Getting ready to build the MPP

[baum7154]

Was looking at a chart I posted awhile back. The .5in x .5in target has an area of about 1.25 percent of the coil area. Maybe the general rule is for the center of the coil at a distance of the coil diameter.

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If my math is right a .5" X .5" = .250 square inch so 46.56 square inches divided by .250 sq in =186 or .00536 of the 7.7" dia. coil area. In other words about 1/2 of 1% unless we are talking about another coil diameter.

Dan

[green]

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If my math is right a .5" X .5" = .250 square inch so 46.56 square inches divided by .250 sq in =186 or .00536 of the 7.7" dia. coil area. In other words about 1/2 of 1% unless we are talking about another coil diameter.

Dan

The chart is for a 5 inch coil. (19.6 sq in)

[baum7154]

Ah yes, that makes more sense.

Thanks,

Dan

[Tinkerer]

Tinkerer,

Fascinating stuff and it gives me something more to figure out. I believe the coil turns are 32 and resistance is 2.2 ohms. IRF 9640 has 0.5 ohm max Rds, and HER208 series diode has about 0.7 voltage drop. So with coil voltage at 12.0 volts (assuming 12.7 volts to the mosfet) and 2.7 ohms resistance = 4.44 peak amps coil current X 32 turns = 142.22 field strength divided by 46.56 = 3.05 field density per Square Inch divided by 16 = .1909 = field incidence on the 1/4" X 1/4' target…If I did this all correctly!

Thanks,

Dan

For calculating the peak amps, you also need the TX ON time. See graph attached.
You can also measure the actual coil current by inserting a 50 to 100 mOhm resistor in series with the coil and measure the voltage drop across it.

[Tinkerer]

Was looking at a chart I posted awhile back. The .5in x .5in target has an area of about 1.25 percent of the coil area. Maybe the general rule is for the center of the coil at a distance of the coil diameter.

Thank you for the chart. Detecting distance has often been defined as a one coil diameter distance.

When the target surface area gets to the size of the coil area and larger, the response changes. This effect comes into play with the ground/soil response and the salt water response in the ocean.

[baum7154]

For calculating the peak amps, you also need the TX ON time. See graph attached.
You can also measure the actual coil current by inserting a 50 to 100 mOhm resistor in series with the coil and measure the voltage drop across it.

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Tinkerer,
The Chance PI transmit is very complex and is multi pulse comprised of 16 small but incrementally widening pulses with active coil shorting between pulses to prevent fly back and then ending with a 120us long pulse ending with fly back. I guess you could say that the 120us pulse is the final TX on time.

Dan

[Tinkerer]

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Tinkerer,
The Chance PI transmit is very complex and is multi pulse comprised of 16 small but incrementally widening pulses with active coil shorting between pulses to prevent fly back and then ending with a 120us long pulse ending with fly back. I guess you could say that the 120us pulse is the final TX on time.

Dan

The 16 small pulses might be a method of generating a linear TX current ramp. The 120us final ON time would then end with a fairly steady peak current. This way the eddy current of shorter targets would be decayed at the end of the TX ON time.
My guess is the peak TX current would then be about 3A. I forgot to ask you for the coil inductance, therefore I surmised 300uH for the calculation.

All the best
Tinkerer

[baum7154]

The 16 small pulses might be a method of generating a linear TX current ramp. The 120us final ON time would then end with a fairly steady peak current. This way the eddy current of shorter targets would be decayed at the end of the TX ON time.
My guess is the peak TX current would then be about 3A. I forgot to ask you for the coil inductance, therefore I surmised 300uH for the calculation.

All the best
Tinkerer

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The coil inductance is 335 uh with with the round coil.

Regards,

Dan

[KRinAZ]

Prob at step 6 & 7

Hi, I'm back! & trying to get my revC MPP finished per the build doc, then mod it. I think I'm having a prob with step 6 & 7 - here's what I've got so far: All components per spec except using TL071/2 instead of TL061/2, & using the basketweave coil, all steps thru 6 check out - in the ball park anyway - pulse widths 52uS instead of 58uS, secondary delay more like 130uS but these will ending up getting tweaked anyway so they seem OK for now but - on Step6 the MPP is functional, responds to targets, but reject makes no diff on iron targets, but does have reject effect on lead targets (exact opposite of what it should do I'm thinking) - on Step7 TP4 level in mv is unchanged by changes at R6 - All components verified & all prior steps verify ok (except TX pulse 52uS instead of 58uS) - Hmmm, thoughts?, this doesn't seem to match the build doc but I'm just not seeing why... Thx for any ideas! Apologies for bad formatting - using my phone to post & it seems to not put in carraige returns even after editing & adding them again

[Qiaozhi]

Hi, I'm back! & trying to get my revC MPP finished per the build doc, then mod it. I think I'm having a prob with step 6 & 7 - here's what I've got so far: All components per spec except using TL071/2 instead of TL061/2, & using the basketweave coil, all steps thru 6 check out - in the ball park anyway - pulse widths 52uS instead of 58uS, secondary delay more like 130uS but these will ending up getting tweaked anyway so they seem OK for now but - on Step6 the MPP is functional, responds to targets, but reject makes no diff on iron targets, but does have reject effect on lead targets (exact opposite of what it should do I'm thinking) - on Step7 TP4 level in mv is unchanged by changes at R6 - All components verified & all prior steps verify ok (except TX pulse 52uS instead of 58uS) - Hmmm, thoughts?, this doesn't seem to match the build doc but I'm just not seeing why... Thx for any ideas! Apologies for bad formatting - using my phone to post & it seems to not put in carraige returns even after editing & adding them again

I think you're getting confused between "discrimination" on a VLF, and "reject" on a PI detector. Discrimination of a VLF detector allows you to eliminate iron. This is called ferrous/non-ferrous discrimination. Reject on a PI allows you to eliminate lower conductivity targets, and iron is a highly conductive target. Which means that it will be the last type of target to be eliminated. As you adjust the Reject pot on the MPP, it first starts to eliminate small gold items, then foil, followed by pulltabs. You cannot reject iron.

[KRinAZ]

I think you're getting confused between "discrimination" on a VLF, and "reject" on a PI detector. Discrimination of a VLF detector allows you to eliminate iron. This is called ferrous/non-ferrous discrimination. Reject on a PI allows you to eliminate lower conductivity targets, and iron is a highly conductive target. Which means that it will be the last type of target to be eliminated. As you adjust the Reject pot on the MPP, it first starts to eliminate small gold items, then foil, followed by pulltabs. You cannot reject iron.

Many thanks, you're correct, I was thinking in VLF mode, now I understand. It appears my MPP is working correctly then, so I guess it's time for the mods...

[KRinAZ]

I think you're getting confused between "discrimination" on a VLF, and "reject" on a PI detector. Discrimination of a VLF detector allows you to eliminate iron. This is called ferrous/non-ferrous discrimination. Reject on a PI allows you to eliminate lower conductivity targets, and iron is a highly conductive target. Which means that it will be the last type of target to be eliminated. As you adjust the Reject pot on the MPP, it first starts to eliminate small gold items, then foil, followed by pulltabs. You cannot reject iron.

Many thanks, you're correct, I was thinking in VLF mode, now I understand. It appears my MPP is working correctly then! Now for the mods...

Anyone know a piece of equipment I can scavenge a MUR460 (or equivalent) from - more specifically than "a TV", e.g. model number? :^> thx

[KRinAZ]

Question on diode for the Tx circuit -

I went thru my diodes, I don't have a MUR460 on hand but I do have several RM11B's - 800v 1.2a 30pF but I don't know it's speed - it's supposed to be a fast switching diode but the speed isn't in the datasheet.

So my question - do I really need a 4 amp diode, what is the minimum required forward current for this diode in this circuit, & if I need more than 1.2a - can I put 2 or 3 or 4 of the RM11B's in parallel to get the current? At 30pF I am thinking the added capacitance would be no prob?

I'd like to try the RM11B & it would be great if I can just use 1 in series with the coil, but I of course don't want to force the magic smoke out of the diode.

Many thx for your input

[6666]

rm11b hard to find the right specs must be a secret.

MUR460 60-70ns

[Chet]

Hi KRinAZ

It is a bit slow at 500ns. The current is an average rating so there is probably no need to parallel diodes. A single one will probably handle the average load. You might give it a try, little to lose.

Chet

http://www.digikey.com/product-detai...buynow&site=us