Replies to Minipulse Plus

[Davor]

Davor I have been reading some old threads and am still confused about the "Flyback" on a PI. Some say make it as large as you can and some use a snubber to cut it off. Whats your take on that?

There is an interplay of pulse duration and the target tau. You may observe every target as a high pass filter and this analogy works for both PI and CW. This means that a looooong pulse would do very little with most targets, but after a corner frequency it is the integral of the pulse voltage that becomes a major player.
It means that if a pulse is not longer than the the observed targets tau, it will stimulate all the targets equally well.
What stimulates targets is the current rate of change, or mathematically di/dt, and such phenomenon translates to the coil voltage. However, with pulses reasonably short, what counts in target response is the integral of the pulse voltage. A pulse with duration 1us and 100V is equally stimulating targets as 100ns pulse with 1000V.
Even if you say it is all the same to you, there are phenomena related to high voltage you may wish to avoid. Arcing is the most obvious one. Noisy MOSFET avalanche is perhaps less obvious but it is there. The least obvious is the energy balance.

Hi Davor, I use a MUR460 and 200v zener for my snubber circuit. The other difference between my circuit and yours, is I have the MUR460 connected directly to the Drain of the Mosfet.

I'm not going for a zener for several reasons, first off is that zeners beyond some low voltages are all avalanche devices, and quite noisy too. The other reason is because I wish to sense inductance changes as a consequence of ferrous objects. The famous "X" component of ferrous targets directly translates to the change in a coil inductance. Flyback voltage after a constant current charge, given some freedom, becomes related to the coil inductance.

[6666]

A view of TP3 with a piece of metal waved over coil, trace was a bit bright.

http://youtu.be/NE6IQeNC_MQ

[Silver Dollar]

Ahh I see. Thanks for the info. One other question. What determines the voltage the flyback will get to
(other than the avalanche voltage of the FET)? Is that set by the LRC of the coil? TX pulse width?

[Qiaozhi]

I have been reading some old threads and am still confused about
the "Flyback" on a PI. Some say make it as large as you can and some
use a snubber to cut it off. Whats your take on that?

Here are my thoughts on the subject:
When you turn on the MOSFET, a current flows in the coil and a magnetic field is established around it. When you turn off the MOSFET, you want the magnetic field to collapse quickly so that the rate-of-change of the field is a fast as possible. The faster the collapse, the more energy is generated in any nearby metal target due to an increased number of flux lines cutting through it, and therefore the stronger the eddy current response. That's why the correct value of resistor is important to achieve critical damping. If the coil is overdamped, then energy is unnecessarily wasted in the damping resistor. Conversely, underdamping allows the coil to ring and lengthens the time taken for the response to decay to zero.

If you try to "hurry along" the decay by using snubber networks or other methods, you are simply diverting energy away from stimulating the target. You might as well put less energy into the coil in the first place, as you're achieving the same end result.

This is the same reason why you also don't want the flyback voltage to get so high that the MOSFET will go into avalanche breakdown, as (again) energy will be unnecessarily wasted as heat.

[Silver Dollar]

"why put energy into increasing the flyback voltage?"

Well the theory is that the change in voltage over time determines the field strength
that energizes the target. Thus a greater V can give a greater target response but that's
only if the flyback does all the work...

[Qiaozhi]

Hello,

Does it matter what type/which way round we connect the capacitor ?

A general purpose ceramic will be fine, and it can be fitted either way round since it's not polarized.

[joop]

Davor I have been reading some old threads and am still confused about
the "Flyback" on a PI. Some say make it as large as you can and some
use a snubber to cut it off. Whats your take on that?

Decay of the flyback voltage seems important to me, so why put energy into increasing the flyback voltage?

[6666]

GL, a 100 picofarad capacitor will be a small non-polarised cap which can be installed any way.

[6666]

MUR460, 20 for $3.00, a handy diode to have in junk box.

http://www.ebay.com.au/itm/20PCS-DIODE-ON-VISHAY-DO-201AD-MUR460-MUR460G-MUR460RL-MUR460RLG-MUR460-E3-54-/360867150207?pt=LH_DefaultDomain_0&hash=item54055bb97f

[greylourie]

The puzzle of how to synchronize the TX oscillator with the LT1054 is now solved!

The solution is ridiculously simple. I had a brainwave and connected a 100pF capacitor between the collector of Q2 and U2 pin7.
Now it's synchronized. Yippee!

Before you do the modification, connect scope channel 1 to TP1, and channel 2 to U2 pin2. You will see the two signals are asynchronous.
Then fit the 100pF capacitor and you'll discover the two signals are locked together. Simple really.

I tested for any unexpected side effects, such as the +5V supply dropping out at a higher voltage, but could not see any difference. There was also no discernible difference in the detector's sensitivity, but at least everyone can stop mourning the demise of the sync pulse. I will update the latest (REV-C) Build Document tonight, and post the fix in both the REV-B and REV-C threads.

Hello,

Does it matter what type/which way round we connect the capacitor ?

[mickstv]

My thought exactly, except that most people are blissfully unaware that there is a pin 7 of NE555 that only sinks current, and thus may come very useful in accelerating MOSFET shutdown.
Here is my attempt on a constant current drive with a snubber. V(n005) is a coil voltage (".step param r list .1 10k" is only acting as a switch)
It is apparent that even with a snubber that nicely shapes the flyback, with shutdown acceleration the voltage gets a bit higher.

Hi Davor, I use a MUR460 and 200v zener for my snubber circuit. The other difference between my circuit and yours, is I have the MUR460 connected directly to the Drain of the Mosfet.

[Silver Dollar]

A TL072 draws more current but has less noise than the TL062.

Davor I have been reading some old threads and am still confused about
the "Flyback" on a PI. Some say make it as large as you can and some
use a snubber to cut it off. Whats your take on that?

Here's the old thread about it;

http://www.geotech1.com/forums/showthread.php?15402-PI-Power-Output

[Qiaozhi]

One thing I noticed: U8 is a TL072 in the kit, the component list has a TL062.

That's not the problem. I've just unplugged the TL062 and replaced it with a TL072, and the performance was exactly the same.

[joop]

The original Minipulse coil is 8" diameter. Also, try moving the coil into different orientations and watch if the noise level changes. It may all be external noise.

I have fitted a 470 uH coil with a smaller diameter. Noise has gone down a bit, but not to the expected level. When I move the coil around the noise level changes but it stays at a high level. It might be environment noise. To be sure, I will take the PCB to another place next week to see if it helps.

One thing I noticed: U8 is a TL072 in the kit, the component list has a TL062.

[Davor]

Hi Davor, a simple solution would be to add the circuit I posted link below, only a few components needed.

http://www.geotech1.com/forums/showt...-current-pulse

My thought exactly, except that most people are blissfully unaware that there is a pin 7 of NE555 that only sinks current, and thus may come very useful in accelerating MOSFET shutdown.
Here is my attempt on a constant current drive with a snubber. V(n005) is a coil voltage (".step param r list .1 10k" is only acting as a switch)
It is apparent that even with a snubber that nicely shapes the flyback, with shutdown acceleration the voltage gets a bit higher.