field test unit no 001 "model T"

[smokingdiode]

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

[6666]

Congrats Smokin

[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.

[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

[bklein]

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

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

Barry

[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

[smokingdiode]

600V N channel MOSFET

http://parts.digikey.com/1/parts/101...n-2sk2792.html

now looking for a P channel

[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.

[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]

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

[moodz]

...front end with the amp fitted.

R20 varies the gain
R18 varies the damping

moodz

[hdphilip]

moodz

now thats what i'd call confidence in the blocking fet design, putting the That 1500 amp on the flyback chopping block.... ,

but, i do notice powering the 1500 from c15 and c 16 ,

Philip

can you post your eagle sch file for it?

[moodz]

moodz

now thats what i'd call confidence in the blocking fet design, putting the That 1500 amp on the flyback chopping block.... ,

but, i do notice powering the 1500 from c15 and c 16 ,

Philip

can you post your eagle sch file for it?

LOL ... I will PM you the file ... the amp circuit is not fully tested yet so I guess that is a work in progress.

moodz

[moodz]

The full motza ...

WARNING : THIS CIRCUIT COULD KILL OR HARM YOU .... DANGEROUS VOLTAGES CAN BE GENERATED.

... having said that ... the fully protected circuit.

[hdphilip]

moodz

how about this idea, make the differential front end as simple as possible, then
we could use separate TX coil, or tinker's tem transmitter idea. the best of both
worlds.....

the white strips on the "cat in the hat" design is where the coils will be.

just a thought


Philip