damping resistor

[green]

(Ok, Mr. green has found the same formula! Unfortunately, no free lunch!)

I find it easier to measure resonance with the damping resistor disconnected than to measure coil resonance, calculate coil capacitance, add cable, driver and shield capacitance to calculate Rd. It's another way to do it, maybe adjusting Rd while monitoring amplifier out is a better way. Rd/(pi L) should equal resonant frequency. Example 400/(pi*300)= .424 Mhz.

[Aziz]

(Ok, Mr. green has found the same formula! Unfortunately, no free lunch!)

I find it easier to measure resonance with the damping resistor disconnected than to measure coil resonance, calculate coil capacitance, add cable, driver and shield capacitance to calculate Rd. It's another way to do it, maybe adjusting Rd while monitoring amplifier out is a better way. Rd/(pi L) should equal resonant frequency. Example 400/(pi*300)= .424 Mhz.

No doubt green! Your simple formula is more practical. The math proof shows, that your formula is correct. Good job!

Cheers,
Aziz

[Qiaozhi]

Interesting that without oscilloscope you can audibly adjust damping resistor to get that particular target from maximum possible distance and be proud with results.
Then get the oscilloscope and see how deeply your coil under dumped
Obvious next step is to reduce damping to get that decay curve to proper shape by the book and loose depth for sure.

Because of my laziness I got stuck is this situation several times.

I think some day some brave explorer will have that methods of using partially dumped coils developed.

You already explored it. Somewhat underdamped coil will work best with almost any classic analog detector. Leaving waveform to bounce in positive direction somewhat, some 20-30% of amplifier output swing (may depend on coil too) will actually prevent early integrator saturation, this positive part will be integrated too, so faster sampling can be used, providing more depth for some objects. With perfect waveform adjusted using scope, integrator will saturate earlier, delay must be increased, you will lose some depth. Cut and try, as usual...

You are both correct. If you slightly underdamp the coil, then sensitivity is improved. I use the scope to get the critically damped value, then do the final adjustment using whatever typical target the detector is designed to detect.

During my investigation and back-engineering of the Pulse Technology Minipulse (see Projects section) I noted that the coil is underdamped. So this technique has been known about from at least the 1980s.

[greylourie]

Is there any simple way we could do this via a control interface, so we could adjust when we change coil or decide to search for a particular target ? Im sure I read somewhere that using a variable resistor is not the best solution, the contact between track and leaf spring contact is susceptible to damage.... and having wire from the front out to a chassis mounted V/R doesn't look good either. Is there a simple solution ?

[golfnut]

The simplest and most likely to succeed are plain mechanical switches which can add or subtract R,s to tune in the field.

[Tinkerer]

Is there any simple way we could do this via a control interface, so we could adjust when we change coil or decide to search for a particular target ? Im sure I read somewhere that using a variable resistor is not the best solution, the contact between track and leaf spring contact is susceptible to damage.... and having wire from the front out to a chassis mounted V/R doesn't look good either. Is there a simple solution ?

There is a patent, quite old if I remember, for controlling the damping with a mosfet.
Recently I accidently made a PI coil with nearly 1200uH inductance, when I used bifilar wind and forgot to reduce the turns by half. Of course I noticed it when I run the TX and wanted to adjust the damping resistor. The high inductance produced a very strong response, so I decided to take advantage and do some testing.
Being able to see a target response with none or minimal amplification, showed that underdamping produces a dead spot, or at least reduces the target response at the time of the residual coil oscillation. Bad for minimal TC targets.

[Waikiki_Sweep]

Also I suspect that underdumped coil moves close to dumped conditions in presence of target because of lowering Q and inductance. That change in dumping has a strong influence on shape of signal making detector working as "off resonance" and pulse induction in one.

Also controlling dumping should be a good idea. Why not to dump fast that high voltage spike than use slower dumping in millivolts range having faster coil decay and high sensitivity to get that small gold?

[Qiaozhi]

Im sure I read somewhere that using a variable resistor is not the best solution, the contact between track and leaf spring contact is susceptible to damage.... and having wire from the front out to a chassis mounted V/R doesn't look good either. Is there a simple solution ?

The variable resistor is not supposed to be a permanent solution. You only use this to determine the correct value of damping resistor, and then you're supposed to replace it with a fixed value resistor of the nearest equivalent value.

Assuming you're only going to build coils for a particular detector, you could include the damping resistor in the coil shell (or in the connector, if there's room). Then changing coils is not a problem.

[Qiaozhi]

By the way, the resistor across the coils is called a "damping" resistor, not a "dumping" resistor.
It's called that because of its function ... to critically damp the coil.

[Tepco]

Dumping is not wrong either, used to “dump” energy from coil.

[green]

For mono coils I've bee using an inverting amplifier with the input resistor being the damping resistor. For induction balance coils an instrument amplifier with the coil across the inputs and 1/2 Rd to ground on each input. Both amplifiers have a gain of 140 and a Fc over 1Mhz. Critical damping seems to work best. After reading some of the replies about under damping I tried the normal non inverting input in LT spice. Under damped is faster. Changing amplifier Fc effects the best damping value maybe up to twice the critical value. The Mur460 diode in series with the coil helps with the circuits I've been using. Using spice and the normal non inverting circuit shorting the diode out caused the resonance to drop from 1Mhz to 500khz, but I think by adjusting Rd and amplifier Fc I could get a similar response. Need to try on the bench. My goal while learning has been to sample as fast as possible. Think I can sample faster but I'm wondering if the high Fc isn't causing more problems than it's worth. I think what I posted about damping resistor is valid for finding Rd for critical damping but worthless for a normal non inverting PI input. Thanks for the replies.

[greylourie]

The variable resistor is not supposed to be a permanent solution. You only use this to determine the correct value of damping resistor, and then you're supposed to replace it with a fixed value resistor of the nearest equivalent value.

Assuming you're only going to build coils for a particular detector, you could include the damping resistor in the coil shell (or in the connector, if there's room). Then changing coils is not a problem.

Sorry Qiaozhi, I'm probably getting myself all confused. But for some reason I thought that having some variable or adaptive control over damping would help to counter some of the effect that variable ground has ?

[baum7154]

You are both correct. If you slightly underdamp the coil, then sensitivity is improved. I use the scope to get the critically damped value, then do the final adjustment using whatever typical target the detector is designed to detect.

During my investigation and back-engineering of the Pulse Technology Minipulse (see Projects section) I noted that the coil is underdamped. So this technique has been known about from at least the 1980s.

When I see the comment "If you slightly under-damp the coil, then sensitivity is improved" is there a percentage of the damping resistance that is typical to run in the under-damped condition? For example if a coil requires 1000 ohms for critical damping to be met then is the under-damped condition perhaps 10% more than 1000 ohms or 1100 ohms? Or is it more like 2% or 1020 ohms? Anyone have an idea on this? I know it probably depends on individual coils but I'm just trying to get an idea of how much more resistance under-damping might be.

Regards,

Dan

[Tepco]

Say about 10% but can vary, must be adjusted from case to case

[baum7154]

Good info TEPCO! The reason I asked is that I re-ran the damping on my CHANCE PI coil this weekend. Originally the detector threshold of operation ran with a 1110 ohm resistor on the coil (Guard Interval set at 10). What I noticed this weekend is that in using the potentiometer/network to optimize I was able to go to a point where the detector displayed what I thought was coil ringing as a signal strength indication. By carefully increasing the resistance further I was able to reduce the signal strength indication to zero. Testing on a 1cm aluminum foil target showed good sensitivity that was not improved by any more damping resistance. The final measured network resistance was 1260 ohms and fixed damping resistance of that value was installed . The detector now runs very quietly on the ground and has the best sensitivity yet.

Thanks,
Dan