Replies to Minipulse Plus

[Silver Dollar]

Well chains are the holy grail for PI machines. That is why there is so much talk about a fast
coil. A chain acts like one link so you need to be able to see small gold to see a chain. So short
TX pulse, a fast coil and sensitive RX are needed to pick them up.

I've been busy building lots of different coils and next will be testing their sensitivities to small
gold targets. I'm guessing a small coil with lower inductance will allow faster sampling. You already
have a 2 stage preamp that makes a big difference. So try lower TX widths and shorter delay to the
first sample to see those chains.

[Qiaozhi]

just finnished my build of the minipulse plus rev c, easy build with the kit from silverdog, i think its better than the surf pi,1.2. however one prob im getting on both detectors is that they are very weak on pure gold, and aspecialy gold chains, as it cannot detect chains, anyone any ideas?

Small gold nuggets and thin gold chains are difficult to detect, because gold is a low conductivity target compared to copper, and both pure gold and thin chains are small items that can only sustain eddy currents for a short period of time. To detect these targets you will need a fast coil, and possibly will need to change a few component values.

josewashere replaced the NE5532 with an LM4562, and gained an extra centimetre in depth. He also changed R11 from 22k to 6k8, and C11 from 2n2 to 1n5. This reduces the sample width to ~15us (down from ~48us). It might also be beneficial to reduce the gain on the preamp to allow very early sampling. Basically, you will have to experiment with different TX pulse widths, sample delays / widths, and preamp gains, until you get the best results.

[gdogg]

thanx guys for the reply, for my minipulse plus rev c, ive used the tx configuration as in the pdf file, im using the
Single strand Copper Wire Tin-plated PVC,

http://www.aliexpress.com/item/Free-...891963060.html

which ive found to be excellent above all for coils. im getting great depth no prob on just about all metals except real gold,

anyone got a good link on how to create this fast coil.. cheers

[Qiaozhi]

anyone got a good link on how to create this fast coil.. cheers

Look here -> http://www.geotech1.com/cgi-bin/page...oils/index.dat

[gdogg]

thanx

[KRinAZ]

KRinAZ - this is a good suggestion by Davor, as clicking on "What's New?" shows all your replies as a single dot. Embedding your replies in the original post also makes it difficult to read.

OK many thanks, will do, seems like I learn something good every day from you all!

[KRinAZ]

KRinAZ, you might want to change your way of quoting by adding your answer below or above the [quoting codes]
Regarding diodes, the capacitance is generally lower than naked MOSFETS provide, so it is usually a benefit. The capacitance changes with voltage, higher reverse voltage -> lower capacitance. It depends upon the barrier geometry, but generally it is not linear. It is almost impossible to obtain a true varicap diode nowadays, and they boasted square law capacitance change, but lots of it works in regular diodes as well. Keeping a constant reverse polarity across a diode keeps a capacitance at low values and leakage current at constant, which is the idea behind my solution.

Diode recovery speed is a completely different animal. It interacts with diode capacitance in switching applications, but it is different.

Many thanks Davor on the posting tip for quotes, & many thanks also on the info on how the capacitance of diodes works and interacts with the MOSFET, I'll be interested to watch the developments of your reverse voltage idea!

[KRinAZ]

I'm wondering if using a MOSFET with the lowest possible output capacitance would be an improvement in the ability of earlier sampling; the reason I ask is that I have in my MPP a plain old IRF740 which the data sheet says is 330pF output capacitance (and no mention of effective output capacitance), I also have an IRF740ALPBF which the data sheet says is 170pF output capacitance with 61pF as effective output capacitance. I'll admit I don't fully understand these specs but will be interested in any comments on this.

[Chet]

I'm wondering if using a MOSFET with the lowest possible output capacitance would be an improvement in the ability of earlier sampling; the reason I ask is that I have in my MPP a plain old IRF740 which the data sheet says is 330pF output capacitance (and no mention of effective output capacitance), I also have an IRF740ALPBF which the data sheet says is 170pF output capacitance with 61pF as effective output capacitance. I'll admit I don't fully understand these specs but will be interested in any comments on this.

Hi KRinAZ, capacitance values in solid state devices change considerably with circuit conditions so the numbers only provide general guidelines to work from. In our previous post we discussed using a high voltage diode in series with the output of the MOSFET to reduce the circuit capacitance.


The three values you have in your case would be reduced by a series diode with 40pf of reverse capacitance as follows:


330pf in series with 40pf = 37pf
170pf in series with 40pf = 34pf
61pf in series with 40pf = 24pf


From these numbers you can see why the diode is very effective in reducing the capacitance across the coil. If you use two diodes in series you would reduce the diode capacitance to 20pf. Using a series diode is more cost effective than changing to an expensive MOSFET.


Since you already have an inexpensive MOSFET go for it.

http://www.calctool.org/CALC/eng/electronics/cap_series

[Davor]

2 diodes in series ... no. It would require equal reverse polarity on both of them (too complicated), or otherwise it will not work as advertised.

[Chet]

2 diodes in series ... no. It would require equal reverse polarity on both of them (too complicated), or otherwise it will not work as advertised.

Hi Davor, you are correct. I tossed the second diode idea in in haste as a possible expansion.


It would take some balancing by either closely matched diodes or some high value (meg ohms) resistors across each diode to equalize the reverse voltage drop across each diode.


Without perfect balancing there should still be a measurable reduction in total capacitance. Measuring the oscillation frequency with the damping resistor lifted with one diode then again with two diodes in series (anode to cathode) would be one way of testing.


I will experiment with this when I get through with some other modifications to my Minipulse.

[sinclairuser]

hi all finally back to full fitness, the other half helped me clean and sort out the workshop yesterday, its amazing how much dust can build up in a room that is closed up.
anyway finally going to complete my mp, i only got to stage 2 before so i have only just started really, i am going to read the thread from the start, when i'm up to speed i will chip in.
also wanted to say thanks for the kind words and encouragement given to me while i was ill, thanks lads you know who you are.

[Qiaozhi]

hi all finally back to full fitness, the other half helped me clean and sort out the workshop yesterday, its amazing how much dust can build up in a room that is closed up.
anyway finally going to complete my mp, i only got to stage 2 before so i have only just started really, i am going to read the thread from the start, when i'm up to speed i will chip in.
also wanted to say thanks for the kind words and encouragement given to me while i was ill, thanks lads you know who you are.

Welcome back!
The important things to read are:
1) The Build Document,
2) and the notes in the relevant sticky thread.

[sinclairuser]

Welcome back!
The important things to read are:
1) The Build Document,
2) and the notes in the relevant sticky thread.

thanks george, my boards are rev b, so ill look to see the differences between b and c, and check the errata etc.
i'm interested in davor's GB mod, but that's for the second board, i just want the first one as build plans.

[CAS]

Well while I am waiting for parts on another md project, I got round to connecting the chipkit Uno up to the MPP. I only had a small coil connected as I am not going for depth but more small fine gold. Still have to program the GB but will try that over the next few days.