coil to plus input

[green]

Maybe I'm looking at it wrong, why doesn't the op amp get zapped thru the back diode across the 1440 when the coil is on?

http://www.geotech1.com/forums/showthread.php?22661-coil-to-plus-input&p=208180#post208180

[greylourie]

Maybe I'm looking at it wrong, why doesn't the op amp get zapped thru the back diode across the 1440 when the coil is on?
http://www.geotech1.com/forums/showt...180#post208180

Maybe the 1N4148 takes the nasties away ? Also curious about the zeners (TVS?) cathode to cathode between gate and source on the datasheet.

[mickstv]

Maybe I'm looking at it wrong, why doesn't the op amp get zapped thru the back diode across the 1440 when the coil is on?
http://www.geotech1.com/forums/showt...180#post208180

All I know is the 1440 is switched off during TX and I've been using this circuit for a long time without blowing up any op amps, so it's obviously reliable.

[greylourie]

hi mick, think its quite an achievement to progress from the conventional resistor and diodes.

hi green, maybe you have read these posts. But will add the links here. http://www.geotech1.com/forums/showt...3634#post83634

http://www.geotech1.com/forums/showthread.php?17106-LM394-on-my-Minelab-PCB&p=125978#post125978

Really looking forward to any possible measurements and results you may share.

[green]

All I know is the 1440 is switched off during TX and I've been using this circuit for a long time without blowing up any op amps, so it's obviously reliable.

Not saying it doesn't work. Trying to understand where my thinking is wrong. To me it looks like the diodes open and the op amp keeps working, or I'm missing something. Seems like if the diodes don't open there would be over 500ma thru the 10 ohm resistor.

[Altra]

I think it works because it only conducts during the TX period. Say 100us. Mick did not
mention the pulse frequency. But the average current would be well below 500mA depending on the duty cycle.
The SD2000 and QED use a P channel to block this current.

[mickstv]

I'm not really sure why it all works, but here's some more screen shots at the connections to the blocking fet and also the output at 100x gain.

All CRO shots are at 20us/div. Also the period between TX pulses is 430us.


1. CRO 20us/div Top flyback block pulse and bottom TX pulse.




2. Source Pin of blocking fet (2 volts/div) and bottom TX pulse. The output on the source pin looks under damped but the circuit still works fine.




3. Output of preamp at 100x gain.




4. Output of preamp 100x gain with target near coil.

[Mechanic]

HI Mick,

Whats the v/div for the last shot?
And also, do you have your probe A/C coupled? Just the whole rx waveform has moved up a bit compared to the no target shot.

Cheers Mick

[mickstv]

G'day Mick,

Both at 5v per div. The target was a 1" piece of coke can resting on the coil.


EDIT : DC coupled, that's just me adjusting the CRO into different spots.

[mickstv]

Fixed.

I couldn't change the last two pictures from my previous post. I took them again, this time without adjusting the CRO.

[Mechanic]

Ahhh, ok! Thought you may have just moved it a bit or the scope had drifted a bit, but thought I'd better check

So your tx is -8v and the output from your preamp during tx is -5v, the source pin of your fet has a bit of voltage on it too(dropping away the longer the tx is on), what v/div for that one?

Would it be possible for you to put a 1r or 10r resistor in series between the fet and preamp and measure the current during the tx period? I wonder if a bit better performance could be gained by using the p fet as well, stopping the diodes in the preamp conducting(well at least conducting so much current) causing a bit of heat and also causing a bit of heat in the fet.

Cheers Mick

[mickstv]

So your tx is -8v and the output from your preamp during tx is -5v, the source pin of your fet has a bit of voltage on it too(dropping away the longer the tx is on), what v/div for that one?

I'm sure that pic was at 2v/div.

Would it be possible for you to put a 1r or 10r resistor in series between the fet and preamp and measure the current during the tx period? I wonder if a bit better performance could be gained by using the p fet as well, stopping the diodes in the preamp conducting(well at least conducting so much current) causing a bit of heat and also causing a bit of heat in the fet.

Cheers Mick

I'll try a 10r resistor and see if I can measure across it.

When I tried a Pch, I had problems with ringing etc. so I gave up on that idea.

I've got two detectors built exactly the same, one in use and the other as a test unit for doing mods. This second one I've had running on the bench continuous for 6+ hours a day over multiple days and hasn't fried anything so far.

[Mechanic]

Did you use a low C, low Vf diode at the junction between the n & p fet to drain off the charge? This tends to be the source of the ringing at first fet turn on, also turn on timing is critical.

Cheers Mick

[mickstv]

Here's the picture across a 10 ohm resistor between the blocking fet and pin 3 of the preamp.

The CRO is 20us/div and 1v/div

[mickstv]

Did you use a low C, low Vf diode at the junction between the n & p fet to drain off the charge? This tends to be the source of the ringing at first fet turn on, also turn on timing is critical.

Cheers Mick

From memory it was a BAV99 or similar.