field test unit no 001 "model T"

[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]

Try this one matched complementary in the same package. .....cheap too.

http://au.element14.com/nxp/phc2300-118/mosfet-np-ch-300v-sot96-1/dp/1758128

Moodz

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



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/bsp225-115/mosfet-p-ch-250v-225ma-sot223/dp/1758084

and

http://au.element14.com/infineon/bss139/mosfet-n-sot-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/phc2300-118/mosfet-np-ch-300v-sot96-1/dp/1758128

Moodz

[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/bsp225-115/mosfet-p-ch-250v-225ma-sot223/dp/1758084

and

http://au.element14.com/infineon/bss139/mosfet-n-sot-23/dp/1056525

The RDS is way different on each Fet.




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]

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]

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




Mick

[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

[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.

[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]

With the above circuit, is this setup inverting ?. Eg. The coil in ON when the dspic pin is low and OFF when it is high.

The mosfet driver between the dspic and the mosfet is inverting ... TC4421 .... so a low input will produce a "high" on the output of the driver thus turning the mosfet on.

moodz

[maca404]

With the above circuit, is this setup inverting ?. Eg. The coil in ON when the dspic pin is low and OFF when it is high.

[moodz]

Of course the actual voltage developed across the upper winding also depends on the coupling coefficient between the two halves of the coil as well. This is bound to be less than one, but then I've been shooting for 5V with a LDO regulator which appears to be viable with the couple of different coils I've made so far.

I did try it out on a big coil and get nearly +10.5 volts at 100 ma loading ripple is 500 mV ...after a regulator this will be alot better. The peak flyback voltage dropped about 15 volts on a 470 volt max but the overall decay was not impacted. This will be useful for powering a diff amp located in the coil head or a pluggable module near or on the coil head. The coils I use are made with magnet wire tightly twisted 2 -3 TPI and coupling seems to be quite good.

moodz.

[AdrianM]

Of course the actual voltage developed across the upper winding also depends on the coupling coefficient between the two halves of the coil as well. This is bound to be less than one, but then I've been shooting for 5V with a LDO regulator which appears to be viable with the couple of different coils I've made so far.