Can the inductive time constant formula be applied to coil?

[msysta]

Hi,

Just one question. Inductive time constant is t=L/R applies for a series LR circuit. Can this be used when thinking of a search coil?

For example, take surfmaster coil, inductance 210 microH and resistance 1,3 ohm. This gives t a value of
0,000209 / 1,3 = 0,000160769231

So it takes that many seconds to achieve 63,2% of the maximum current level.

and if we think about Surfmasterer's duty cycle, it is only 50 microseconds.

[bbsailor]

Hi,

Just one question. Inductive time constant is t=L/R applies for a series LR circuit. Can this be used when thinking of a search coil?

For example, take surfmaster coil, inductance 210 microH and resistance 1,3 ohm. This gives t a value of
0,000209 / 1,3 = 0,000160769231

So it takes that many seconds to achieve 63,2% of the maximum current level.

and if we think about Surfmasterer's duty cycle, it is only 50 microseconds.

Msysta,

The short answer is yes. But, you need to also include in the resistance value all the resistance that is in series with the coil including:
1. Coil resistance
2. Any current limiting series resistor in the TX circuit
3. MOSFET on-resistance
4. Coil coax lead resistance

Also note that the flyback voltage amplitude is also governed by the pulse width which is also a factor of the coil's charge time constant. Therefore, critical damping of the coil is based on a particular flyback voltage amplitude range that is being damped by the value of the damping resistor in parallel with the coil.

Look for a free program called miscEL.zip on the web and use the coil charge and discharge calculator. It has a nice graph output where you can see the effect of changing the pulse width time. Somewhere just after about 1 TC the slope of the current rise will start to rotate from being more vertical at lower fractional TCs to being more horizontal with higher TCs.

If you want to see the coil discharge TC using miscEL, use the value of the damping resistor as the discharge path.

Do a search on this forum on "damping resistor" to see more details about this.

bbsailor

[msysta]

Msysta,

The short answer is yes. But, you need to also include in the resistance value all the resistance that is in series with the coil including:
1. Coil resistance
2. Any current limiting series resistor in the TX circuit
3. MOSFET on-resistance
4. Coil coax lead resistance

Also note that the flyback voltage amplitude is also governed by the pulse width which is also a factor of the coil's charge time constant. Therefore, critical damping of the coil is based on a particular flyback voltage amplitude range that is being damped by the value of the damping resistor in parallel with the coil.

Look for a free program called miscEL.zip on the web and use the coil charge and discharge calculator. It has a nice graph output where you can see the effect of changing the pulse width time. Somewhere just after about 1 TC the slope of the current rise will start to rotate from being more vertical at lower fractional TCs to being more horizontal with higher TCs.

If you want to see the coil discharge TC using miscEL, use the value of the damping resistor as the discharge path.

Do a search on this forum on "damping resistor" to see more details about this.

bbsailor

Bbsailor,

My greatest thanks to your reply. It was very clearly put to understand and with an excellent reference. The MiscEl looks amazingly useful for me! Seems to be a true versatile multi-tool.

This issue was bothering me because I have an old Surfmaster as a project and I´m learning by planning about what to do with it to make it more powerful -my ultimate goal is however to gain understanding of how that machine works in detail. I am planning to add adjustment for 555 freq, duty cycle length and for damping resistor plus to add pinpointing; actually to take soil balance out of use. Also the original TIP will go.

I made couple of experimental coils, wanted to test with something that is not normally used and put one together using 1mm solid copper wire. Radius is 40 cm and it is a monotype circular coil. L according to my multimeter is 350mH. Resistance is of course low compared to regular one, 0,5 ohms or so, and it comes back to my original question which is now more clear. As can be predicted by those who know, the coil was not a huge success with the original control box. But now I know better what part to adjust and, which is more important for me, why to.

Thanks and Best Regards,
Markus

[mikebg]

Formula for damping resistor

Simplified equivalent circuit for damping of monocoil at open switch S. To obtain the actual equivalent circuit, must include an impedance Z, which is shown not connected from the side. One part of it, which represents RX Front end, appears to be in parallel to resistor R. The other part of it, which is time (frequency) dependent, represents environment. It is in series of coil resistance r.

[mikebg]

What is "design timeconstant" of a coil

Despite that (R)EMI group no longer exists and since July Morning my posts are no longer checked in advance by a bunch of experts on EMI theory and on design of ham radio, these experts continue to operate as a group for theoretical knowledge and continue to express dissatisfaction with the imperfection of the circuits designed by me and with low quality of my two-year job as their spokesman. They gave me a long list of started by me but unfinished actions.
I endlessly love and respect these people because they are great teachers. It is very valuable to have a corrective, someone to indicate your errors and to leads you to perfection. The bad thing is that I'm almost not a perfectionist, I'm used to doing things only with approximate accuracy. If I were a teacher, I wuld explained to the students lessons only in summary form, making them look for themselves the necessary detail knowledge and ask me for the vague things only as a last resort case where they are already exhausted all other sources.
The expers on EMI theory mean that I had to explain the term "Mutual inductance" and its specific case "Self-inductance" in order ham designers to understand why Monocoil is the worst sensing head for all kinds metal detectors (BFO, Frequency Shift, PI etc).
Especially for this thread is supposed to replace the incorrect term "damping" with "dumping" and to explain the term "design timeconstant" of a coil because there is no such term in the WEB, but it is very important for design of lightweight search head.
Here is my posting not edited by experts:
DESIGN TIMECONSTANT of a coil is a tool for designer to analyse price, volume and weight of designed coil. The coil timeconstant is calculated by formula T=L/r where L and r are self-inductance and resistance of the coil. Large timeconstant have coils with ferrite potcore. If designed by you coil for search head (without ferrite core) has timeconstant more than 10ms, your coil will be too expensive, large and weighty. In this case you should change your circuit diagram to accept and operate with a coil having smaller timeconstant. Soon I will post in the forum an example design for low noise and lightweight RX coil.