Replies to Minipulse Plus

[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

[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.

[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]

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!

[gdogg]

thanx

[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 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-Shipping-300m-0-5mm-Orange-inner-0-25mm-Single-strand-Copper-Wire-Tin-plated-PVC/1891963060.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]

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.

[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.

[gdogg]

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?

[6666]

I can list the diodes I have discovered so far if anyone wants to see the list...

That could be a good reference to have thanks.

[Qiaozhi]

KRinAZ, you might want to change your way of quoting by adding your answer below or above the [quoting codes]

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.

[Davor]

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.

[KRinAZ]

I was playing with a PI Tx circuit with a diode, and I think I discovered a sort of "final frontier" of technology.
I have a thought here - that doesn't address the "diode leakage gradient problem" but I wonder if it may reduce capacitance and help early sampling some - since I am salvaging components from equipment (TV's and such) I get free - & so that I can recognize diodes of interest for the Tx circuit - I have started compiling a list of diodes listed as super/ultra fast, and at least 500V reverse voltage, and have found a wide difference in both capacitance (from 12pF to 185pF) and speed (from 25nS to 250nS) among them, also some (but not all) list total capacitance as well as "effective capacitance" on the data sheet - and I noticed on the data sheets that the MUR460 speed is 75nS but capacitance is not listed - I wonder what it is? and how it compares to the range? and if there is significant variance (and maybe the reason it's not listed)?
I can list the diodes I have discovered so far if anyone wants to see the list...

.

[bernte_one]

who has finished this project already and can Show a Video of the machine in action