How do you determine the value of the dumping resistors for a balanced coil without an oscilloscope?

[wes0008]

How do you determine the value of the dumping resistors for a balanced coil without an oscilloscope? I am planning on making a 'OO' coil and the inductance a for tx will be 317uh and rx will be 450uh. Oh, and my detector is a mini pulse rev-c if that helps!

[kt315]

an ringing at last pic http://chemelec.com/Projects/Oscilloscope/Scope.htm

[wes0008]

What do you mean? So I'm assuming it can't be done without an oscilloscope? If I need one I don't think I could be bothered making one!

[Qiaozhi]

The only accurate method is to use an oscilloscope. However, you can calculate the desired value from:

---------- [1]

or:

------------ [2]

BUT ... equation [1] requires you to know the self-resonant frequency (f) of the coil, and equation [2] requires the self-capacitance (C).

The bottom line is that you need to make the damping resistor adjustable:
10k pot + 200R series resistor, both in parallel with 1k2 resistor.
Adjustment range is between 171R and1k1.

and (using an oscilloscope) adjust the signal at the preamp output for critical, or very slight under-damping.

Alternatively you can take a guess at the final value (say 470R for the MPP), and set the damping resistor adjustment tool to that value. Then (by trial and error) adjust the pot for best results.

In both case, you need to measure the final resistance, and replace it with a fixed resistor of the closest preferred value.

[wes0008]

I think I'll give that alternate method a whirl! If only there were self adjusting caps and resistors in detectors so all you had to do was plug in any coil and let it do the work!!! Thanks again Qiaozhi!

[kenny444]

Hello I am reading this with reference to a Surf PI 1.2 kit, do you know where I can pick up the signal for a scope on this circuit please. In the mean time I will look for damping signals within the forum. Grateful for you input.

[Silver Dollar]

Look here in #5 for pictures of damping; http://www.geotech1.com/forums/showt...uda-and-SurfPI

#3 and #4 are good. #1 and #2 are no good.

[kenny444]

Very grateful, thank you.

[kenny444]

Hello Qiaozhi regarding equation [2] can we find L and C just using a LCR meter? Thank you.

[Qiaozhi]

Hello Qiaozhi regarding equation [2] can we find L and C just using a LCR meter? Thank you.

You can directly measure the inductance (L) with an LCR meter, but measuring the self-capacitance (C) is more tricky.
One method is to put a known capacitance (C2) in series with the coil, and connect this combination to the LCR meter. This effectively puts the self-capacitance (C1) of the coil, and C2 in series. Since you will then know the total capacitance (CT) from the LCR meter, and the value of C2, you can calculate C1 from:

[kenny444]

Thank you, always happy to learn and share.