field test unit no 001 "model T"

[mickstv]

I just noticed that the descrim works when I have the TX set to 50us or less, but for some reason anywhere between 55 and 100us all targets get detected the same.



Mick

[moodz]

mick
where you have a picture with this caption ...
"The picture below is of fet sife of the coil and below is the tx pulse."

on the fet side of the coil your flyback pulse should be positive ... but it is going negative from the looks of it ... something is wrong .... make sure the centre connection of the coil is connected to the 7.4 volt rail.

moodz.

[mickstv]

mick
where you have a picture with this caption ...
"The picture below is of fet sife of the coil and below is the tx pulse."

on the fet side of the coil your flyback pulse should be positive ... but it is going negative from the looks of it ... something is wrong .... make sure the centre connection of the coil is connected to the 7.4 volt rail.

moodz.

Hi Paul, yep the centre connection of the coil has 7.4volts on it. I should clear up that was with the CRO directly connected to the coil so it also has the diode in line with the fet.

[moodz]

Hi Paul, yep the centre connection of the coil has 7.4volts on it. I should clear up that was with the CRO directly connected to the coil so it also has the diode in line with the fet.

Disconnect the blocking FETs completely and check the positive and negative flyback pulses on both sides of the coil ... should be about +/- 25O volts with that coil.

moodz.

[mickstv]

Disconnect the blocking FETs completely and check the positive and negative flyback pulses on both sides of the coil ... should be about +/- 25O volts with that coil.

moodz.

No worries. I won't get a chance now but I'll check it tomorrow morning and post the results including pictures of both sides of the coil without the blocking fets attached. I'll also mark on the pictures the CRO gnd centre lines.


Mick

[mickstv]

Hi Paul, here are the extra shots I tool with out the blocking fets.

[moodz]

hmmm ...

Mick,
your TX is mostly OK except that after the flyback the voltage is low ???? for a period ... this is definitely wrong ... see red area on your scopeshot. you can see a step in the voltage during the RX period this is not OK.



moodz

[mickstv]

Mick,
your TX is mostly OK except that after the flyback the voltage is low ???? for a period ... this is definitely wrong ... see red area on your scopeshot. you can see a step in the voltage during the RX period this is not OK.



moodz

[ATTACH]17031[/ATTACH]

The only difference between the your second schematic and my design are the frontend diodes, mine are HER208 and yours are FS18G. I can't even find a match in google for FS18G what are they ?

I'll change them for BYW96e tomorrow and see what happens.


Apart from that I'm not sure what else to try.



Mick

[mickstv]

Hi Paul, I may have found the problem area I removed the descrim circuit including the IRF740 and the 100ohm resistor I then added a 470ohm resistor across L1 side of the Diff coil. The waveform now settles as you would expect and the descrim between ferrous and non ferrous as read on the cro works perfectly.

Now just have to sort out the problem with the descrim circuit.

All the checks are on the coil input no blocking fets fitted.



Mick

[moodz]

Hi Paul, I may have found the problem area I removed the descrim circuit including the IRF740 and the 100ohm resistor I then added a 470ohm resistor across L1 side of the Diff coil. The waveform now settles as you would expect and the descrim between ferrous and non ferrous as read on the cro works perfectly.

Now just have to sort out the problem with the descrim circuit.

All the checks are on the coil input no blocking fets fitted.



Mick

That's funny I just logged on to suggest the same thing. Might sound obvious but check that the descrim mosfet is not upside down.
Moodz

[mickstv]

That's funny I just logged on to suggest the same thing. Might sound obvious but check that the descrim mosfet is not upside down.
Moodz

Hi Paul, I just checked again I have the source pin RHS going to coil + "7.5volts" and the drain going via a 100ohm resistor to L1 coil. I did notice that if I removed the 10uf cap from the gate, the waveform was also normal but couldn't be adjusted.


Mick

[moodz]

Hi Paul, I just checked again I have the source pin RHS going to coil + "7.5volts" and the drain going via a 100ohm resistor to L1 coil. I did notice that if I removed the 10uf cap from the gate, the waveform was also normal but couldn't be adjusted.


Mick

Measure the gate voltage as you adjust damping ... also measure the voltage on the cap connected L2 via diode ... should be something like 14 volts or so. The FET might be punched through ...though I supposed you changed it already.
moodz

[mickstv]

Measure the gate voltage as you adjust damping ... also measure the voltage on the cap connected L2 via diode ... should be something like 14 volts or so. The FET might be punched through ...though I supposed you changed it already.
moodz

From memory the cap on L2 had about 12-13 volts. I haven't replaced the fet yet I'll do what in the morning and see what happens.


Mick

[mickstv]

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

[mickstv]

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.


Mick

Further to the the story about the A4 sized lead sheet, I cut off a piece that was 2cm wide and 15cm long this smaller piece tracked on the cro the same way as the lead sinkers and copper.

Also the TX pulse width seems to effect the result I have found the higher you go the more likely the targets are going to be detected the same way on the CRO.


Mick