Minipulse Plus Rev My time has come!

[6666]

Searching the web for a decent LF357 for my DP Legend but eBay is full of Chinese rubbish..Equivalents are hard because bom says 50V/us slew rate and above grrrrrrrr0

While I have not tried it, I think it was on the TI site they suggested a CA3130, but check the pins

[mushaba]

Looked at Rev=E schematic. Rin connected to op amp -input. If op amp was fast enough, shouldn't need Rd(Rin should damp).

Hello green,are you reffering to R1 (1K) as Rin?

Wondering what TP3 looks like if a 4148 diode were connected across R8(cathode to amp out) and Rd was adjusted for best decay.

A diode parallel to R8 (cathode on opamp out)..Will do that and take measurement to see what will happen.


In general speaking what is the main feature that a coil named "fast"...Is it its low capacitance and/or its inductance to resistivity ratio?

[mushaba]

The main issue is the NE5532 op-amps can not recover from over load (over Voltage) any faster.
This recover time has been discussed much with a few newer op-amps suggested but not much real data on how well they work.

Im open for suggestion and willing to experiment in my board,my knoledge is building step by step but im far away from the point to choose an opamp suitable for this md...

10us is a great short delay for any mono coil and simple front end.
This is fast enough for all but the smaller gold nuggets.

You say its not fast enough to detect gold nuggets?Then i have to make a faster coil since the MPP has the ability to detect them with a fast coil..

I know that coils discussions and posts are a lot but with short guidance from all of you i will try to make a faster one

[dbanner]

So with this procedure we can calculate a coils srf and capacitance..But should have in mind capacitance from cable and from diode..

How can I use this coil to it's max capabilities?I mean what tx pulse width delays etc..and if it is a fast coil why can't I sample below 10us?
Tommorow I will wrap a spiderweb just to practice with the thinks I've learned today and finalise my build.

Searching the web for a decent LF357 for my DP Legend but eBay is full of Chinese rubbish..Equivalents are hard because bom says 50V/us slew rate and above grrrrrrrr0

If you are unsure of genuine LF357AN (I even got fake chips from genuine local dealer, seems they were also fooled), You can use NTE937M.(NTE Electronics) This is genuine direct equivalent replacement for LF357. I use this in my delta pulse. It has slew rate of 50V/uS.
You know it is genuine because it comes in official sealed plastic bag direct from NTE Electronics. You can check out their website for an address of closest dealer to your location.
They have a huge catalog of parts and an on line cross reference database. They specialize in manufacture of direct replacement parts and they are always 100% genuine.

[mushaba]

To excercise yesterdays lesson about coils and Rdump calculations i wrapped a new coil




Measured SRF with oscope and probe loop precedure,counts 2.38MHz

Rd=pi*L*SRF --> Rd=3.14 * 0.00032 * 2380000 --> Rd=2391.4 Ohms

Rd*Rd= L / 4*C --> C= L / Rd*Rd*4 --> C= 0.00032 / 2391.4 * 2391.4 *4 --> C = 0.00032/5718794 --> C = 5.59 e-11 --> 55pF

Conclusion:As SRF goes higher the Rd goes higher therefor capacitance goes higher.So we supose to lower the SRF but how???

And my embarassing moment of the day,what do you mean by "lead" ??

... Possible with a short lead.

... Lead can exceed 1pf/inch

..so measuring coil resonance with leads should be close

... Total parallel coil capacitance including lead capacitance

[mushaba]

If you are unsure of genuine LF357AN (I even got fake chips from genuine local dealer, seems they were also fooled), You can use NTE937M.(NTE Electronics) This is genuine direct equivalent replacement for LF357. I use this in my delta pulse. It has slew rate of 50V/uS.
You know it is genuine because it comes in official sealed plastic bag direct from NTE Electronics. You can check out their website for an address of closest dealer to your location.
They have a huge catalog of parts and an on line cross reference database. They specialize in manufacture of direct replacement parts and they are always 100% genuine.

Thanks a lot dbanner i looked at tme store i buy parts from and sadly its out of stock...I will enquiry them about the stock...Ultra thanks!!

[waltr]

You say its not fast enough to detect gold nuggets?Then i have to make a faster coil since the MPP has the ability to detect them with a fast coil..

I know that coils discussions and posts are a lot but with short guidance from all of you i will try to make a faster one

I said "smaller" gold nuggets. My HH2 will detect natural gold nuggets down to about 5 grams but none smaller.
The returned signal of these are dependent on their size and these are what a Fast coil and very short first delay is about
.

To excercise yesterdays lesson about coils and Rdump calculations i wrapped a new coil

[ATTACH]46209[/ATTACH]
[ATTACH]46210[/ATTACH]

Measured SRF with oscope and probe loop precedure,counts 2.38MHz

Rd=pi*L*SRF --> Rd=3.14 * 0.00032 * 2380000 --> Rd=2391.4 Ohms

Rd*Rd= L / 4*C --> C= L / Rd*Rd*4 --> C= 0.00032 / 2391.4 * 2391.4 *4 --> C = 0.00032/5718794 --> C = 5.59 e-11 --> 55pF

Conclusion:As SRF goes higher the Rd goes higher therefor capacitance goes higher.So we supose to lower the SRF but how???

And my embarassing moment of the day,what do you mean by "lead" ??

No, the capacitance goes lower if the SRF is higher. The equation of resonance for an LC is: fc = 1 /[2PI*SQRT(L*C)].
So if L is constant and Fc increases then C must have decreased.

"lead" is the wires or cable from the coil to the circuit. This 'lead' has capacitance and becomes part of the coil (RLC) when measured or installed.

[mushaba]

No, the capacitance goes lower if the SRF is higher. The equation of resonance for an LC is: fc = 1 /[2PI*SQRT(L*C)].
So if L is constant and Fc increases then C must have decreased.

So capacitance is 14pF...How low must it be to consider a coil "fast"

[waltr]

So capacitance is 14pF...How low must it be to consider a coil "fast"

I would consider that fast.
A coil with an SRF greater than 1MHz is fast.
The coils I use with my HH2 have SRF of 300-600kHz and not fast so first delay is greater than 12-15usec.

[mushaba]

I would consider that fast.A coil with an SRF greater than 1MHz is fast.

I try to understand why a 320uH coil made from magnet wire has better SRF than a 3DSS made from PTFE...

Also i want to make a coil that damps near 300 - 400 Ohms just to have it as reference..

[waltr]

Also i want to make a coil that damps near 300 - 400 Ohms just to have it as reference..

A simple bunch wound enamel wire coil around 400uH should get you there.
These are what I use. C ~ 120pF and SRF ~700kHz needing ~500 Ohm Rdamp.

[godigit1]

Waltr
"I said "smaller" gold nuggets. My HH2 will detect natural gold nuggets down to about 5 grams but none smaller."
Is that grains not grams? 5 grams that would be totally unaccpetable to me . stock Mpp revd 1 gram 4 to six inches 6x9 figure 8 coil.

[waltr]

I think it was grams but going from memory. I'll check my notes at home tonight and post back.
It could have been grains or grams but the idea is that the smaller the target the earlier sampling needs to be to detect it.

Also, most gold targets are not pure gold so the conductivity is quite low giving a small TC.

[mushaba]

Tommorow I will paint and make the final mount the coil to the detector chassis...Still haven't figure out the battery compartment...
Anyway,I will take the outer wire insulation from an rg cable to insulate the "leads"coming out of the coil case.I will solder them to an rg62 cable,is it possible soldering affect the coil reading?Also i believe all parameters will change(coil leads+rgcable+rg connectors to MD)is there a way to measure capacitance from inside the detector?For sure I will connect a pot to check for critical damping..and at last is there any ideas how to make TP3 output waveform more stable ?

[green]

1. Picture of two coils I posted awhile back. SRF measured the way I suggested with scope connected to amplifier out of operating PI. Lead wires used between coil and circuit that I have measured have a capacitance of a least 1pf/inch. A 3/4m lead would have 3times the capacitance of the coil. Don't know how fast a 20cm diameter 300uH coil could be made. I think anything around 3MHz SRF without the lead is fast enough. Interested in the specs of coils others have made. Repeated the tests today using procedure reply#110 with scope lead clipped to ground lead making a pickup loop. I've been shorting the not wanted coil when measuring SRF of IB coils(separate Tx and Rx). Tried again today wit the flat radial and flat basked overlapped. Reads close to single coil reading. Maybe there is a better way?