Need help about calculating damping resistor

[solmer2]

Hi there, I'm wondering if there is a calculator or formula to calculate the damping resistor I should use when building a new coil. I know I could use an oscilloscope but need to know if I can approach the "right" damping resistor should I use as starting point without using an oscilloscope.

Thanks in advance.

[vasster]

The general formula for critical damping of a parallel RLC circuit ( wikipedia ) is
R=(sqrt(L/C))/2 so for L=300 uH and C=60 pF then R= 1118 Ω

[Carl-NC]

You can, but you may still need an oscope. We have a parallel RLC circuit, and we want to achieve critical damping. This occurs when:



L is the coil inductance (measure it), so all you need is C, which is the total parasitic capacitance of the coil, cable, and circuitry (mostly the MOSFET). The cable and circuitry can be estimated, but you will need to measure the self-capacitance of the coil. This is usually done by measuring the self-resonance with an oscope.

As an example, a 300uH coil has 100pF of self cap, so

= 866 ohms

That's in the ballpark of most damping R's.

- Carl

[porkluvr]

Carl, what program did you use to imbed your graphics in the text field?

I saw a name for something, didn't write it down, and promptly forgot where to find the reference.

Please?

[Carl-NC]

It's LaTeX, and is embedded as a part of the forum. So typing in this:

[latex]R = \frac{1}{2}\sqrt{\frac{L}{C}}[/latex]

gives you this:

[porkluvr]

Thank you.

Now all I've got to do is learn ciphers and cognitive thunk and I can be a genius.

[Carl-NC]

Do a google for a LaTeX manual.

[Rudy]

It's LaTeX, and is embedded as a part of the forum.

Carl, is the embedding something you had to do or is it part of the vBulletin software?

[Carl-NC]

It's an add-on, which is one of the reasons I chose vBulletin for these forums. It has strong support for add-ons (vBulletin.org).

[Aziz]

Online LaTeX Equation Editor

http://www.codecogs.com/latex/eqneditor.php

After editing the formula, change into Wordpress markup (bottom) and copy & paste the field.

Aziz

Test:

[mikebg]

Formula for analysis od critical damping

This is the equivalent circuit diagram of TX at pulse induction and formula for analysis of critrical damping.
R is the resistance of critical damping resistor, r is coil resistance, L is coil inductance, C is equivalent capacitance of the circuit (not only coil capacitance).
Note that resistor R depends also on coil resistance.
The formula in WIKIPEDIA is incorrect, because we can't make coil without resistance.

[porkluvr]

Something didn't look right. Mikebg's and Carl's formulae did not look like they would jibe. But when I made comparative calculations using what I thought were realistic variables, the two formulae agreed to within about 1%. So I set out to try and prove equivalency.

[I kept getting negative results until I realized that does not equal (arrooo).]

After working the proof below, I see that Carl's and mikebg's formulae are approximately equivalent. Mikebg formula includes an error term and is slightly more accurate. But that does not make Carl's formula wrong, only simplified.

I refreshed some basic algebra skill, and got a lesson in LaTex all at the same time.
(and it's time for a break)

[bklein]

Does anyone switch in the damping resistor after the TX pulse ends?
Worth considering?

[DOOLEY]

OR , poss better , switch on damping AFTER flyback peak , only a thought.

[Carl-NC]

Does anyone switch in the damping resistor after the TX pulse ends?
Worth considering?

I did that in a super-low-power PI detector design. Works fine. Otherwise not much reason to do so.

OR , poss better , switch on damping AFTER flyback peak , only a thought.

Tried that, but could never get the timing precise enough to make it work worth a darn. It might be more feasible by using a variable damping R (MOSFET) and ramping it in. There is a Carl V Nelson patent that covers the use of a MOSFET as a variable damping R.

- Carl