PI SAMPLE DELAY QUESTION

[baum7154]

For calculation of the dumping resistor Dan´s (Baum7154) variant is correct. Attached is a picture with the calculated 324 Ohm dumping resistor.

But for quick finding if a coil is a good one, i think Joseph´s (bbsailor) variant is the best.

So we should meassure some coil´s in both variants to see a ratio from Joseph´s to Dan´s variant to find quick (and dirty) the right dumping resistor.

The ratio for my 10" Coil is: 625 kHz : 377 kHz = 1,78

So for calculation the dumping with joseph´s variant: 625:1,78 x PI x 295uH coil = 325 Ohm (Dan´s variant: 377 kHz x PI x 295uH = 349 Ohm)
I have to test it with other coil´s, but think it is close enough.

Had one big mistake in my conclusion. The ratio depending on how you built your pi. So this factor is only right for my pi. So we should use joseph´s variant to compare coils without the factor of different pi-builds.

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In my original calculation based on your scope traces I used 350 kHz not 377 kHz, and the calculated result was 324 ohms. The ratio between 625 and 350kHz =1.78. If I multiply the 324 damping resistance by the ratio it is 579 ohms. Calculating the 'coil only' resistance PI X .625 X 295 = 579 ohms. The difference is the effect of the capacitance added by your detector electronics. The 'coil only' or 'coil with feed line' self resonant frequency is a good indicator of coil speed but as Joe points out that speed is significantly diminished by the added capacitance of the detector electronics. Therefore the damping resistance must be determined with the coil in circuit.

Did you try the variable resistance network in place of your damping resistor in the detector? In my opinion that is the final word because it optimizes damping with all variables of the detector system in play. It will be interesting to see how different the optimized resistor value is from the original 390 ohm damper.

Best regards,

Dan

[deemon]

deemon - What value resistor do you have in series with the coil?

There are 2 resistors of 4,7 Ohm in series , plus 1,6 Ohm of the coil wire .... so I am limiting the current at 2,2 A approx.

[Qiaozhi]

There are 2 resistors of 4,7 Ohm in series , plus 1,6 Ohm of the coil wire .... so I am limiting the current at 2,2 A approx.

I thought you were using a 24V supply (see post #143). In which case your coil current should be limited to 3.8A.

[baum7154]

Hello Joseph!

Thanks for this great tip. I have meassured my coil now with your instruction.
Now i get 1.6 uS~625 kHz.
Scope-Settings are 50V/Div and 1uS/Div.
I will test some other coil´s to see differences with shield and without.

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mschmahl, Does your surf PI use an IRF740 power mosfet in the transmit stage? If so you may be able to reduce the effect of the mosfet COSS by adding a series HER208 or similar diode between the mosfet and the coil with the anode toward the coil. It will put the capacitance of the diode (30pf) in series with the capacitance of the IRF740 mosfet (206pf) and result in a reduced capacitance (87% or 26pf total) which will help to speed up your coil a lot. Just a thought.

Dan

[deemon]

I thought you were using a 24V supply (see post #143). In which case your coil current should be limited to 3.8A.

I use 24 V , and 2 resistors 4,7 Ohm in series gives 9,4 Ohm , and 1,6 Ohm coil wire , so I have 2*4,7+1,6=11 Ohm overall resistance . With 24 V power supply my current is 24/11=2,18 A , exactly what we see on the scope - because vertical sensitivity was 0,1 V/div ( in my circuit it's equivalent to 1 A/div ) . I used 0,1 Ohm current sense resistor , connected to +24 rail , and the ground clip of the scope probe connected to +24 too , this is why the current looks negative on the upper trace .

[Qiaozhi]

I use 24 V , and 2 resistors 4,7 Ohm in series gives 9,4 Ohm , and 1,6 Ohm coil wire , so I have 2*4,7+1,6=11 Ohm overall resistance . With 24 V power supply my current is 24/11=2,18 A , exactly what we see on the scope - because vertical sensitivity was 0,1 V/div ( in my circuit it's equivalent to 1 A/div ) . I used 0,1 Ohm current sense resistor , connected to +24 rail , and the ground clip of the scope probe connected to +24 too , this is why the current looks negative on the upper trace .

Thanks, now I understand. I thought you meant the two resistors were a 4R7 in series with the coil resistance 1R6.
Now the results make sense. More later .....

[green]

Recorded some data with a DD coil. Coil TC 5.2usec, target TC 57usec. The excel chart shows the same TC decaying during the coil on and off times. Had to absolute value to plot log so you need to look at the third picture second row of the scope pictures for polarity. Other targets might show different results. I plan on looking at a couple targets. If anyone sees something I could do different or have a target suggestion I would try. I think it looks similar to the LT spice simulation except the signal goes positive the first few usec and then goes negative, opposite the coil off decay signal when the coil current flattens out.

[Qiaozhi]

I use 24 V , and 2 resistors 4,7 Ohm in series gives 9,4 Ohm , and 1,6 Ohm coil wire , so I have 2*4,7+1,6=11 Ohm overall resistance . With 24 V power supply my current is 24/11=2,18 A , exactly what we see on the scope - because vertical sensitivity was 0,1 V/div ( in my circuit it's equivalent to 1 A/div ) . I used 0,1 Ohm current sense resistor , connected to +24 rail , and the ground clip of the scope probe connected to +24 too , this is why the current looks negative on the upper trace .

deemon - Can you explain what's happening with the receive signal? Previously you mentioned something about a transistor being used to clamp the coil after switch-off. I'd like to understand what's going on there, as it's difficult to interpret the RX images in the scope shots.

So far I can calculate the tau of your coil charging circuit as 38.18us, which means that at 200us the coil is charged to 99.47%, and at 300us it's 99.96%. This is only an increase of 0.49%, which doesn't account for the ~10% increase we can see in the RX signal. The tau during discharge is much faster at 1.07us, which explains why the current drops so quickly at switch-off.

If the ~10% increase was truly due to a target charging effect, the only postulate I can come up with is that the copper plate becomes aligned to the [static] magnetic field of the coil, and the collapse of this [target] field might enhance detection. However, if this was the case, then a 0.49% increase in coil current would still not give a 10% increase in target response. So (IMHO) there must be some other explanation. That's why I need to understand how your coil clamping works, as this could simply be a side-effect of the operation of this circuit.

[deemon]

deemon - Can you explain what's happening with the receive signal? Previously you mentioned something about a transistor being used to clamp the coil after switch-off. I'd like to understand what's going on there, as it's difficult to interpret the RX images in the scope shots.

There are only 2 transistors in this front-end switch - one for negative spike suppression , and another for positive ... it's better to draw the circuit . Tomorrow I'll do .

[Tinkerer]

Both TC's need to be considered. The TC of the coil and the TC of the target. In the case of the solid copper ashtray, the TC is probably 1000us or more.
When the Coil current has reached the flat top, the eddy currents in the ashtray keep increasing until the TX time has reached 5 TC's of the target.

After 5 TC's the target eddy currents start decaying because there is no more di/dt within the target. The decay will take another 5 TC's to reach near zero.

If you switch the TX OFF before that time, the di/dt of the switch OFF transient first has to kill the eddy currents in the target before it can build the new eddy currents which are of the opposite polarity, thus reducing the target response.

[mschmahl]

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Did you try the variable resistance network in place of your damping resistor in the detector? In my opinion that is the final word because it optimizes damping with all variables of the detector system in play. It will be interesting to see how different the optimized resistor value is from the original 390 ohm damper.

Best regards,

Dan

I try today with Resistance network and get 334 Ohm. So you were very close.
My next try is the HER208, how you suggested. The PI is no Surf-PI. It is Mirage-PI, a variant of Hammerhead from Smitty and Sven.

[porkluvr]

Both TC's need to be considered. The TC of the coil and the TC of the target. In the case of the solid copper ashtray, the TC is probably 1000us or more.
When the Coil current has reached the flat top, the eddy currents in the ashtray keep increasing until the TX time has reached 5 TC's of the target.

After 5 TC's the target eddy currents start decaying because there is no more di/dt within the target. The decay will take another 5 TC's to reach near zero.

If you switch the TX OFF before that time, the di/dt of the switch OFF transient first has to kill the eddy currents in the target before it can build the new eddy currents which are of the opposite polarity, thus reducing the target response.

Thanks, Tinkerer. That is a very good explanation of an obscure phenomena.

[baum7154]

I try today with Resistance network and get 334 Ohm. So you were very close.
My next try is the HER208, how you suggested. The PI is no Surf-PI. It is Mirage-PI, a variant of Hammerhead from Smitty and Sven.

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with the HER208 in circuit it will be very interesting to see how much more resistance it will take to get critical damping!
Good Luck!

Dan

[mschmahl]

With HER208 i get 12 cycles/20uS on Pin 6 from the NE5534 without damping resistor. So I have around 600kHz. Dampingresistor with your formula should be 556 Ohm. Critical dumping with network is 761 Ohm. One good sideeffect from the HER208 is that the curve on Pin 6 /5534 is cleaner now, finding a clear Frequencysetting of the mirage is now easier.

[deemon]

deemon - Can you explain what's happening with the receive signal? Previously you mentioned something about a transistor being used to clamp the coil after switch-off. I'd like to understand what's going on there, as it's difficult to interpret the RX images in the scope shots.

Here is the circuit - it's primitive . As I said before , here are 2 switches - Q2 shunts the output wire to the +24 V rail during the ON pulse ( negative ) , and Q3 does the same with the positive exponential flyback pulse . Duration of the flyback blanking interval I can set by the VR1 resistor , in order to allow signal transmission when our flyback pulse has been dissipated to near zero - so I can set the maximum scope sensitivity without input overload .