Minipulse Plus Rev My time has come!

[bbsailor]

As you said, everything is a trade-off. A lot depends on what you're searching for. If it's small gold nuggets, then you may want to increase the TX pulse rate, and use a series resistor to limit the maximum current in order to achieve flat-topping, but (as a consequence) lose a lot of depth. In that case it's probably unavoidable with the standard PI setup. For coin searching, cutting off the current before it flat-tops may be the best solution if you don't want to lower the TX pulse rate. For instance, try 1000pps with a 100us pulse width and a 3R3 series resistor. Then do the same test (same pps and pulse width) but with a suitable series resistor to allow the current to flat-top. The former test will have the best result on coins, as the magnetic field will be much greater, and it more than compensates for the opposing field from any eddy currents in the target.

Qiaozhi,

A paper that Eric Foster posted (that I can not find) mentioned that the coil discharge Time Constant (TC) should be 5 times faster than the desired target TC to fully stimulate it. The paper stated that turning off the coil discharge TC any faster did not stimulate the target any more than doing it 5 times faster than the target TC. I am just adding this to the discussion so as to fully address this little spoken of variable.

The coil discharge TC is governed by how vertical the discharge spike is. More vertical is faster. This discharge spike slope is governed by the coil inductance divided by the damping resistor (Rd) value and any other resistance that is in parallel with Rd such as, in a mono coil, the series resistor to the op amp plus the clamping diodes that parallel the Rd value with the op amp input resistor value while the diodes are conducting. All mono coil discharge slopes have a kink in the discharge slope at the point until the clamping diodes open and then resumes a more vertical discharge slope.

If we are doing target and pulse characteristic analysis we need to assume that the desired target is being fully stimulated in order to calculate any other variables.

What is your opinion on this?

Joseph J. Rogowski

[Qiaozhi]

Qiaozhi,

A paper that Eric Foster posted (that I can not find) mentioned that the coil discharge Time Constant (TC) should be 5 times faster than the desired target TC to fully stimulate it. The paper stated that turning off the coil discharge TC any faster did not stimulate the target any more than doing it 5 times faster than the target TC. I am just adding this to the discussion so as to fully address this little spoken of variable.

The coil discharge TC is governed by how vertical the discharge spike is. More vertical is faster. This discharge spike slope is governed by the coil inductance divided by the damping resistor (Rd) value and any other resistance that is in parallel with Rd such as, in a mono coil, the series resistor to the op amp plus the clamping diodes that parallel the Rd value with the op amp input resistor value while the diodes are conducting. All mono coil discharge slopes have a kink in the discharge slope at the point until the clamping diodes open and then resumes a more vertical discharge slope.

If we are doing target and pulse characteristic analysis we need to assume that the desired target is being fully stimulated in order to calculate any other variables.

What is your opinion on this?

Joseph J. Rogowski

Another factor that governs the coil discharge is the gate driving circuit of the MOSFET. Using active turn-off (as opposed to passive) can dramatically boost the flyback voltage, and hence the speed of turn-off. In a circuit I was experimenting with recently, I was able to increase the value of the damping resistor after implementing active turn-off. Of course you have to be careful not to push the MOSFET into avalanche mode.

I did a quick search the paper you mentioned, but couldn't find it either.

[bbsailor]

Another factor that governs the coil discharge is the gate driving circuit of the MOSFET. Using active turn-off (as opposed to passive) can dramatically boost the flyback voltage, and hence the speed of turn-off. In a circuit I was experimenting with recently, I was able to increase the value of the damping resistor after implementing active turn-off. Of course you have to be careful not to push the MOSFET into avalanche mode.

I did a quick search the paper you mentioned, but couldn't find it either.

Qiaozhi,

Please tell us what the higher value of the damping resistor was in your experiment compared to the original value? I would assume that using active turn off eliminates the COSS of the MOSFET and other sources of capacitance and loading as seen by the flyback pulse?

Reducing capacitance as seen by the flyback pulse reduces the energy in the oscillations that require an Rd value to quickly get to the point where the RX circuit can be turned on, otherwise known as delay time.

Since the Rd value determines the coil discharge slope, this becomes very critical for those seeking targets with less than a 2 micro second target decay.

The optimization of TX and RX pulse parameters comes from reverse engineering the PI parameters based on the desired targets time constant.

Do you agree with my above observation?

Joseph J. Rogowski

[bbsailor]

Another factor that governs the coil discharge is the gate driving circuit of the MOSFET. Using active turn-off (as opposed to passive) can dramatically boost the flyback voltage, and hence the speed of turn-off. In a circuit I was experimenting with recently, I was able to increase the value of the damping resistor after implementing active turn-off. Of course you have to be careful not to push the MOSFET into avalanche mode.

I did a quick search the paper you mentioned, but couldn't find it either.

Qiaozhi,

Please tell us what the higher value of the damping resistor was in your experiment compared to the original value? I would assume that using active turn off eliminates the COSS of the MOSFET and other sources of capacitance and loading as seen by the flyback pulse?

Reducing capacitance as seen by the flyback pulse reduces the energy in the oscillations that require an Rd value to quickly get to the point where the RX circuit can be turned on, otherwise known as delay time.

Since the Rd value determines the coil discharge slope, this becomes very critical for those seeking targets with less than a 2 micro second target decay.

The optimization or TX And RX pulse parameters comes from reverse engineering the PI parameters based on the desired targets time constant.

Do you agree with my above observation?

Joseph J. Rogowski

[mushaba]

Another factor that governs the coil discharge is the gate driving circuit of the MOSFET. Using active turn-off (as opposed to passive) can dramatically boost the flyback voltage, and hence the speed of turn-off. In a circuit I was experimenting with recently, I was able to increase the value of the damping resistor after implementing active turn-off. Of course you have to be careful not to push the MOSFET into avalanche mode.

I did a quick search the paper you mentioned, but couldn't find it either.

Taken from a study os some Chinese dudes.. https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8409349


Conclusion : In order to solve the problem of voltage spikes and oscillations caused by the high frequency switching of MOSFETs in power electronic converters, the NAGD is proposed to suppress voltage spikes and oscillations in the turn-off process. The simulation and experimental results show that under the threshold of the gate voltage, the higher the gate voltage is, the better the suppression effect of voltage spike and oscillation will be. At the same time, for higher parasitic inductance and bus voltage, not only can voltage spike and oscillation be effectively suppressed, EMI at high frequencies can also be effectively suppressed. In addition, the control of the NAGD is simple and requires fewer additional components, which is beneficial to the realization of the circuit.

[Qiaozhi]

Please tell us what the higher value of the damping resistor was in your experiment compared to the original value? I would assume that using active turn off eliminates the COSS of the MOSFET and other sources of capacitance and loading as seen by the flyback pulse?

I was experimenting with a 1mH coil wound with enameled wire, so it was not supposed to be a fast coil. The transmit circuit was using active turn-off, and I was surprised to find that the coil could be critically damped with a 1k resistor. I did test this coil quite a long time ago with a Hammerhead circuit using passive turn-off, but cannot remember the value of the damping resistor. However, I think I would have remembered if it was as high as 1k. The active turn-off on this circuit is quite aggressive, so I suspect that it's simply dumping the gate charge very fast. Since Coss = Cds + Cgd; and Cgd (the Miller capacitance) is affected by Cgs, that this is the underlying cause.

[mushaba]

Hello!!
Back again,last 10 days were crazy around here left me no time to work with my MPP..

Read a lot all post from this topic and lots of others,still some things blur to me so be,ready for some rookie questions again

1)To calculate my TC i should measure resistance of the coil plus the "current setting"resistor (R10).Also plus the 5 Ohms resistor i added to reduce my spikes before the mosfet goes into avalance. Made a coil (3dss 24mm OD) 390uH 1.4 Ohms. 1.4+3.3+5=9.7 Ohms. The TC=40.2us mutiplied by 3 so tx pulse should be around 120us.I added the 1 Ohm resistor to power supply of the mosfet and the current flats out at 100us give or take. Now to criticaly damp(maximum reading at coil side and no ringing at amp out side) i use a 5k pot,is it right or wrong??My damping comes out at 1.8k

2)I want to place the damping resistor inside the coil housing (parallel with coil ),will it affect the coil reading?Because of solder (as a metal) ??How come VLF machines i've seen have aluminium brackets and mounts and they are not mess up the coil readings???

3)Found that Vishay has a 1N4448 named diode similar to 4148 with half the recovery time (2 ns) do you think is worth the trouble?

4)Also found that RG62 has half the capacitance of the RG58,think im gonna use that..

5)Made the Krinaz mod for the audio threshold (removed the 1K from each side and put 470K and 100k trimmers,)the adjustment is much stable.Also placed a 5k trimmer in series with R21 for sensitivity adjust,at the air tests i didnt see much difference..

6)Last but not least about timmings..The "cycle" beggins at TX Pulse rising,then we have a delay for coil to settle down,then we sample,then we delay a little more and finally we get the 2nd sample.Adding all time i think there is plenty left for us to increase the frequency of the Tx pulse right?I have set about 2,5Khz..I assume it will be more energy consuming,mosfet will get slightly hotter but what is the factor of selecting the frequency?Also noticed that if we change the frequency of the tx pulse we should have the EFE and the SAT circuit quicker (i didnt do that)..

Your advice for once again is needed!!Thanx!

[waltr]

1) Calculations are good and a 1.8k damping resistor indicates low C in the coil and cable.
Later you can try experimenting with other pulse widths but a 120us pulse will be good for many targets.
The important thing is not the allow the MOSFET to avalanche.

2) I would not put the damping R near the coil. Best place is inside the coil cable connector at the circuit housing. Then you can switch coils since the damping R is matched to each coil.
Any metal near the coil increases the decay in a PI detector. VLF detectors can get away with this if tuned with the metal near the coil.

3) Don't know so try it and let us know.
4) Lower C cable is good.
5) ok.

6) Experimenting with the timing is good. In theory the faster the pulse rate the faster the integrator output raises which means more sensitive.
Is the MOSFET getting warm now? If not then faster pulse rate will not be a problem, it just increases the pulse duty cycle which means a higher average current draw, shorter battery life. This probably will not be a problem if you get 6 hours instead of 7 hours. It is not hard to measure the average current draw with a DVM. Just put a 1 Ohm resistor is series with the battery and measure the Voltage drop which then equals the current.
I run my HH2 at 1500Hz with a 100us pulse, about 800mA peak, and the average current is 180mA. I get a good 5-6 hours with a 3-cell, 1800mA-Hr LiPo battery.

Construction looks good. Do hope those are temporary metal bolts on the coil mount.

Note: A lot of PI builds try going for the Fastest coil and shortest sampling delay. This is needed IF looking for small gold nuggets but not if looking for coins, jewelry, etc.
I typically run a delay of 15usec with bunch wound coils and find even small 2gm ear rings.

[green]

Calculations are good and a 1.8k damping resistor indicates low C in the coil and cable.

Wondering what the coil specs are. A 300uH coil would need 23pf total capacitance to critical damp with a 1.8k resistor. Possible with a short lead.

[mushaba]

390uH 1.4ohms.What do you mean by "short lead".Also how did you calculate the capacitance?

[green]

390uH 1.4ohms.What do you mean by "short lead".Also how did you calculate the capacitance?

For critical damping, Rd=(L/4C)^.5 C=L/4/Rd^2. C=390uH/4/1800^2=30pf. Lead can exceed 1pf/inch. A fast 390uH coil will probably exceed 10pf, less than 20inch lead if the circuit doesn't add any capacitance. A MUR460 diode in series with the mosfet reduces the circuit capacitance a lot but still adds a small capacitance for me.

Rd=pi*L*SRF You could remove Rd and the input resistor like Carl suggests and measure SRF with a scope as the coil decays after Tx off.

[mushaba]

1) Calculations are good and a 1.8k damping resistor indicates low C in the coil and cable.
Later you can try experimenting with other pulse widths but a 120us pulse will be good for many targets.
The important thing is not the allow the MOSFET to avalanche.

Hello waltr,i have the R-Limit 5 ohms in series with coil to control MOSFET not to go avalanche and that solution you gave me serves me very well!!!!I start to believe i have some problem in my board or my coil has low resistance because all others finished the MPP hadn't problem with MOSFET spikes.

2) I would not put the damping R near the coil. Best place is inside the coil cable connector at the circuit housing. Then you can switch coils since the damping R is matched to each coil.
Any metal near the coil increases the decay in a PI detector. VLF detectors can get away with this if tuned with the metal near the coil.

So PI fans are doomed to use non magnetic materials as chassis..Yes i plan to put the Rdamp resistor in the coil connector side.

Is the MOSFET getting warm now? If not then faster pulse rate will not be a problem, it just increases the pulse duty cycle which means a higher average current draw, shorter battery life. This probably will not be a problem if you get 6 hours instead of 7 hours. It is not hard to measure the average current draw with a DVM. Just put a 1 Ohm resistor is series with the battery and measure the Voltage drop which then equals the current.
I run my HH2 at 1500Hz with a 100us pulse, about 800mA peak, and the average current is 180mA. I get a good 5-6 hours with a 3-cell, 1800mA-Hr LiPo battery.

I can tell (by touching) that at this moment (2.5KHz) no heat on the irf..

Do hope those are temporary metal bolts on the coil mount.

Yes they are temporary bolts to hold parallel the plastic mounts until i glued the on the coil housing..

I typically run a delay of 15usec

OK another embarassing moment for me:what do you mean by i run a delay of 15us????I read all people mention this and i dont understand which delay is this..

[mushaba]

For critical damping, Rd=(L/4C)^.5 C=L/4/Rd^2. C=390uH/4/1800^2=30pf. Lead can exceed 1pf/inch. A fast 390uH coil will probably exceed 10pf, less than 20inch lead if the circuit doesn't add any capacitance. A MUR460 diode in series with the mosfet reduces the circuit capacitance a lot but still adds a small capacitance for me.

Thanks a lot for the math green!! the ^.5 means?? For example 2^.5= 1.41421356237 ???? (Please excuse me but math wasnt favorite when i was at school - that fool i was..)

Rd=pi*L*SRF You could remove Rd and the input resistor like Carl suggests and measure SRF with a scope as the coil decays after Tx off.

So you want me to remove the R Limit and measure CH1 coil hot side and CH2 TP2 ???[/QUOTE]

[waltr]

OK another embarassing moment for me:what do you mean by i run a delay of 15us????I read all people mention this and i dont understand which delay is this..

The time from TX pulse OFF to the start of the first sample is the delay. It is the 'first delay' and typically is user adjustable in the field and is the delay value always talked about.
Any other delays, times between sampling are typically fixed.

[godigit1]

[QUOTE=mushaba;256815]Thanks a lot for the math green!! the ^.5 means?? For example 2^.5= 1.41421356237 ???? (Please excuse me but math wasnt favorite when i was at school - that fool i was..)

Good question Mashuba.

Ive been afraid to ask. I know I should have all the prerequisite skills to do this so ive been trying to teach myself advanced math but understandind of the different ways symbols can be used is difficult. Best Ive been able to find out is it is the symbol for a exponential ^.
So I research that But how it fits into the math I dont know.
I try to reverse the math and to get 30 Pf i have to * by 568
Rd =L/4C =97.5
C=l/4/Rd=.0541
C= 390 /4 /1800=
so from here if times by 568 I get 30.7288Pf
Im not sure how ^.5 Or ^2 turns to 568

Green Im sorry for asking such basic questions but could you break down the basic math for us?
This has been a stumbling block for me and many other beginners here on the forum Im sure. maybe some think we shouldnt be here without the prerequisite skills. But we all want to learn new things and dont have enough life left to catch up to you guys.(stop learning start dying) But learning enough to function we are able play with and gain a better understanding of our favorite past time which is or should be why we are on a metal detecting forum in the first place.
Some of us may never really contribute to anything but cant blame us for trying.
Coils are why I came here in the first place. I was tired of drooling over every new coil for my Sd or TdI that cost in excess of $500.
I figured I can twist some wire.
Little does one know what a envolved science it is untel you undertake it.
Please

Mashuba Thanks for starting your thread Im sorry ive piggybacked on to asking questions but the information here is the missing link for me.
Best Regards