coil transmit booster question

[Davor]

Actually ZL equals a square root of a sum of RL squared and XL squared
For high Q it turns to be ~ XL anyway

The biggest point with inductance vs NI is that inductance rises with a square of turns, while NI is obviously rising in linear fashion, and therefore with the same applied voltage you get less NI with coils with more turns, but your Q factor improves.

Many oscillators won't start with coils that have Q<10 and even if they do the waveform is not nice.

[golfnut]

Its perhaps an over simplification, but at resonance there is no XL only residual coil R. Assuming there is no dielectric losses in the cap ( tan delta )



So 8.5V/4R = about 2Amps RMS ??



Struggling to pull basics from the 80's... S

[Davor]

No.
In a parallel tank the coil current is always limited by ZL.

[Qiaozhi]

Actually ZL equals a square root of a sum of RL squared and XL squared

I bet you just want me to re-hack the Latex code.

Actually you're correct. I was just blindly following Carl's equation without thinking. That probably also accounts for a small discrepancy I had with the LTSpice simulation. Will check later.

[Carl-NC]

Properly speaking, ZL = RL + jXL, and mag(ZL) = sqrt(RL^2 + XL^2).

[Qiaozhi]

Correcting the schoolboy error I made earlier:

The general equation is:

and the magnitude is:

The results then follow as:

(approximately equal to the inductive reactance for a high-Q coil)





(power available to create the magnetic field)

(power lost as heat in the dc resistance of the wire)

From an LTSpice simulation, the results are:





(power available to create the magnetic field)

(power lost as heat in the dc resistance of the wire)

Pretty damn close!

[Carl-NC]

It's worth noting that the Pmag component isn't really "power" as ideal reactance doesn't dissipate power. If it's a good design, all that "power" gets dumped onto a cap to be recycled over & over. That's the funny thing about induction transmitters I was alluding to earlier... an ideal coil transmits zero power, so it doesn't make much sense to talk about "transmit power". Obviously if a metal target couples in, then there is power transfer to create the eddy currents, but in general to have a metal detector with a high transmit power you need to design a bad transmitter.

[Qiaozhi]

It's worth noting that the Pmag component isn't really "power" as ideal reactance doesn't dissipate power. If it's a good design, all that "power" gets dumped onto a cap to be recycled over & over. That's the funny thing about induction transmitters I was alluding to earlier... an ideal coil transmits zero power, so it doesn't make much sense to talk about "transmit power". Obviously if a metal target couples in, then there is power transfer to create the eddy currents, but in general to have a metal detector with a high transmit power you need to design a bad transmitter.

Another way of explaining this issue of reactive versus resistive loads, is that power in a resistive load is always positive, resulting in power dissipation in a [real] resistance. But for a purely [imaginary] reactive load, the power swings between positive and negative values, which causes the power to alternate between being sent to the reactive load and being returned to the source, with the result that zero power is dissipated. When you have a load (such as a metal detector coil) that contains both reactive and resistive components, only the coil resistance dissipates power.

I suppose it's actually more correct to say that Pmag is the power [potentially] available for transfer to the target, in the same way as a transformer transfers power from its primary to the secondary windings. The [small] amount of power involved initially establishing the magnetic field around the coil is bundled in with the resistive losses. Since Pmag is really potential energy, then it's just waiting to be used. When no metal targets are present, there is no power transfer.

Bringing a metal object close to the coil will then cause an increase in coil current as power transfer takes place through flux linking with the target, and eddy current generation takes place.

Once you can get your head around it, everything becomes clear.

[golfnut]

oversight there, we were not talking of a resonant case.



just a coil, yes the R is the bottle neck.



Silver plated wire can help, most military equipment uses it to keep the losses down. I used to work at Marconi and the military and most civilian avionic kit had decent silver plated wire..

I never saw enamelled silver-plated copper, but either silver plated thick wire for RF inductors, some plated tube rather than solid. Or ptfe covered multi strand silver plated.

[Skippy]

Quote:"Silver plated wire can help"
...but it's not necessary, as you could just increase the wire diameter by about 3%, giving you 6% more cross-sectional area, approx the resistivity difference between silver and copper.

[muddy fingers]

Most of this thread, especially the maths bit, unfortunately is way above my head, but has what's been said point to the fact that detector manufacturers that claim to have a "boost" switch that gets more depth just using a misleading marketing ploy? If it is not extra power going to the coil to achieve extra depth then what is being done to achieve it? Is it a slight change in frequency? I know that lower frequencies are better for detecting deeper targets. Otherwise, is it just a higher Gain setting (an op amp on the circuit) that can be momentarily flicked on when checking a target to give a louder amplified signal which the operator is led to believe is caused by the fact that his detector can now go deeper with it's signals.

[Silver Dollar]

I know some of the "boost" switches allow a lower threshold so you can hear deeper targets.
The machine was always hearing them but not letting you hear.

[muddy fingers]

Thanks for your reply Silver Dollar. I do remember reading about this before somewhere- so the lower threshold causes a bigger spike (difference) between it's level and the signal level (which remains the same) meaning it can now be more easily heard? I'll have to read up on this but thanks again.

[muddy fingers]

I found an explanation from 01/07/ 2011 title: Doc's audio amp The whole thread, "Metal Detector Coil Booster" was interesting to me. I think all these "booster" gadgets were suitable for the Minelabs as they have their meters positioned mid- coil cable which is unusual for a metal detector.