Chances of blowing-out metal detector

[bigrex]

Hello again,

Here is a simple question, although I don't know how simple the answer may be. I'm just wondering what the chances are of "blowing-out" a metal detector by hooking up a homemade coil if the resistance and inductance are both correct for the detector. Does impedance play any role that could be dangerous in the coil set-up? Just wondering and worrying since I don't want fry a $3,000 metal detector...Happy 2006. If the ohms are the correct resistance level will that take care of any impedance "issues"?

[bigrex]

No, I won't sue anyone if my detector blows, and just for the record it was a used unit on eBay that was a very good deal.

[Carl-NC]

With many detectors, you can dead-short the coils with no damage. Some of the digital transmitters (DFX, CZs, most Minelabs) might not like a dead short, tho. However, if your coil is even +/-50% to spec, it would be highly unlikely you could damage anything.

- Carl

[Reg]

Hi BigRex,

I have to agree with Carl, it is highly unlikely you will hurt anything by building and trying a coil, providing you stay within reasonable limits.

As I mentioned before, I have not built a coil for a ML but have built several PI coils for other machines. Also, I have not tried to take a ML apart to see just what is inside, so I don't know for sure just how they are built. I just wanted you to know that my answer is based upon information I have received from people who have built coils for the ML's and not from personal experience.

From what I am told, the FET used is an extremely heavy duty one, meaning it will be very difficult to blow one. This certainly makes sense. Now, about the only way one might hurt the FET would be to just short out the coil completely, which is highly unlikely if you take care.

If your inductance is off a little because of too many or two few windings then the worse that should happen is the coil will simply not work. Even a cheap LCR meter should get you in the ball park of 300 uh. To be sure, I would check the coil at the plug connections verify the inductance. This will eliminate any possibility of a short problem.

BTW, I use a BK 878 LCR meter but do have a cheaper Wavetek that reads basically the same as the 878 on all the coils that I have compared. The difference is a couple of uh max. Now, I am considering purchasing one of the very cheap models off Ebay and comparing it just to see how accurate it is. This unit I am referring to is an "all in one" multimeter, LCR, etc, type unit and sells for $50 or so. If it works, it would reduce the number of meters I would carry.

Has anyone tried one of these cheaper units?

Remember, if you do purchase a cheaper LCR meter then you can check the inductance of a standard ML coil to get an idea of the accuracy of the meter. The ML coil should be 300 uh and most do not vary much from this. However, I am told that some of the special coils made by Coiltek may be about 280 uh or so.

Reg

[bbsailor]

Reg,

Look at the LC meter at this link. http://www.aade.com/

I built it and it is very accurate. I have a standard, calibrated 500uH inductor and this LC meter is right on! I also cut several standard pieces of coax 1 ft long and compared the capacitance (shield to center conductor) to the published specifications...again, right on!!!

bbsailor

[bigrex]

Thanks/LC meter

Thank-you again for calming my coil fears. Here is a link to the LC meter I purchased. It was a good deal and it is also accurate:

http://www.multimeterwarehouse.com/vc6243f.htm

Regards,

Rex B.

[Charles (Upstate NY)]

On the ML I can tell you I have subjected my test bech Explorer to some serious shorts during late night projects when the brain was tired and its still working. I have cross wired the transmit to the receive, to household ground, wired the battery backwards, etc. Most of the time the detector just shuts itself off.

One time I thought I had killed it. It shut down and would not restart. The next morning like magic it started up again, go figure.

If your specs are in the ballpark I don't think you are going to hurt one but it pays to make some test fixtures to avoid the cross wiring ooopses.

[robby_h]

Hi Bigrex,
You shouldn't have any problems re harming the GPX as long as you keep the coil reasonably close to specs.
The early model such as the 2000 is a worry in that it has physically small fets (these aren't the two main tx fets which are IRF740s or similar) in the rx damping circuit and as these look directly at the spike then they can be harmed by excess ringing. The later models appear to have fets that can take a lot more abuse so you should be safe. The danger, if any, would be in going wildly high with the inductance or not fitting a suitable inbuilt damping resistor to a DD rx coil.
Apart from any damage scare, the detector should start up and settle down in a time similar to when using a stock coil. The coil is very heavily clamped at start up and when this clamping settles down then the active damping circuit should have also taken over from the passive damping. If the detector doesn't settle down in a reasonable time then it is an indication of a possible fault.
If all appears ok at switch on then a test on bad ground will tell whether you have it right.
If you have trouble measuring the coil/cable resistance then you might try this simple idea. I haven't tried it but it looks ok, at least for doing comparisons. Hopefully it doesn't do weird things when looking at an inductor.

http://www.users.tpg.com.au/pschamb/lom.html