damping resistor

[baum7154]

Fascinating specs on the original CHANCE planar basket coil. If 36" of RG62u cable was used with it, the added capacitance would only be about 40.5 pf. I presume your coil is wound with enameled wire. When I look at all the parallel enameled wires in the original coil design it appears that there should be a lot of wire to wire capacitance on each face of the coil. The 4 mm thickness of the plexiglass does minimize capacitance between wires on opposite faces of the coil. Seeing all those close spaced 22 awg enameled wires it is also hard to believe the original coil has only 30 pf of capacitance even though that is how it calculates. Makes me wonder what dynamics are at work here.

I have a number of coils that I will run SRF on tonight, including the original design from CHANCE wound with 26 ga teflon insulated wire.

Thanks

Dan

[green]

Hope this can be interesting to someone, instead of simulation I made few measurements. Original Chance coil is used, spider design, nominal 400uH, measured 397uH, made from 0,7mm solid wire on 4mm plexiglass former. I tested SRF, (coil alone, and with cable and tween lead) and minimal sampling speed and this is what I get:

Coil alone SRF is very high, 1.46MHz, indicating only 29pF self capacitance. Connected to 1.8m RG 58 cable it drops to 595kHz, now total capacitance is 182pF. With 93pF\m cable capacitance should be 167pF, I measured it to 172pF using resonant LC meter, close enough, but simplified calculation, just adding coil self capacitance, require only138pF. With tween lead SRF is 835kHz or 92pF in total, but I measured cable alone to 72pF (this may be questionable due to nature of instrument used), 63pF is required with simple calc. Probe capacitance is very small (some 3.5pF x10 probe), I ignored it.


Now, even best part. This is connected to TX stage, working at 100uS pulse at 1kHz, with IRF740 switch, and observed using 2 stage preamp, known to be fast enough, tested well below 10uS before. Using RG58 cable and original 390R damping I get nicely damped waveform at around 17uS, but interestingly, increasing this value to over 1K waveform remained about same, at very high value (1020R) some very fast ringing appear. Without Rdamp coil ringdown is 310kHz (!) indicating unrealistic 660pF capacitance (!) Fast diode in circuit not changed waveform at all when added. Adding very small (ten's of pF or less) capacitance at TX end produce underdamped waveform and increase speed to less than 14uS. Same setup using tween lead cable produce maybe 1uS faster waveform, diode again failed to produce any effect, now ringdown without Rdamp is around 500kHz, indicating more realistic but again too high 250pF total circuit capacitance.


Conclusion: forget simulations, build it and measure! Number of factors are involved, coil is not simple lumped LC circuit but distributed parameter , cable too, TX stage behavior is highly nonlinear (simulation may account for this to some degree), coil own eddy current decay counts, material used and geometry...Not that simple, setting up complete model to account for all this is very messy business.

Interesting. Not what I expected, I did an experiment this morning. (Coil with twisted pair lead) tested on my coil tester, 220uh, 2.15 Mhz SRF. Connected to PI driver circuit (IRF740 switch, Rd disconnected, 1k resistor with diodes across coil, 390 khz SRF. With MUR460 diode in series with coil, 1.58 Mhz SRF. With 1N4937 diode in series, 1.58 Mhz SRF. I had thought the 1N4937 didn't work as good, but the test showed no difference. Based on the test I would expect a lot faster response with the diodes.

[baum7154]

Just ran SRF on 4 coils

#1. 8" 335uh spider coil with 33" feed of twisted pair continuation of coil wire. Wire is 600 Volt PTFE insulation 24 AWG 19 strand twisted silver plated wound on .093" thick polycarbonate toroidal form. No shield
SRF =1.056 mhz.
NOTE: I was mistaken that this coil was wound with 26 AWG until I put a micrometer on it tonight...It is really 24 AWG wire. I'm sure 26 AWG would make it even faster.

#2. 8" 371uh Original Chance coil design with 48" RG-62B/U coax (13.5 pf/ft). Wire is 23 AWG solid copper with PTFE insulation on .093" polycarbonate form. No shield
SRF =1.065 mhz

#3. 8" 378uh Original Chance coil design with 35" continuation of twisted coil wire as feed. Wire is 26 AWG 7 strand twisted silver plated PTFE insulation on a .055" thick polyethylene form with No shield
SRF =1.251 mhz

#4. 8" 410uh spider coil with 34" of RG-62B/U. Wire is 26AWG enameled copper wire wound on a .093" thick polycarbonate toroidal coil form. Shielding is varnish/graphite applied at 1300 to 1500 ohms per inch.
SRF =825 khz

Dan

[baum7154]

the coil/feed capacitance calculated from the SRF above are:

#1 68pf,
#2 60pf
#3 43pf
#4 91pf

Removing the calculated feedline capacitance based on 13.5pf/foot of RG-62B/U and 1 pf/foot for PTFE insulated twisted pair gives the following coil capacitance:

#1 35pf
#2 6pf
#3 8pf
#4 52.75pf

#2 & #3 are the Original Chance coil design. It is hard to believe the capacitance numbers for the coils calculated without the feedlines. I'll run SRF on the #3 coil tonight with the feedline removed.

Dan

[baum7154]

Removed feedline from #3 coil last night. Twisted feedline (26 AWG PTFE insulation)measures 31pf with my LC meter. Coil w/o feed measures 368.8 uh with LC meter. Did SRF on coil only - 2.435 mhz. Coil capacitance calculates from this to be 12 pf. Adding the feedline capacitance back into the calc gives SRF @1.263, within reasonable margin of error. Conclusion is that this form of coil with this wire is about 12pf.

[baum7154]

the coil/feed capacitance calculated from the SRF above are:

#1 68pf,
#2 60pf
#3 43pf
#4 91pf

Removing the calculated feedline capacitance based on 13.5pf/foot of RG-62B/U and 1 pf/foot for PTFE insulated twisted pair gives the following coil capacitance:

#1 35pf
#2 6pf
#3 8pf
#4 52.75pf

#2 & #3 are the Original Chance coil design. It is hard to believe the capacitance numbers for the coils calculated without the feedlines. I'll run SRF on the #3 coil tonight with the feedline removed.

Dan

OOPS! NOT 1pf per foot...Correct is 1pf per INCH of twisted pair...sorry for the goof!!

[baum7154]

Ran coil #3 (an original CHANCE pattern coil) this AM on my CHANCE PI and with an adjustable network set at 1360 ohms this coil would run at a Guard Interval setting of 11, but never at the minimum of 10. Had to unwind a turn off the coil (now it is 368 uh) to get the solder joints out of the coil in order to get it to run at 11. Discrimination and sensitivity were not good. Large Gold Ring was represented at about 75% up the scale. This coil definitely needs shielding at the low GI settings.

Decided to put the prescribed 390 ohm damping resistor in and see how it behaved. The lowest Guard Interval it would run at was 18. At this setting it would not see 8.8 grain dental gold. It does see a 10.8 grain piece of dental gold at 1.5". Discrimination on a 6mm wide men's 10K wedding band is odd. Face on to the coil it displays 75% up the scale, but edge it is solidly on the left end as Gold. Detection distance on the ring face on is about 9" and on edge to the coil is about 7"

[baum7154]

I'm still trying to learn. Would appreciate being corrected where I'm wrong. Rd for critical damping doesn't change with stored energy. With spice I see a small increase in time to damp to the same volt level if I double the current. The hardware circuit I'm using clamps the fly back at 450 volts. Increasing the current increases the clamp time which adds to the total time, with the decay time staying the same. Maybe other circuits act different. Including spice simulation showing effect of changing Rd for critical damping and under damped. The first plot the amplifier Fc is above coil resonance which measured 900 khz with Rd disconnected. Rd for critical damping equals pi * l * Fr = 850 ohms. The second plot I tried to simulate the CHANCE amplifier. Divided OP37 BW by gain of 470 = 170 khz. Shunted 470k fdbk resistor with a capacitor for same response. Maybe not correct. Playing with the circuits in spice has been fun. Making changes and measurements is a lot quicker. Don't know how good spice works for non inverting input simulation.

I think the BW of the OP37 is 63mHz. So if you divide it by the 470 gain the result is 134kHz. Does that sound right?

[baum7154]

I am seeing the adjustable resistor network in my CHANCE PI drift in resistance with heating of the damping resistors in the network. Using an IR temp sensor the heating appears to be about 10 degrees Farenheit. Has any one else run into this and if so how is it addressed? I'm thinking of using better resistors/pot in the network.

[bbsailor]

Baum7154,

Try using a metal film resistor that is about 5 Watts and is about 5 to 10 percent higher in value than the optimum critical damping value. Then use a variabe pot in series with a resistor that combined is 10 times higher in value to fine tune the damping resistor value. The series resistor is there to limit the current going through the pot so as not to have a arc at the sliding pot contact.


bbsailor

[baum7154]

Thanks Joe,

The current network I use is a 3900 ohm /1 watt in series with a 5k /1/2 watt pot. This pair is in parallel with a 2 watt 1270 ohm resistor. This network gives a resistance range of 958 ohms to 1112 ohms. While this gives a nice,less touchy, tuning range it appears from your comments that too much current may be flowing in the series leg with the pot. At the midpoint of the pot the ratio of current distribution is 5:1 for the main resistor vs the pot/resistor in parallel.

The drift that I saw seemed to be related to thermal rise of some of the resistors that naturally occurs in damping the coil. Maybe I'll have to abandon the goal of easy tuning adjustment in favor of more robust component performance.

[satdaveuk]

Hi
Maybe some of you are a little over caution with regards using pot/pre set for damping adjustments
on both Barracuda builds I used 5k multi turn pot series with half watt high stab 560ohm resistor parallel with a 2w high stab 1.2k which gives between 382ohms-987ohms damping drops in at 466ohms with all coils on both machines.
I personally leave the pre sets in once fitted, everything runs cold really cant see a problem, did the same procedure back when doing the mini pulse 3 only difference was the pot was mounted on the front display panel together with ISP input to save taking it apart all the time .
both first build barracuda and mini pulse 3 which deals with higher voltages have been flogged to death with no problem , just make sure as said earlier on here the bulk of the power is not getting dissipated through the pot/pre set and also bare in mind the bulk of the voltages are dissipated through the search coil so don't worry
Regards

[baum7154]

I have rebuilt the network with much better and higher power components and have had no problems at all. In fact the adjustment knob on the back of the case allow me to optimize damping over wet or highly mineralized soil.

Regards,

Dan

[6666]

Dan just for interest have you measured the variation in the value of the damping resister from soil to soil ?

[baum7154]

Yes I have done some rudimentary comparisons of damping resistor settings for different/wet soils. My results are at home and I'll try to post some info later.

From what I remember my calculations comparing open air damping vs damping near wet soil seemed to indicate a nominal 10pf capacitance increase (subject to how far from the soil the coil was held) with a corresponding change in damping resistor using a 335uh 8" fast coil.

Regards,

Dan