Replies to Minipulse Plus

[6666]

MUR460, 20 for $3.00, a handy diode to have in junk box.
http://www.ebay.com.au/itm/20PCS-DIO...item54055bb97f

[6666]

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

[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?

[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.

[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]

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]

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?

[6666]

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

http://youtu.be/NE6IQeNC_MQ

[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.

[Davor]

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?

Ideally it is:



It is the rate of change that plays it, and in case of MOSFETs it is always related to capacitances.
If we can agree that 1us is reasonably short flyback pulse, you'll end up with reasonable voltages despite some capacitances.

[bbsailor]

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?

Let me take a stab at answering this. All current rises by the amount of time that the TX pulse is on. Take a coil that is 3 ohms plus a MOSFET that is 0.5 Ohms On-Resistance and 0.1 ohms for the lead wire resistance makes a total TX circuit resistance of 3.6 ohms. Assume that the supply voltage is 12 Volts. Assume that the coil inductance is 300 micro Henries. The coil charging Time Constant (TC) is 300/3.6 or 83.3 micro Seconds (uS). So with a TX pulse width of 83.3 uS the coil current will rise to 63 percent of the maximum current which is 12V/3.6 Ohms or 3.33 Amps but at a TX pulse of 83.3 uS only 63 percent of the maximum or 3.33 X .63 or 2.098 A. Add another 83.3 US to the TX pulse and you now have a TX pulse of 166.6 uS for a current of 85 percent of max at two TCs; 3.33 X .85 is 2.83A. Add another 83.3uS to the TX pulse width and you are at the 3 TC point on the current rise graph or about 95 percent of max. All current rises by the Time constant of the coil based on coil inductance, coil resistance and TX pulse width. Longer TX pulses create higher current in the coil and produces a higher flyback spike the needs to be damped to begin sampling as soon as the flyback spike is damped to zero.

The coil discharge TC is determined by the coil inductance divided by the damping resistor value. Typically to optimize a coil for a specific target TC you want the discharge TC to be 5 times faster than the TC of the target you are seeking. A 300uH coil with a 300 ohm damping resistor (Rd) has a 1us discharge time, good for target TCs of 5uS or higher. Now take a low capacitance coil with techniques to reduce the TX circuit capacitance including reducing MOSFET capacitance and shielding capacitance, coil winding capacitance, coax capacitance and you may be able to make a coil that critically damps with 1000 ohms. Now the 300uH coil has a discharge TC of 300/1000 or 0.3 uS optimum for targets with a TC of 1.5us such a fine gold chains or smaller gold nuggets. Measure the coil resonance at the end of the coax and the coil with the highest resonant frequency will use the higher value damping resistor.

So, as you can see, there is a balancing act between all the elements mentioned above.

Look up on the internet: "current rise Time Constant" to see that the laws of electronics are universal for all coils driven by various pulse lengths.

It helps to keep the peak of the flyback pulse below the rated MOSFET voltage otherwise the MOSFET will clamp the flyback voltage, cause the MOSFET to heat up, and delay the time when the flyback pulse returns to zero when you can sample.

I hope this helps?

Joseph Rogowski

Go to this web link to obtain an electronics calculator the will show you current growth curves and flyback voltage.http://www.miscel.dk/MiscEl/miscel.html

[Silver Dollar]

A very good explanation! Thanks for the input!

But is there a way to tell how much voltage to expect on the coil as I have a malfunctioning PI
detector and don't want to burn my scope with too much voltage. Say we had 3 amps in a 300uh
coil with a 390 ohm damping resistor...

[Davor]

If you drive a coil with a MOSFET and have no idea on a coil performance, just check the MOSFET avalanche voltage, and that's the maximum voltage you can ever get, regardless of everything else.

[Qiaozhi]

But is there a way to tell how much voltage to expect on the coil as I have a malfunctioning PI
detector and don't want to burn my scope with too much voltage.

If you're using a 10x probe, then you will not be in danger of frying the scope. For example, the MPP uses a MOSFET with an breakdown voltage of 400V with a flyback of about 350V, and my scope has a maximum input voltage of 300V. But with a x10 probe the voltage from the transmitter is reduced to 35V max, so there's no problem.

[Silver Dollar]

Well my FET avalanche voltage is 500V and the scope probes say 300V so I wasn't sure
if I needed a voltage divider or high voltage probe to see what's going on (as it isn't working
so good). The scope is a loaner, I got it because it's previous owner kept blowing channels
up on it, I don't want to do the same...