field test unit no 001 "model T"

[moodz]

...front end with the amp fitted.

R20 varies the gain
R18 varies the damping

moodz

[moodz]

that's pretty clever

if I'm understanding it correctly, the 330 ohm resistors are used as the primary damping , and the 1k/Q4 acts as a fine tune damping circuit with the adjustment of R18.
It would be insteresting to see the different waveforms, no target/ ferrous/ iron

Philip

Thanks ... You are right about about the damping ....oh and I did not indicate that the damping MOSFET Q4 is an IRF740 and all diodes are 1n4004 ...no fancy parts required LOL.
R18 should be a multiturn pot.
This frontend is now the "reference standard" frontend for differential pi detectors and it is not tied to microprocessor based designs ...you could easily put a surf pi type backend on with some small mods. The only things you have to remember is that the fly back voltage must not exceed the breakdown of the blocking fets and that VPP or battery pos is your analogue "GROUND".
I am going to skip over mentioning the discrim function for now and will revisit when the back end and sampling stage is bolted on.
Then we can publish some Waveform data.

Moods

[hdphilip]

that's pretty clever

if I'm understanding it correctly, the 330 ohm resistors are used as the primary damping , and the 1k/Q4 acts as a fine tune damping circuit with the adjustment of R18.
It would be insteresting to see the different waveforms, no target/ ferrous/ iron

Philip

[moodz]

BLOCKING FET FRONT END

...dilemma solved .... I wont include the protection circuit .... the blocking fets will not blow if the circuit below is utilised and ... TX PULSE must not exceed 100 us ... this allows a healthy safety margin by limiting coil current.

The circuit has been tested and verified ..... works like a charm.

moodz.

[smokingdiode]

600V N channel MOSFET

http://parts.digikey.com/1/parts/1015852-mosfet-n-ch-600v-4a-220fn-2sk2792.html

now looking for a P channel

[Tinkerer]

Mounting the blocking FETS on a small plugin module should an unfortunate event occur .. changing the module is much quicker than soldering ...... also it allows trying out different types of FETS and diodes.

[ATTACH]19184[/ATTACH]

The coil current reaches 4.6 amps peak in a 260 uH coil .....the mathematically gifted readers should be able to calculate the peak flyback

Below is the coil current sampled across a small resistance.

[ATTACH]19185[/ATTACH]

and now THE DILEMMA....in order to prevent the blocking FETS from self destructing but without inhibiting the blocking action ( which is excellent BTW ) the use of a Tank Capacitor was required with a very unfortunate side effect that it builds up large charge ( in the order of 600 volts ). This is enough to give you a severe shock or worse ... I realised this when I accidentally touched the cap myself. The second problem apart from the danger is that this voltage cannot be simply discharged as the high potential is required for the blocking protection to function as it should.
So the bottom line is that I will not publish ( or email ) a circuit where there is the potential for someone to kill themselves ... I will be thinking about how to get around this.

moodz.

With 4.6A and a 260uH coil and a Flyback duration of about 10us, you will have about 600V peak during the Flyback.
You will need at least a 650V Mosfets to go safe.

Tinkerer

[bklein]

series of high voltage TVS diodes and a larger value cap?

or, how about using a LTC4366 that I just learned about....

Barry

[mickstv]

Mounting the blocking FETS on a small plugin module should an unfortunate event occur .. changing the module is much quicker than soldering ...... also it allows trying out different types of FETS and diodes.

[ATTACH]19184[/ATTACH]

The coil current reaches 4.6 amps peak in a 260 uH coil .....the mathematically gifted readers should be able to calculate the peak flyback

Below is the coil current sampled across a small resistance.

[ATTACH]19185[/ATTACH]

and now THE DILEMMA....in order to prevent the blocking FETS from self destructing but without inhibiting the blocking action ( which is excellent BTW ) the use of a Tank Capacitor was required with a very unfortunate side effect that it builds up large charge ( in the order of 600 volts ). This is enough to give you a severe shock or worse ... I realised this when I accidentally touched the cap myself. The second problem apart from the danger is that this voltage cannot be simply discharged as the high potential is required for the blocking protection to function as it should.
So the bottom line is that I will not publish ( or email ) a circuit where there is the potential for someone to kill themselves ... I will be thinking about how to get around this.

moodz.

Maybe a bleed resistor ? and a huge warning sign on your circuit.




Mick

[moodz]

...small updates and a dilemma..

Mounting the blocking FETS on a small plugin module should an unfortunate event occur .. changing the module is much quicker than soldering ...... also it allows trying out different types of FETS and diodes.



The coil current reaches 4.6 amps peak in a 260 uH coil .....the mathematically gifted readers should be able to calculate the peak flyback

Below is the coil current sampled across a small resistance.



and now THE DILEMMA....in order to prevent the blocking FETS from self destructing but without inhibiting the blocking action ( which is excellent BTW ) the use of a Tank Capacitor was required with a very unfortunate side effect that it builds up large charge ( in the order of 600 volts ). This is enough to give you a severe shock or worse ... I realised this when I accidentally touched the cap myself. The second problem apart from the danger is that this voltage cannot be simply discharged as the high potential is required for the blocking protection to function as it should.
So the bottom line is that I will not publish ( or email ) a circuit where there is the potential for someone to kill themselves ... I will be thinking about how to get around this.

moodz.

[6666]

Congrats Smokin

[smokingdiode]

Philip, I didn't need to raise supply rail - 5v only.

[hdphilip]

hello simon

good job!

just wondering, did you have to raise the cpu vcc by .2 volt by any chance?


philip

[smokingdiode]

The dsPIC30F4012 code worked first time! Pulse waveform easily controllable with the rotary switch setup. Nice. Just waiting for matched FETs to arrive before rigging the differential stage. Interesting project Moodz, can't wait to see how the benchmarks turn out on coins and rings. Does anyone have any preliminary benchmarks (eg. an Australian $2 coin at 19cm in air).

Planning to eventually identify a set of parameters (eg pulse width, gain parameters, etc) and algorithmic control variables and put them in a genetic algorithm / neural network optimizer. Hopefully I'll be able to share some good parameter sets that will be tweaked for specific target sets eg a coin set, a gold set, a ring set etc. The fitness function will comprise of a complete system simulation involving moving parts! Two stepper motors will be connected to large (1 meter) wooden wheels that rotate parallel to each other and are spinning in the same direction at slightly different speeds. At equal spaces on the perimeter of each wheel will be tied a combination of good and junk targets (eg bottle caps, rings, coins, tabs). The coil will be positioned a few inches away from the perimeter of the wheels so that as the wheel spins, new targets will automatically applied (come in range) of the coil. Note, because there will be two wheels different combinations of targets will be presented to a coil to simulate the situation where a good target is coincident on a junk target as well as an isolated good and an isolated junk target. A "fitness" evaluation of a sample parameter set will be an assessment of the ability of the system to positively identify good targets and to reject interference and junk targets. Of course, I'll share all code. Will probably have to run the code on the PC and commicate down the RS232 line as genetic algorithms consume considerable RAM.

Good collaboration.

Simon

[hdphilip]

great news moodz!

I'll be looking forward to giving it a go!

Philip

[moodz]

...revisit the blocking FETS

I said I would revisit the blocking FETs and the problem with blowing them up all the time is solved ....
... I will give it a good testing prior to publishing anything ...

... below is a pic of the blocking FETS in action ( scuse my camera shake ) .... neatly snipping the top and bottom off a 500 volt peak flyback with perfect tracking of the base line signal voltage. The coil used here is an 18 inch differential coil. No amplfication .. we are sampling directly off the output of the blocking FETS. The dynamic range and sensitivity is an order better than with crowbar diode blocking The flyback is neatly blocked to a 8 volt peak to peak signal ... so it wont blow the input to the sampling demods .... . the neato thing is that the blocking FETs draw almost no current when blocking .... unlike diode crowbars.

The white trace is the +ve flyback ... the yellow trace is the -ve flyback ... the green trace is the TX mosfet gate drive signal.

moodz