Damping dilemma

[lucifer]

If I understand correctly when use low R damping value, target receiving higher current stimulan in short period while high damp value will get higher flyback for longer stimulan periode but what target receives low current which is less deep. High damp value for shorter TC before start to ringing / vice versa. Wish I can explain better. To me, it's better with lower Q for coil.

Target stimulation does not depend on the flyback voltage. It depends on the current through the coil and how quickly it changes. Peak current through the coil doesn't really change with different values of damping resistor. Higher value damping resistor makes it possible to better detect targets with lower conductivity (including gold). A too low value of the damping resistor will make your detector find only ferrous objects.

[Op04]

Target stimulation does not depend on the flyback voltage. It depends on the current through the coil and how quickly it changes. Peak current through the coil doesn't really change with different values of damping resistor. Higher value damping resistor makes it possible to better detect targets with lower conductivity (including gold). A too low value of the damping resistor will make your detector find only ferrous objects.

I'm talking about the energy saved and spill out back from coil after turn off, yes I forgot to mention how fast it changing.

[lucifer]

I'm talking about the energy saved and spill out back from coil after turn off, yes I forgot to mention how fast it changing.

Right, a low value damping resistor will make the coil current discharge slower. It's better to discharge the coil as quickly as possible.

[Teleno]

Right, a low value damping resistor will make the coil current discharge slower. It's better to discharge the coil as quickly as possible.

The coil's rate of current discharge dI/dt determines the EMF induced in the target. The EMF generates a current in the target that takes three target time constants (tau) to reach 90% of its maximum possible value.

As a consequence, in order to maximize the response, the discharge needs to last for at least 3 target taus. This time and the rate of discharge determines the peak current at the Tx coil.

For example, at a discharge rate of 1A/us maximizing the response of a target with a tau of 4usec requires 3 x 4 = 12 us. Maintaining said discharge rate for 12usec would require a peak current of 1 A/us x 12 us = 12 A.

But if you're looking for low conductivity targets, say tau = 1us then your peak current would be 1A/us x 3us = 3A.

So depending on your maximum available peak current and the targets of your interest, it might be better to apply a slower discharge rate for a longer time than a faster rate for a shorter time.

Lots of compromise to be made.

[Tinkerer]

The coil's rate of current discharge dI/dt determines the EMF induced in the target. The EMF generates a current in the target that takes three target time constants (tau) to reach 90% of its maximum possible value.

As a consequence, in order to maximize the response, the discharge needs to last for at least 3 target taus. This time and the rate of discharge determines the peak current at the Tx coil.

For example, at a discharge rate of 1A/us maximizing the response of a target with a tau of 4usec requires 3 x 4 = 12 us. Maintaining said discharge rate for 12usec would require a peak current of 1 A/us x 12 us = 12 A.

But if you're looking for low conductivity targets, say tau = 1us then your peak current would be 1A/us x 3us = 3A.

So depending on your maximum available peak current and the targets of your interest, it might be better to apply a slower discharge rate for a longer time than a faster rate for a shorter time.

Lots of compromise to be made.

So, if for an AMX TX coil we have a rate of change of 2A in 3us = 0.66A/s at 50turns = 33 Ampere turns u/s?

[Carl-NC]

The coil's rate of current discharge dI/dt determines the EMF induced in the target. The EMF generates a current in the target that takes three target time constants (tau) to reach 90% of its maximum possible value.

As a consequence, in order to maximize the response, the discharge needs to last for at least 3 target taus. This time and the rate of discharge determines the peak current at the Tx coil.

For example, at a discharge rate of 1A/us maximizing the response of a target with a tau of 4usec requires 3 x 4 = 12 us. Maintaining said discharge rate for 12usec would require a peak current of 1 A/us x 12 us = 12 A.

But if you're looking for low conductivity targets, say tau = 1us then your peak current would be 1A/us x 3us = 3A.

So depending on your maximum available peak current and the targets of your interest, it might be better to apply a slower discharge rate for a longer time than a faster rate for a shorter time.

For a given peak current, a faster turn-off time will always win, though not by a whole lot. It is true that a 12A current turned off at 1 A/us will beat 1A turned off at 1A/us, but 12A turned off at 2A/us will be even better.
See this post for diagrams.

[Ism]

Right, a low value damping resistor will make the coil current discharge slower. It's better to discharge the coil as quickly as possible.

This comment I don't understand.
Isn't the damping resistor also acting to discharge the coil? Wouldn't a lower resistance discharge it faster?

[Carl-NC]

This comment I don't understand.
Isn't the damping resistor also acting to discharge the coil? Wouldn't a lower resistance discharge it faster?

The coil, its capacitance, and the damping resistor form an RLC circuit. You want this to be critically damped. A too-high resistor will make it ring, a too-low resistor will make it slow.

[bbsailor]

This comment I don't understand.
Isn't the damping resistor also acting to discharge the coil? Wouldn't a lower resistance discharge it faster?

The value of the damping resistor has a lot of influence on the coil delay or the time the coil must wait to fully damp the TX coil oscillations and turn on the RX circuit. Longer delays will typically miss low time constant small targets because most of small target induced stimulated currents fade away during the delay time. That is why fast coils attempt to reduce as much coil seen capacitance as possible by selecting coil wire type, wire insulation type, wire insulation thickness, low COSS MOSFET, low coil to shield capacitance, low coax capacitance to allow higher damping resistor values that create a lower coil delay.

Make sure that your damping resistor is not getting too hot as it’s value tends to drift higher in resistance value due to the heat.

Joseph J. Rogowski

[Ism]

Thanks Carl and Joeseph.

I was confounded why my Tektronix analog scope wasn't showing the waveform I see in videos about damping (10x probe).
While I was away from the forum, I was doing some more reading and testing.
I remembered I have a little pocket digital scope I bought about 5 years ago so I tried it on the coil (10x probe).

First pic is the waveform without any damping. I'm not sure why it doesn't show the ringing that my Tektronix scope does.
Second is the waveform with damping adjusted so the waveform is flat with the grid line. That is what we want to see right?
Over damp and the waveform dips under the grid line then comes back up.

If I add the shield and measure with the digital scope, the variable damping resistor has no effect on the waveform. (Scratching my head over that one.)
A 600 Ohm resistance gives me the waveform you see in pic #2. The coil with shield works nicely down to 10us on the TDI.

I am going to try adding a little more resistance to the damping (slightly under damped per your recommendation Carl) and see how the coil performs.
However I am encouraged with the improvement I see in the coil performance.