help me with PI metal detector

[lou0908]

hi everybody
i have encountered some problem with my pi metal detector, my goal is to use a micro-controller to control a MOSFET driver which in association with a N-MOS so that the MCU can control the N-MOS indirectly.
the waveform I have captured is shown in the attachment , form my point of view, the two Diodes will clip the flyback voltage between -+0.7 v , unfortunately, the result is differ far from my excepted (in the picture the channel 2 of the oscilloscope is connected to the point 1), I have checked the Diodes and resistances, they are all in good state.Have somebody have encountered this problem I mentioned above yet. I need your help and technical assistance.
Thank you very much.Regards.

picture:

[mickstv]

Try a lower value resistor for R1, around 400 ohms and see what happens. Also reduce the TX to under 100us.

[lou0908]

Try a lower value resistor for R1, around 400 ohms and see what happens. Also reduce the TX to under 100us.

I add a capacitance whose value of 470uF between 12v and GND, and there is a little improve to this problem.
I thank that the power source cannot supply enough circuit to my test board upon the micro-controller switch on the MOSFET. Am I right?
This is a waveform picture I have captured from oscilloscope. I wounder whether it is right or wrong?
attchment:

[mickstv]

use 2200uf or higher and like I suggested reduce the TX to less than 100us.

[lou0908]

how can I calculate the value of the dumping resistance? what is relationship between the dumping resistance with the coil? And what width the pulse can I use to get high performance? Have it an accurate formula which is used to calculate these related parameter mentioned above ?
thank you!

[traverso40]

R_damp_eq = VDSmax_Mos / IL_peak
R_damp_eq = 1/(1/R1)+(1/R2):

in your case

VDSmax_Mos(IRF740) = 400V
IL_peak = 3A
R2 = 1Kohm
suggested R1(effective damping resistor) = 150 To 160 ohm

[kt315]

how can I calculate the value of the dumping resistance? what is relationship between the dumping resistance with the coil? And what width the pulse can I use to get high performance? Have it an accurate formula which is used to calculate these related parameter mentioned above ?
thank you!

all has INSIDE THE METALL DETECTOR. please read this book. you can order this book from George and Carl DIRECTLY ON THIS SITE GEOTECH, the book is stiil accessable
for everybody.

[Carl-NC]

The damping resistor is found by trial-and-error as it depends heavily on parasitic capacitances. Your first pic made no sense at all, I don't see any way you got the waveform on 'point 1'. The second pic looks normal.

[lou0908]

all has INSIDE THE METALL DETECTOR. please read this book. you can order this book from George and Carl DIRECTLY ON THIS SITE GEOTECH, the book is stiil accessable
for everybody.

I can not find this book in chinese amazon. where can I get it in china? Thank you very much!

[lou0908]

R_damp_eq = VDSmax_Mos / IL_peak
R_damp_eq = 1/(1/R1)+(1/R2):

in your case

VDSmax_Mos(IRF740) = 400V
IL_peak = 3A
R2 = 1Kohm
suggested R1(effective damping resistor) = 150 To 160 ohm

Thank you very much for your help, have some papers introduced this related knowledge to these formulas in details? If so, can you recommend some to me? Regards

[kt315]

I can not find this book in chinese amazon. where can I get it in china? Thank you very much!

I do not in course where you may get the book in china. visit this page slide at its down and look there addresses you can order it where directly.
http://www.geotech1.com/forums/conte...metal-detector

[Qiaozhi]

Thank you very much for your help, have some papers introduced this related knowledge to these formulas in details? If so, can you recommend some to me? Regards

Here are the ordering details -> Inside the METAL DETECTOR - Published September 2012
Asia, Australia Paypal $34.95 + $24 postage to [email protected]