field test unit no 001 "model T"

[moodz]

I've found the problem with the descrim front end, the 78L05 that I had fitted was drawing too much current. Once replaced with a new one the input voltage went upto 15-16 volts and the Descrim section started damping correctly.

Without the blocking fets I've been mucking around with various targets. Copper and lead sinkers go one way and Iron targets go the opposite way when looking at the CRO.

The only exception was a sheet of plumbers lead it's amplitude went the same way as iron, not sure why that would be.

Next will be fitting the blocking fets, Paul you were walking about mods to the frontend just wondering how that's going and whether the blocking fet setup is different to the second circuit you posted. If it is I'll wait till you post up the next circuit.



Mick

That is good news ... I have not tested a large piece of lead ... was it rolled up ? that could affect the descrim.

You can fit the blocking fets and diodes as per the last schematic but no amplifier yet and check that blocking should now be working.
The input of the THAT amplifier has diodes fitted and I not sure that these are interacting somehow with the front end ... looking into it.

Otherwise the channel mods are coming along ... but I have been busy at work so not as fast as I would like.

moodz.

[mickstv]

That is good news ... I have not tested a large piece of lead ... was it rolled up ? that could affect the descrim.

You can fit the blocking fets and diodes as per the last schematic but no amplifier yet and check that blocking should now be working.
The input of the THAT amplifier has diodes fitted and I not sure that these are interacting somehow with the front end ... looking into it.

Otherwise the channel mods are coming along ... but I have been busy at work so not as fast as I would like.

moodz.

Hi Paul, the lead was basically flat with a slight wave in it. I did have one lead sinker 3cm diameter that detected the same way as iron also but all other smaller sinkers detected the same way as copper.

Have you done tests with different TX widths did you find the disc changing.




Mick

[moodz]

Hi Paul, the lead was basically flat with a slight wave in it. I did have one lead sinker 3cm diameter that detected the same way as iron also but all other smaller sinkers detected the same way as copper.

Have you done tests with different TX widths did you find the disc changing.




Mick

I have not had time to sit down and test all combos of pulse timings vs targets ... I imagine there could be some change ...

I generally run Tx at 100 us.

moodz.

[mickstv]

Some more pictures with the blocking fet package attached. No amp attached. Descrim running at 710.

For some reason it's taking approx 20us to 0 volts. I can get it faster but the damping code has to be set to 10 on the display.

[moodz]

Mick ... I think you still have a problem ... the output of the blocking fets should look like below .... if the coil is sitting at 7.4 volts then that is what the voltage should decay to ...

I have wired up a board for field tests ... just finalising the amplifier ....



moodz

[mickstv]

Just took this shot the cro is set to DC input 2 volts p-p taken on one of the blocking fet output's. If this picture isnt correct I don't know whats going on I must mention that I may have done previous measurements on AC input.

[moodz]

Mick there seems to be some current flowing ... if you have diodes connected to the ouputs of the blocking fets disconnect them then measure.

moodz

[mickstv]

Mick there seems to be some current flowing ... if you have diodes connected to the ouputs of the blocking fets disconnect them then measure.

moodz

[ATTACH]17143[/ATTACH]

Ok shall do will post up result in about 1/2hour.




Mick

[mickstv]

This seems to be looking worse every time I do something to it. no diodes on output of blocking fets

[Midas]

Hi Mick,

I think what your seeing there is the Miller effect.
http://www.richieburnett.co.uk/mosfail.html
It shouldn't normally be a problem and it indicates that your not driving the gate of your coil drive mosfet low hard enough.
Which raises the question how are you driving the gate of your main coil driving mosfet?
I noticed the mosfet driver dropped off the schematic a while back.. I'm not sure why. I hope you didn't make the not so unreasonable assumption that meant you could drive the mosfet directly from the PIC.
If you are using a suitable driver then as a next step I'd recommend scoping the mosfets gate and make sure it is working as it should driving the gate high and low nice and smoothly.

I hope this helps you get it working properly, I'm keen to see someone else replicate moodz's excellent work.

[moodz]

Hi Mick,

I think what your seeing there is the Miller effect.
http://www.richieburnett.co.uk/mosfail.html
It shouldn't normally be a problem and it indicates that your not driving the gate of your coil drive mosfet low hard enough.
Which raises the question how are you driving the gate of your main coil driving mosfet?
I noticed the mosfet driver dropped off the schematic a while back.. I'm not sure why. I hope you didn't make the not so unreasonable assumption that meant you could drive the mosfet directly from the PIC.
If you are using a suitable driver then as a next step I'd recommend scoping the mosfets gate and make sure it is working as it should driving the gate high and low nice and smoothly.

I hope this helps you get it working properly, I'm keen to see someone else replicate moodz's excellent work.

Hi guys ... that is something to check out ( Thanks Midas ).... I am using an IRF740 and it drives OK off the PIC but I guess this issue can happen.

I have got myself some of those blocking FETs you are using Mick so I will look into this on the weekend ... Just a thought ... your "top" diode is not the wrong way round is it ? ( not FET end diode ).

moodz.

[Midas]

Hi Moodz,
Yeah right, sorry it was rude of me to assume that it wasn't a design decision. I understand you're trying to keep it as simple as possible.
But I definitely think the mosfet driver is worth the extra complexity. Even your implementation may be perilously close to unwanted mosfet turn on possible only needing a IRF740 (they have a wide gate threshold variation 2-4v) from a different batch or longer traces to tip the balance. The other issue with direct drive is that the IRF740 is far from being fully turned on at 5v either which will reduce maximum coil current capability and power efficiency. Then of course switching times are increased also extending the time the fet spends in its linear region wasting more power.

[mickstv]

Hi all, I've pretty much tried everything including different coils, I also tried running it with a TC4422a driver IC made no difference to the outcome.

All other components bar the blocking fets are exactly the same values as Paul's second design. So unless the frontend schematic has a mistake in it I have no idea why the circuit isn't working correctly for me, well unless the analogue CRO isn't capable of displaying the responses correctly.

Maybe I should wait until Paul has the frontend finished including the diff amp then try again.




Mick

[moodz]

Hi Moodz,
Yeah right, sorry it was rude of me to assume that it wasn't a design decision. I understand you're trying to keep it as simple as possible.
But I definitely think the mosfet driver is worth the extra complexity. Even your implementation may be perilously close to unwanted mosfet turn on possible only needing a IRF740 (they have a wide gate threshold variation 2-4v) from a different batch or longer traces to tip the balance. The other issue with direct drive is that the IRF740 is far from being fully turned on at 5v either which will reduce maximum coil current capability and power efficiency. Then of course switching times are increased also extending the time the fet spends in its linear region wasting more power.

No ... no offence taken .... it could be what you mentioned. ... there isn't much else to try out unless I actually send Mick my working example ... I will be trying out Micks setup on the weekend ... I have previously been using out of production mosfets for the blocking function. ... maybe there is a "feature" on the NP mosfet pack that Mick is using.

moodz

[mickstv]

Hi Paul, do you have an old Analog CRO that you could use to compare my pictures ?

The PHC2300 packs have protection diodes between drain and source maybe this is the problem ?



Mick