Replies to Minipulse Plus

[greylourie]

I tried using an oscilloscope. But I could not get a dancing decay like in 6666's video. The curve "shrinks" when a metal object is brought close. Does the attached image look ok ?

[Qiaozhi]

With improvised coil am getting 30mV difference when a pair of mini side-cutters are brought close.

Please be aware that the +1V DC offset I measured at TP3 was taken from a single build. The second MPP I've constructed has an offset of -30mV, so it appears that the absolute value will vary from unit to unit. The important thing is that you actually see a voltage drop at TP3 when a metal target is placed near the coil.

[greylourie]

Sorry, my thinking often skews reality. With improvised coil am getting 30mV difference when a pair of mini side-cutters are brought close.

[Qiaozhi]

In step 5 of construction, what if, instead of a drop in voltage at TP3 there is an increase of voltage when bringing a metal object close to the coil ?

Eddy currents are generated in any metal that is close to the coil. This has the effect of lengthening the decay time, and thereby causing the voltage to move in one direction only. The result is that all metal targets produce a decrease in voltage at the pre-amp output.

[greylourie]

In step 5 of construction, what if, instead of a drop in voltage at TP3 there is an increase of voltage when bringing a metal object close to the coil ?

[Qiaozhi]

Well I built the simplest PI detector and it doesn't work. I want to experiment with different FETs
and adjust the damping resistor so it would be nice to see what I'm doing...

Adjustment of the damping resistor is usually done by monitoring the pre-amp output. With a 10k pot in series with a 270R resistor, and both in parallel with a 1k2 resistor; connect this across the coil, and adjust the pot until you achieve critical damping (no ringing in the pre-amp signal). Disconnect the resistor network and measure the resistance, and then fit a damping resistor of the nearest preferred value.

[Silver Dollar]

Well I built the simplest PI detector and it doesn't work. I want to experiment with different FETs
and adjust the damping resistor so it would be nice to see what I'm doing...

[Qiaozhi]

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...

As it's a loaner, I can understand why you're being cautious, especially since the previous owner has already had similar problems.

In that case, is there any reason why you need to probe the TX voltage directly? Is it just that you want to check if the MOSFET is breaking down?

[Davor]

Your probe is ~1M impedance, so you may put a divider to go on a safe side. If the probe says 300V, it is probably because of the potential arcing that could damage things.

[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...

[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.

[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.

[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...

[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

[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.