PI sensitivity Problem

[Qiaozhi]

Done! http://imageshack.com/a/img909/9527/FtXQ9k.jpg

OK - I'll put you out of your misery.
Here's a snippet of the PI-5 schematic from ITMD that uses an NMOS to drive the coil, and creates a dual voltage supply for the opamp from a single battery pack.

[xpcam]

Ok? I see how the diode arangement and how i should bring the second end to the ground, but how can i set the offset of the opamp (by the way i do not have an NE5532 opamp) so i want to use ones that i have laying around (soic8/sot23-5).

Christian

[DOOLEY]

unfortunately the Pi - 5 schematic was pointed out to be drawn wrong months ago !!!

the arrangement around the back to back diodes send all fly-back to the rail splitting op-amp.......
it may not like it..........

poss best print another diagram, sorry George.

[xpcam]

Im confused. Not a good solution i guess :-) uhm???

[Qiaozhi]

unfortunately the Pi - 5 schematic was pointed out to be drawn wrong months ago !!!

the arrangement around the back to back diodes send all fly-back to the rail splitting op-amp.......
it may not like it..........

poss best print another diagram, sorry George.

I think you'll find that the schematic is correct.

You're probably getting confused with the generic "Inverting Opamp" diagram in Fig 11-11 (Chapter 11, page 149), where the bottom end of the clamping diodes should be connected to 0V, and not to the negative battery terminal. The way it is drawn at the moment will cause the preamp output to be driven up to the positive supply rail.
The PI-5 design in Chapter 12 on page 171.

[xpcam]

Hi, i have been learning a bit about opamps yesterday and today and i build a new cirquit (not tested yet) but i think this is going to work http://imageshack.com/a/img633/787/SUB1GF.jpg First opamp is a buffer. Second the preamp. (All with respect to the ground).

[DOOLEY]

well, thought I was right................



just run a simulation on the Pi-5 ............

and a spike of 7.75 amps of fly-back goes straight into the output
of the rail splitting op-amp.

firstly, there is no "in-line " resistor after the coil,

secondly the protection diodes should be after the in-line resistor and go to whatever voltage the coil is
powered from.

I am only an amateur, but have learnt much over the last 6 years of tinkering master.........

rekon my inserted picture below is how the Pi-5 was intended to look.......
could be wrong I guess......



below is 2 "standard" arrangements of Pi front ends to help out xp-cam
on his journey...............(assume he's a he, may not be.......)







on the inverting diagram, the 1k on the op-amp to the top rail may not be needed,
esp if only amplified lightly, ie: 100x.

but can be used with another resistor to the neg rail to "tweek" the off-set.
just in case the op-amp used doesn't have that facility.

anyways,
call me old fashioned, still living in the dark ages,
but,
of all the op-amps ive tried for pre-amps I still revert back to
and like using a good ole TL071 / TL072.
hardly any bias to have to worry about,
quite quick at 13uS also,
oh well, I like e'm anyways.

[xpcam]

below is 2 "standard" arrangements of Pi front ends to help out xp-cam
on his journey...............(assume he's a he, may not be.......)

Yeah, im still breaking my mind about it Dit you checked out my last drawing?! in the mean time i've changed some small thing on the offset by
adding a 10K instead of 220K and twist the pot wit the resistor. Im going to try it tomorrow. What do you think about that setup!?

[DOOLEY]

xpcam,

your feedback resistor is still going to the non-inverting (+) input of the (2nd) op-amp,
needs to be to the inverting (-) input.

you need to power the op-amps (usually) with more voltage than the signal it's going
to see,
ie: more than 12v+

can't see the need for the 1st op-amp as a voltage follower.

and your upper 10k to +12v doesn't need to be there,
remove and link to +12v

please try either the non inverting or inverting "standard" set-ups like I posted
earlier.

there's lots of ways to arrange a Pi front end that work,
the ones I have posted are but just a few.

oh, your bottom 10k just before the diodes will work,
but from experience its going to be as "noisey as hell"
most designs use 1k, or less.

[xpcam]

Mmmzz, ill try it tomorro, but why is it then that i need to bring in the signal to the minus?! Is this beqourse the reflected pulse is in there?! I could change the 10K resistors but is that then still with respect to the ground while the tvs breaks the signal down to 33 volts?! The input voltage wont be higher then 3.3 volts at this way. I have done some other (5 volt circuit) with a tvs on it and it gived a cool output that i simply could see if it is ferro or non ferro. I will try the schematic you made. Or am i wrong?!

[6666]

Whats a TVS ?

[xpcam]

Whats a TVS ?

Transient-Voltage-Suppression Diode

[xpcam]

well, thought I was right................

Well i have tested the 'inverting' schematic from you but the detection output is very poor on the oscilloscope, the opamp is stable but i almost cant see if i detected something. I also tried my schematic and that one didnt do at all. (too mutch interferents :-( ) with my first test (page 1 first message) i could see the detection of a gold ring around 2 cm, i tested a the pi-5 schematic and my gold ring was visible at 0.5 cm!?!?!? Mmmzz
so many tries so many different results.

Photo's Pi-5 Inverted

http://imageshack.com/a/img909/754/OQoO2n.jpg
http://imageshack.com/a/img905/9289/Wdbyor.jpg

[DOOLEY]

Xpcam,I'm not surprised it don't work !!!On your bread board, and this is just a guess.......You have fed your signal through the 1k,Then into the diodes........But,The diodes should then go to + rail,Not your opamp input !!!So,Your opamp has now seen full flyback voltage.

[xpcam]

Xpcam,I'm not surprised it don't work !!!On your bread board, and this is just a guess.......You have fed your signal through the 1k,Then into the diodes........But,The diodes should then go to + rail,Not your opamp input !!!So,Your opamp has now seen full flyback voltage.

I did precise like on your schemetic except the 680R i used a 1K. For the rest its exactly the same. If it was straigth to the opamp i couldnt make a picture of the wave on my oscilloscope :- ) smd opamps die realy quikcly on high voltages/spikes. :-) a dip8 takes around 5 seconds a soic8 les than 1 second.

http://www.geotech1.com/forums/attac...4&d=1422468849

What it could be is that i neet to turn my opamp 180 degrees LOL