound by 0,8 mm wire) pulse width 120us is enough to saturate them. Frequency should be 100, 200 or 300Hz for slow sweeping, normal and fast. The same parameters I use in 2nd ChancePI project.
Low resistance coils are able to saturate faster.
Maximum power consumption for fast sweeping and 120us together with detector logic is 110mA and MOSFET is little bit warmer than hand without any heatsink. Power source is 12V gel battery - 13,8V when fully charged)

In my opinion in your case 70us pulse width won't saturate your coil properly and you use frequency much higher than 400Hz. Frequency is the reason your IRF is overloaded and one is hot.

People excessively want to overdrive their PI by pulse width and frequency but it is wrong way. The coil can store some limited energy and nothing more.
After coil saturation the rest of energy is converted into a heat in the weakest point - it means IRF
After that people say that PIs consumes too much energy

Is it clear now for you?

Hi dantech.......My tx frequency is around 3.2 khz and i tried a tx width of 70 uS and around 15v supply but as i stated after about twenty mins i lost in air depth...after that the mosfet never recovered fully and permanant loss of depth even from cold...i use 3.2 khz as i thought its best for gold rings which are low conductive.

[daverave]

put a resistor in fet source 1-2 watt 1-2 ohm. it must take some heat energy from fet on itself.

Hi kt...thats a good idea...although my coil is around 2.6 ohms.

[dantech]

Wowwwww...

Why 3,2kHz but not 10kHz?

That is source off your problem. You PI is very bad in this moment.

- coil is under saturated constantly
- mosfet is overloaded

Like I said 400Hz and 140us should be maximum. Pulse witdh little bit varies depending on coil parameters.
Using higher frequencies in the PIs is bad. It gives overheating, high battery drain and reflected signal distortion only.
Serial resistor is prosthesis.
Change it ASAP.

What is total power consumption in your case?
1 or 2 Amps?

To check you coil resistance use technical method. I mean voltage and current source.
Multimeters do not measure low resistances with proper precision.

[dantech]

I use 3.2 khz as i thought its best for gold rings which are low conductive.

It is not true. Frequency gives you only sweeping speed comfort, nothing more. 200Hz is enougt in most cases but 300Hz gives you comfort
You need fast coil - proper coil saturation only in short pulse as is possible (120us)
Look onto movie with 120us/200Hz pulse

https://www.youtube.com/watch?v=7vIdAO51E_c
https://www.youtube.com/watch?v=uYxHxEehncY
https://www.youtube.com/watch?v=PDnZ3_BuyM0

[daverave]

Wowwwww...

Why 3,2kHz but not 10kHz?

That is source off your problem. You PI is very bad in this moment.

- coil is under saturated constantly
- mosfet is overloaded

Like I said 400Hz and 140us should be maximum. Pulse witdh little bit varies depending on coil parameters.
Using higher frequencies in the PIs is bad. It gives overheating, high battery drain and reflected signal distortion only.
Serial resistor is prosthesis.
Change it ASAP.

What is total power consumption in your case?
1 or 2 Amps?

To check you coil resistance use technical method. I mean voltage and current source.
Multimeters do not measure low resistances with proper precision.

when i was running detector at 70 uS tx/3.2khz freq/15v supply/2.6 ohm coil...i was drawing from the supply about 300 mAH.

[eclipse]

I suggest trying lower voltages on the Mosfet - try 9/10/12V.

[kt315]

Hi kt...thats a good idea...although my coil is around 2.6 ohms.

ok. i grasp with 3.2kHz you want to make C-Scope 6PI... but then a coil must be with 1mH value... NOT 300uH.
irfd110 has also less low R in open state... compare datasheets on both 9640 and it. so heat energy on irfd110 is very low, a coil
also has high ohm vs 300uH coil, also in serial with coil sets a resistor... all those gives low heat on the FET.
and all that was keeping in consideration by you.

[kt315]

if you do not believe me look this pic and on this resistor in serial to coil

[daverave]

I suggest trying lower voltages on the Mosfet - try 9/10/12V.

Hi eclipse...ive tried 12v but back emf spike is still 250v........at 9v the back emf is 200v but not good depth proformance...im a bit confused with frequency cause i thought 3.2 khz is better than say 400-800 hz for finding small gold items
if you take for example white's tdi sl which i read is using around 3.3 khz tx...then why does a top line deep detector like this use a 3.3 khz frequency ?????