field test unit no 001 "model T"

[moodz]

Protecting the front end amplifier ...

Instead of the usual "crowbar" diode and resistor arrangement that shorts out high level voltage into the amplifier a series blocking FET arrangment is proposed where the FET becomes an open circuit when a high voltage is present a the input thus protecting the amplifier.

The Pic below shows the circuit in operation where the incoming flyback pulse green is knocked down to 20 volts peak yellow which the amp can deal with. The white trace is the gate drive for the TX MOSFET.

moodz

[moodz]

rolling on ....

Series blocking to the input amplifier is a success.

Pics below show blocking signal at input to amplifier and .......


descrimination info


and I have not even begun to amplify it yet.

This working on the UNIPI chip with the damping circuit and series blocking FETs. The series blocking FETs dont require any control signals ... they block depending on the voltage signal at the input ... basically high negative or positive voltage = OFF.

The three pics are in order 1. No target 2. Ferrous target 3. Non Ferrous target

I dont have to tell you where I will be sampling do I ??

moodz.

[mickstv]

This is all very interesting particularly the ability to descriminate between ferrous and non-ferrous objects. Does the dampening circuit related to the ability to descriminate or is it to do with the blocking fets.


Cant seem to get a handle on how it's being applied.



Mick

[mickstv]

Hi Paul, just wondering what the part numbers are for the FET's your using in the blocking circuit.




Mick

[moodz]

DESCRIMINATING PI FRONTEND

Below is the descriminating PI frontend with series blocking FETs and damping control compatible with the UNIPI chip.

This is a major development in descriminating PIs and free for everyone to use

Enjoy ... moodz.

[mickstv]

Hi Paul, brilliant thanks for posting that up. The design looks so simple obviously your way smarter than the rest of us.

Relating to the circuit D1 and D2 are they just standard small signal diodes ?

I'll have to find a supplier for the blocking fets and wire it up and see what happens.



Mick

[moodz]

Hi Paul, brilliant thanks for posting that up. The design looks so simple obviously your way smarter than the rest of us.

Relating to the circuit D1 and D2 are they just standard small signal diodes ?

I'll have to find a supplier for the blocking fets and wire it up and see what happens.



Mick

Thanks Mick ... power mosfets will work as blockers for demo purposes however the large capacitance will slow the damping down... the damping control is absolutely necessary as it is used to balance the descrimination function ... you will see what I mean when you play with it. The damping connects to pin 22 on the UNIPI. The diodes can be small sig types but I actually have 16 volt back to back zeners fitted. If you are testing don't connect your amp till you are sure the blocking function is working. Buy a few of the mosfets (try to get lowest capacitance and highest voltage. P and N) they blow up easily if you touch the wrong bit of the circuit. The damping circuit is driven by a PWM signal from the UNIPI and takes about a minute to settle after powering up. ..after that you can change it using the menu. I find the best value is around 1300 to 1900. A future feature isto switch the damping circuit off the coil after fly back is done and maybe an active filter on the damping gate control voltage.
As for the simplicity of the circuit I have to admit it surprised me that it actually worked. I must credit AdrianM for the technique of obtaining the differential power supplies for the amp by connecting a diode and cap to the other side of the diff coil. You can use an alternative to theTHAT1510 specified here but gain should be low if sampling early.

Paul

[mickstv]

Paul here's a couple of SMD fet I found just wondering if you could have a look at the links and see if they would be suitable please.

http://au.element14.com/nxp/bsp225-1...223/dp/1758084

and

http://au.element14.com/infineon/bss...-23/dp/1056525

The RDS is way different on each Fet.




Mick

[moodz]

Paul here's a couple of SMD fet I found just wondering if you could have a look at the links and see if they would be suitable please.

http://au.element14.com/nxp/bsp225-1...223/dp/1758084

and

http://au.element14.com/infineon/bss...-23/dp/1056525

The RDS is way different on each Fet.

Mick

Try this one matched complementary in the same package. .....cheap too.
http://au.element14.com/nxp/phc2300-...6-1/dp/1758128
Moodz

[mickstv]

Try this one matched complementary in the same package. .....cheap too.
http://au.element14.com/nxp/phc2300-...6-1/dp/1758128
Moodz

Thanks Paul, will order them tomorrow....looks like 6-7days wait. Might be a next weekend build.



Mick

[moodz]

Thanks Paul, will order them tomorrow....looks like 6-7days wait. Might be a next weekend build.



Mick

I am going to get some also for testing. They have a lower gate threshold of 2 volts which means the blocking threshold can be made lower also

[mickstv]

I am going to get some also for testing. They have a lower gate threshold of 2 volts which means the blocking threshold can be made lower also

Yeah for the price i'll order 10 because knowing my luck lately i'll blow the first few up.


EDIT i've noticed the damping circuit is only across one of the two coils whereas the previous version using a damping resistor was across the two ends of the diff coil. Wouldn't the other half of the diff coil have undampened coil pulse on it or am I looking at it the wrong way.



Mick

[moodz]

Yeah for the price i'll order 10 because knowing my luck lately i'll blow the first few up.


EDIT i've noticed the damping circuit is only across one of the two coils whereas the previous version using a damping resistor was across the two ends of the diff coil. Wouldn't the other half of the diff coil have undampened coil pulse on it or am I looking at it the wrong way.



Mick

The unbalanced damping improves the fly back time and the descrimination signal. ....arranging for variable damping on both sides of the coil is more difficult. You will see hopefully when you build one that damping only one side provides near perfect damping for both sides of the coil providing the coils are made as specified in my coil posts. I guess there is plenty of room for investigation here though.
Paul.

[mickstv]

The unbalanced damping improves the fly back time and the descrimination signal. ....arranging for variable damping on both sides of the coil is more difficult. You will see hopefully when you build one that damping only one side provides near perfect damping for both sides of the coil providing the coils are made as specified in my coil posts. I guess there is plenty of room for investigation here though.
Paul.

Just ordered the parts needed hopefully they will all turn up by Friday. But in the mean time I don't seen any reason why I couldn't at least get the damping circuit installed and tested on the existing setup.

Back on your previous version, did you ever get a chance to test the diff setup over heavily mineralised ground and hot rocks. Just wondering how it may have performed.


Mick

[moodz]

Just ordered the parts needed hopefully they will all turn up by Friday. But in the mean time I don't seen any reason why I couldn't at least get the damping circuit installed and tested on the existing setup.

Back on your previous version, did you ever get a chance to test the diff setup over heavily mineralised ground and hot rocks. Just wondering how it may have performed.


Mick

I have several kilos of different ferrites and minerals here but I cant get any of them to produce a response ... the differential coil only seems to respond to conductive items like metal. ( ie induced currents ) Anything that is not conductive is invisible. I have previously done some experiments with DD coils and these targets do respond so I know they are good test rocks. Wet clay / damp salty ground may be a different story though.

Paul.

EDIT: I should say that the hotrock testing has been done on the previous version of the circuit with resistor / diode "crowbar" amplifier protection ..... the results may be different for this version with blocking FETs fitted as the voltage is not clamped off at 0.6 volts as it was with the diode / resistor arrangement.