New Homebrew Metal Detector

[Power]

air tests

25x25cm copper plate at 90cm.Is this enough or can i make it better?
Also the current is 20 to 40 mA without vco, when i have sound the total current is about 100 to 120 mA.Is anything wrong?

[zisme]

Hi,
I suspect the reason your FET is getting hot is because you don't have a proper drive circuit for it. I had this problem until I used a good gate drive circuit. FETs in this mode want to be turned on or turned off and nothing in between so they must be swithed on and off very fast (ns) and hard. I found this was my problem after putting in larger FETS, heatsinks, adjusting pulse rates, etc and then I finally did some reading and analyzing of what was going on and realized the problem. I thought it was the high current I was pulsing but that wasn't it.
Good luck

PS-now design one using a single 9V battery.

You are absolutely correct about switching FET's on and off. The faster the better. I have been designing power MOSFET circuits for years at the last place I worked. Here, I was driving automobile headlights and other high current lighting. I was using high and low side driver/conditioner IC's which work very well. Some of them used charge pumps to create the driving voltage for N channel devices that could supply upwards of 100 amps to the load. Driving high current FET's was hard because of the high gate capacity these devices have. You need fast switching times, but you also need high current drive to overcome this internal capacity.

However, with the P.I. detector drive, there is a current limiting resistor in series with the gate (R27) that does slow down the switching times somewhat. The advantages of using R27 is to reduce the noise on the circuit board. So, there is a little heat generated at the trade-off of less noise.

With the pulse width and frequency combination I am using, the FET does not get hot or even warm. I allowed space for heatsinks on the circuit board so you could experiment with the timing without worrying about over-heating the FET's. Notice the board that's mounted in the box has no heatsinks at all. As you know, increasing the TX pulse width and/or increasing the frequency will eventually reach a point that will warm up the driving FET no matter how fast they are switching on and off. You could replace R27 with a short, it won't hurt anything, but you will notice an increase in noise.

I have played around with lower voltage circuits, but I didn't get the depth I get with 12 volts. If I could, I would be using 24 or maybe even 36 volts! But you would need a pack mule to carry it around.

Thanks for the comments,

Mike

Getting warm

Hi,
I suspect the reason your FET is getting hot is because you don't have a proper drive circuit for it. I had this problem until I used a good gate drive circuit. FETs in this mode want to be turned on or turned off and nothing in between so they must be swithed on and off very fast (ns) and hard. I found this was my problem after putting in larger FETS, heatsinks, adjusting pulse rates, etc and then I finally did some reading and analyzing of what was going on and realized the problem. I thought it was the high current I was pulsing but that wasn't it.
Good luck

PS-now design one using a single 9V battery.

[zisme]

hi
did you have field or lab tests for it.
1 euro coin from ? cm
and do you have big coil

I only have my own air tests which are as follows:

U.S. penny: 12 to 15 inches

U.S. nickel: 9.5 to 10 inches

automobile: 4 to 5 feet

These are the absolute minimum detection distances using both sound and meter.

The 9.5 inch coil is the only size I have tried on this detector.

Mike

[okantex]

hi
did you have field or lab tests for it.
1 euro coin from ? cm
and do you have big coil

[zisme]

Hello, you can put more resolution for the PCB because I can not view whit details the lines and the holes of your circuit-
thanks

Carlos

Carlos,

Try right clicking, saving the picture then printing it at a smaller size. I will try to improve the image when I get a chance.

Thanks for the feedback,

Mike

[Carlos]

PCB

Hello, you can put more resolution for the PCB because I can not view whit details the lines and the holes of your circuit-
thanks

Carlos

[zisme]

I just posted the plans for my new P.I. detector at: http://oldradiobuilder.com/MDET.html Let me know what you think of it. Thanks, Mike