field test unit no 001 "model T"

[moodz]

Clarification

Regulated voltages all round and some nice blocking

moodz

blocking the the flyback ... +ve only shown here ..


The voltage excursions a the input are 400 volts and busted down to this with no resistors or diode clamping No target.

A small pair of side cutters about 2 inches from the coil ....

A small piece of copper PCB about 2 inches from the coil .....

Those voltages are at the INPUT to the diff amp

[mickstv]

Could you post up a picture of the response from that huge iron target please.



Mick

[moodz]

Could you post up a picture of the response from that huge iron target please.



Mick

Large iron ( metal not ferrite ) target at 2 inches ...

It just rails the coil output .... what ?? ... 6 volts of spread .... and thats at the ouput of the blocking FETS ...BEFORE the amplifier ....

moodz

[mickstv]

Thanks for posting the pix.

The PHC2300's turned up, did you end up ordering some yourself, if not do you want me to send you a couple of them.



Mick

Large iron ( metal not ferrite ) target at 2 inches ...

It just rails the coil output .... what ?? ... 6 volts of spread .... and thats at the ouput of the blocking FETS ...BEFORE the amplifier ....

moodz

[ATTACH]16974[/ATTACH]

[mickstv]

Hi Paul, how important is the design of the diff coil. I'm using about 28turns of single pair cut out of cat5 cable and it's about 30cm diameter.

This coil then goes into the newly finished frontend with the blocking fets. When I monitor the output of the blocking fets, all objects waved at the coil whether they be ferrous of non ferrous show the same response. I have tried the descrim balance control but still no good.

What to try next ?



Mick

[moodz]

Thanks for posting the pix.

The PHC2300's turned up, did you end up ordering some yourself, if not do you want me to send you a couple of them.



Mick

I am still using my BSP/N's they are out of production I think ... I will get some of the PHC's if I lay out a SMT board though.

No need to send me any ... but thanks very much for the offer.

moodz.

[moodz]

Hi Paul, how important is the design of the diff coil. I'm using about 28turns of single pair cut out of cat5 cable and it's about 30cm diameter.

This coil then goes into the newly finished frontend with the blocking fets. When I monitor the output of the blocking fets, all objects waved at the coil whether they be ferrous of non ferrous show the same response. I have tried the descrim balance control but still no good.

What to try next ?



Mick

Can you post a pic of your coil and the scope shots across the output before and after the blocking FETs with no amp connected.

moodz.

[mickstv]

Can you post a pic of your coil and the scope shots across the output before and after the blocking FETs with no amp connected.

moodz.

Hi Paul, I'll have to do it tomorrow as i don't have a camera handy at the moment. So I'll post up some pictures tomorrow arvo.

How critical is the 7.4v rail.

FYI the damp/descrim circuit seems to work, if you have it adjusted down too far you get ringing on the cro.



cheers
Mick

[moodz]

Hi Paul, I'll have to do it tomorrow as i don't have a camera handy at the moment. So I'll post up some pictures tomorrow arvo.

How critical is the 7.4v rail.

FYI the damp/descrim circuit seems to work, if you have it adjusted down too far you get ringing on the cro.



cheers
Mick

I use a 7805 with two 1.2 volt zeners in series on the ground leg .... you could just use 4 diodes ( 4 x 0.6 volts ) however for testing the 5 volt reg on its own should work ... I only use a lower voltage like 7.4 otherwise the voltage peaks cause breakdown of the blocking or switching mosfets. Mine runs from 400 to 600 volts peak to peak and depending on where the damping is adjusted.

moodz

[mickstv]

I use a 7805 with two 1.2 volt zeners in series on the ground leg .... you could just use 4 diodes ( 4 x 0.6 volts ) however for testing the 5 volt reg on its own should work ... I only use a lower voltage like 7.4 otherwise the voltage peaks cause breakdown of the blocking or switching mosfets. Mine runs from 400 to 600 volts peak to peak and depending on where the damping is adjusted.

moodz

I actually found a 7808 in an old board so I set it up with that and a BYW96e on the output pin to drop the voltage. The output seemed smooth and didn't vary.

I also used HER208's on the FET and the other side of the diff coil.



Mick

[moodz]

I actually found a 7808 in an old board so I set it up with that and a BYW96e on the output pin to drop the voltage. The output seemed smooth and didn't vary.

I also used HER208's on the FET and the other side of the diff coil.



Mick

That sounds fine ... must be the coil ...

[mickstv]

That sounds fine ... must be the coil ...

I know you have a thread on the diff coil but at the moment what diameter coil are you testing and how many turns.


Also did you end up ordering the PHC2300 fet pack IC's or do you want me to send you a couple for testing in your circuit.



Mick

[moodz]

I know you have a thread on the diff coil but at the moment what diameter coil are you testing and how many turns.


Also did you end up ordering the PHC2300 fet pack IC's or do you want me to send you a couple for testing in your circuit.



Mick

22 cm coil with 32 turns. I wound a coil using cat5 and it runs a little better than my usual coil ... I can sample at 5us. Descrim is good.

Watch out for the amplifier .. protection diodes at the input can upset the bias on the blocking fets and either blow them up or sink current which will stuff up the coil performance. This is the area I am fiddling with now ...previously I could barely run the amp at a gain of 10 without overload ... Now it is running at gain of 2000. ( the noise actually improves on these amps with higher gains as the bandwidth decreases )



I would like to see how you go with NP fet pack ... I am still moving things around on the front end circuit to get the demodulator channels completed ... the blocking fets are still in place so dont worry ... however I blow a set up every day on average ... the ones I am using bsp254 and bsn254 I have quite a few so I will try the fet pack when I stop probing.

moodz

[mickstv]

22 cm coil with 32 turns. I wound a coil using cat5 and it runs a little better than my usual coil ... I can sample at 5us. Descrim is good.

Watch out for the amplifier .. protection diodes at the input can upset the bias on the blocking fets and either blow them up or sink current which will stuff up the coil performance. This is the area I am fiddling with now ...previously I could barely run the amp at a gain of 10 without overload ... Now it is running at gain of 2000. ( the noise actually improves on these amps with higher gains as the bandwidth decreases )



I would like to see how you go with NP fet pack ... I am still moving things around on the front end circuit to get the demodulator channels completed ... the blocking fets are still in place so dont worry ... however I blow a set up every day on average ... the ones I am using bsp254 and bsn254 I have quite a few so I will try the fet pack when I stop probing.

moodz

Good to see the high gain on the THAT ic, it will be interesting to see the circuit mods once you've finished.

The circuit I build is exactly the same as your second schematic, only I haven't fitted the THAT ic yet, till I get this front end running correctly. What I'll do before I post up any pix tomorrow I'll wind another coil the same as the one you just made and try it. I'm still only on the first fet pack, who knows maybe they will be more robust.

I won't put the THAT ic in till you post up the possible changes, because as you would know it isn't cheap.



Mick

[mickstv]

Well here's what has happened. I've built another coil pictured below 24cm dia and 29 turns, wire from cat 5 cable. All Cro shots are 5v p-p at 50us per div.

With the new coil and once I looked in the right spot I can see the cro showing the difference between ferrous and nonferous objects.






The picture below is of fet sife of the coil and below is the tx pulse.




The picture below is FET side of diff coil and bottom picture is after the n-channel blocking fet.




The picture below is the other side of the diff coil top and after the p-channel blocking fet below.




This final picture is of the output top N-channel and bottom P-channel blocking fets.