Getting ready to build the MPP

[KRinAZ]

& by the way, with no diode & on the bench anyway, it looks to me like the signal recovery is stable & could be sampled at about 8.5uS from when the coil signal just starts to collapse - again observed at TP3 - & right about 11.5uS from when the TX pulse at TP1 just begins to fall. Seems to me the important part is how long after the [TX] coil begins to collapse - can I start to sample the signal for targets & from when the coil signal just begins to fall/collapse at TP3 to when stable - I'm under 10uS now.

[KRinAZ]

Well I tried 2 different diodes, they make no difference in the recovery of the pulse at TP3 - I tried the RM11B, then a RH1S (600v 1a trr=200ns) & even verified with my diode checker they are good, but the waveform is no different when either of these is added in series between Q1 & R3. I was expecting faster recovery and/or higher ohms for the damping resistor - the diode addition I did matches the RevD schematic. Any thoughts? Am I correct in what I'm expecting from the diode addition? Is the MUR460 going to make a difference over these diodes I tried? Thx for any ideas!...

[Achillion]

OK another question - this time about the pole setup for my MPP - metal or non-metal lower rod (the rod that attaches to the search head)? I've noticed a lot of the cheaper VLF units (that would be cheap donors of rod parts) have a lower rod of aluminum with just a small plastic mount at the bottom that attaches to the search head. To me this seems like it would be a bad thing & that we would want a non-metal lower rod - anyone have thoughts & especially have experience with metal and with non-metal lower rods for the MPP PI detector?

I'm using a carbon monopod (used for photo camera) which works well. It's strong and the lower shaft is carbon which is ok for metal detecting. Brand name is Benro.

[KRinAZ]

OK another question - this time about the pole setup for my MPP - metal or non-metal lower rod (the rod that attaches to the search head)? I've noticed a lot of the cheaper VLF units (that would be cheap donors of rod parts) have a lower rod of aluminum with just a small plastic mount at the bottom that attaches to the search head. To me this seems like it would be a bad thing & that we would want a non-metal lower rod - anyone have thoughts & especially have experience with metal and with non-metal lower rods for the MPP PI detector?

[KRinAZ]

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

Thx Chet for that link - interesting - it has a separate field for speed (500ns) and a separate field for trr, which doesn't have a value, hmmm, like 6666 I would think the trr would be the "speed" so I'm not sure what that's supposed to mean as a separate measurable from trr...but oh well like you say what the heck, so I'll try it & see what the change in recovery is like & if the critical damping ohms goes up. The circuit as built & with my basketweave coil has a critical damping value of 565 ohms right now with no diode.

[6666]

I thought it was the Reverse Recovery Time (trr) the time the diode switches from conducting to non-conducting state the important spec

[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-detail/en/RM%252011B/RM%252011B-ND/4454348?WT.z_cid=ref_octopart_dkc_buynow&site=us

[6666]

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

MUR460 60-70ns

[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

[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]

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...

[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]

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

[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

--------------------

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

Regards,

Dan

[Tinkerer]

------------------------
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