Damper Resistors Calculate!

[albert]

Hello to all ~
one question : is there anybody that explain to me about" Damper Resistors " that situated in parallel to Coil and how we can calculate their ?
i have a problem to accurate calculate them and lead to decrease my Metal detector depth.

Many Thanks for your guide

[mikebg]

Albert, dont calculate the dumping resistor! I published in the forum a correct formula, but it is useless because needs to know equivalent stray capacitance of TX coil network. You need an oscilloscope and a potentiometer 470 ohm 1W to find the correct value.

[WM6]

Hello to all ~
one question : is there anybody that explain to me about" Damper Resistors " that situated in parallel to Coil and how we can calculate their ?
i have a problem to accurate calculate them and lead to decrease my Metal detector depth.

Many Thanks for your guide

Way better than calculate (cause of many parameters to take in account) is to observe signals on scope using pot as suggest mikebg. Read about this topic in forums. In short: reverse induced signal have to fall as quick as possible and by most settled line to the bottom. Once you reach this your damper resistor is set proper and you can replace pot by fixed resistor.

[albert]

Damper Resistor

Many Thanks ~
you mean i should reduce the discharge time ?
please explain briefly about this issue and discharge time.

[Carl-NC]

To achieve the earliest sampling, you want the flyback decay to be as fast as possible. The best way to find the right damping resistor for this is with an oscope. There have been lots of discussions about this in the Tech forum.

[authere]

Dampening Resistor

Hi All,
As it has been pointed out to me before, that is, most coils need a different D/R so why do we persist with a fixed D/R to this day.? Woody has shown us how to fix the problem on minelab machines, it gives the user a min and max res. thru using fixed resistors at the lower end and a pot with a built in res. for the max., someone else might want to fully explain, but it doesn't make sense to have fixed resistor on a machine built to take many coils.
So how about it wizz kids, some people weren't happy with Woody's design and Woody has already mentioned there is other ways to go about it.
Don't knock it until you have tried it, mostly you will get more distance and obviously be more sensitive to smaller nuggets, isn't that what we are trying to achieve.
Regards Ron

[Carl-NC]

If you are trying to achieve exact critical damping then, yes, different coils will require different damping resistances. But all you really need to do is ensure the preamp output is linear at the earliest desired sampling point. It really doesn't matter if it is overdamped or underdamped. So there is room for a fair amount of variation without affecting performance.

If you find yourself wrestling with the damping R in order to squeeze out a slightly earlier sampling time, then I suggest it's time to re-visit the choice of loop inductance.

- Carl

[authere]

Damping Resistors

Hi Carl,s
I don't know this person you speak of, LOOP INDUCTION, in the meentime i will google and i hope someone can explain this phenomenom because i am electronically eliterate.
It's a bit like being directionally dislexic as is my missus.
Regards Ron

[Mechanic]

G'day Ron,

Loop = coil,

Induction = inductance

I'm sure you know where to go from here....

Mick

[authere]

Loop Inductance

Hi All,
Gee i wish you bloody yanks would speak english!!!

[authere]

coil dampening

Hi Carl,
Hope i didn't offend!!,but when i was playing around with the dampening of a coil it made a lot of difference to not only small nuggets but all sizes, as all depths increased, the overall sensitivity of the coil was increased and to top it off i found that every coil was different and needed that tweek.So why don't they incorporate this into all detectors?

If you wanna play wiff me(loop induction?) fair enough but i am only trying to learn about this mystery you's call electronics, normally one puts up a smilie when one is playing with someones intellect.

Besides all this the HH are only learning dummies as they don't work in the real world as a metal detector as such

Regards Ron

[Carl-NC]

"loop" == "coil", so I meant coil inductance. It's been a long time since I've done experiments with damping, but my recollection is that it doesn't have to be exact if the preamp is far enough away from saturation. If you get a lot of variation, look at the preamp output and see what it is doing.

- Carl

[authere]

Coil dampening

Hi Carl,
This all started when i mentioned that when i attached my coax i found that there was a specific length that made the coil function better(more responsive) when hooked up to my ...inelab machine.
Then Joe(BB Sailor) says to try and fit a variable resistor instead of the fixed one used for damping and it went from there to Woody coming up with a practicle solution to the problem with photos i might add,It all prooved the point that i had made in the first place that there needs to be adjustability for every coil we use.
Now you have the theory and i have the practicality of it all, so what can be done about it, i have seen a patent for the automatic adjustment of the dampening resistor is this what is holding people back or am i missing the point.
Ron
PS:Looking at the pre-amp output would only show the fact that the coil has or hasn't reached its peak voltage which is exactly what i'm trying to say, it hasn't with 99% of coils.

[Carl-NC]

Because of the high voltage flyback, a potentiometer is not practical as it is susceptible to arcing. You could use a MOSFET per the patent, but that is patented. And really, you don't need adjustability, but rather custom damping per coil. A method that has been suggested before is to put most of the damping in the detector, and a "tweak" amount in the loop that compensates for loop-to-loop variation.

[authere]

coil dampening

Hi Carl,
I'm new to electronics and i respect your views on things as i am obviously a beginner, but while i'm on a roll,the pot if it was adjusted whilst having a large current going thru it would certainly cause the "brushes to burn" but as they are hooked up to resistors that set a maximum and minimum, i can't see that being a problem, but would like to be enlightened!!

With puting a resistor in each coil to "peak" dampen each coil gets thrown out the window when you have several detectors that these coils are used on, as i recall Joe mentions of internals that have a bearing on the overall dampening of a coil and its curcuitry.

So i beg to differ on the fact that each coil needs to be dampened to fit a machine ,it's the machine needing adjustability to take any coil that is fitted to it.

You mentioned that a Mosfet could be used, i am all ears in regards to my ..inelab machines as i make my own coils as well as using bought ones and know i can get more out of them IF i had this adjustability that was reliable.
Regards Ron