field test unit no 001 "model T"

[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]

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]

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]

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, 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.

[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.

[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

[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]

[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]

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



Mick

[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

[moodz]

some results ...

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 .....

[moodz]

Hi Paul, how does the that1510 cope with the variation in voltage. Would it be better to have a second regulator before the 5v reg for the amp ic and for the protection diodes.

Still waiting for the fets to turn up, maybe tomorrow then I can test I few ideas I've had.

Mick

The problem with regulating the amp supply is that because one side of the coil is sitting near ground during TX it would "short" out one of the power supplies via the N mosfet via the protection diode. The amp also has protection diodes on the input.

The power suppy rejection on these amps is very high and additionally all the fluctuations on the supply are synchronised to the TX. The descrim balance has no rejection though and was being affected by large target field suck outs on the coil.

moodz

[mickstv]

Hi Paul, how does the that1510 cope with the variation in voltage. Would it be better to have a second regulator before the 5v reg for the amp ic and for the protection diodes.

Still waiting for the fets to turn up, maybe tomorrow then I can test I few ideas I've had.




Mick

[moodz]

UNIPI Descriminating PI FRONTEND v230 Revision002

The control signal previously referred to as a damping control is more correctly termed a descrimination balance control .... you will see it how it works when you build it

Its beneficial and cost efficient that the split supply rail power for the frontend is provided by the differential coil however when a big target is near the coil like a large piece of iron alot of energy is lost and the voltage varies ... because the descrimination balance control is fed from this voltage it causes the descrim to "unbalance" ... and because of the low pass filtering delay ... there is a "memory effect" .... ie a target response remains after the target is removed.

So to fix this a 5 volt regulator for the descrimination balance supply has been added .... I suppose a zener diode would work also however I personally prefer the regulator ...

Also I have cleaned up the protective diode arrangement .... the previous schematic was not good in this area.

Updated schematic below ..

Moodz.