Coil Saturation in a PI?

[bklein]

Yes, lets forget about the core saturation one unless we start talking about ferrite core pinpointers...

[ivconic]

...Example is on my HH2 increasing the pulse time from 100usec to 200usec increased the response of 'silver' coins in particular US quarters which have a TC of about 170us...

I have similar experiences with FelezJoo Pi. Pulse width from 180-200uS brings up the coins to respectable distances from coil.
Everything up to 210uS behaves alright. Above 220uS becomes "linear" counterproductive... at least with small fast coils.
I am a bit handicapped because i don't know how to perform TC math so i have to rely on what i see in such tests.

[ivconic]

...At least I started an interesting conversation, please continue :-)

Educational too. Sometimes man can learn more from these kinds of conversations.
I learned few things, for sure.

[ivconic]

https://www.powerstream.com/Wire_Size.htm

Am i wrong to assume that this kind of "saturation" we can take as "100% skin depth" for used wire gauge in a coil?

[kt315]

Yes, lets forget about the core saturation one unless we start talking about ferrite core pinpointers...

i connect erase heads (take 250uH and more) from video cassette players and audio cassete players to DP and they work good.
if i increase a power to 500us they can fastly roast and burn. good testing microcoils for checking your PI.

[eclipse]

bbsailor (Joseph Rogowski) has posted useful information on coil saturation which I've put on a PDF so I can read it while traveling.
All you need to know is there

[waltr]

Waltr is correct, a longer pulse width is better for long TC targets. The reason is it allows the "on-time" eddy currents to decay out, so they have less cancelation effect on the "off-time" eddy currents.

However, a longer pulse width also creates a higher flyback with longer decay settling, meaning you will need a later sample delay. That's OK if you are trying to enhance long TC targets anyway. So for short TC targets, you want a short pulse width but with still reasonably good flat-topping (add a series R) and lower flyback for faster settling. Minelab's solution is to use a wide TX pulse plus 3 narrow TX pulses.

Thanks for replys from you both Waltr and Carl


Carl do you recall when they take target and efe samples ? is it after all the long and short tx pulses finish ? thanks

Here is a timing diagram of the SD2000. sampling is done after each TX pulse. From my experiments a 240us pulse is good for all long TC targets whereas 60us is enough for short TX targets and also decreases fly-back Voltage as Carl points out and allows earlier sampling.


In my HH2 I also have a 10 Ohm resistor is series with the coil & MOSFET to reduce the Tau of the coil during TX. This increases the rate of the current into the coil and gets to a constant current sooner (flat topping). This reduces the finial current in the coil and reduces the peak fly-back Voltage.
I have experimented with the series resistor value, lower value to have higher coil current, but found NO increase of target detection distance.

[bklein]

bbsailor (Joseph Rogowski) has posted useful information on coil saturation which I've put on a PDF so I can read it while traveling.
All you need to know is there

Wow, no kidding! This is great. I read a third of it so far and got overwhelmed. :-)
One area I am weak understanding the implications of is increasing the TX voltage. For a given coil you want to reach the 5T current level but this is reached faster if the TX voltage is increased. The GP Extreme is doing this.

[Carl-NC]

One area I am weak understanding the implications of is increasing the TX voltage. For a given coil you want to reach the 5T current level but this is reached faster if the TX voltage is increased. The GP Extreme is doing this.

Coil settling time is independent of the TX voltage, it only depends on the coil tau which is L/Rs, where Rs is the total series resistance. The max current at which it settles depends on VTX and Rs. Minelab uses a low voltage and low Rs for long TX and a high voltage and high Rs for short TX to balance the peak currents, which produces equivalent GB points.

[bklein]

I have experimented with the series resistor value, lower value to have higher coil current, but found NO increase of target detection distance.

Why is this?

[ivconic]

bbsailor (Joseph Rogowski) has posted useful information on coil saturation which I've put on a PDF so I can read it while traveling.
All you need to know is there

Indeed is.
From the tons of useful and very educational infos; i found this as somewhat amusing:
http://www.learnabout-electronics.or.../dc_ccts45.php

"... Why 63.2%?If the current never reaches its steady state value, this presents a problem of how to measure the time taken to fully charge.
This is why the idea of a time constant, (the time it takes to charge by 63.2%) is used. Why choose 63.2% when there are easier
numbers such as 50% that could be used? Well 50% would be nice but would create an awkward formula with which to calculate
the time taken..."

[waltr]

I have experimented with the series resistor value, lower value to have higher coil current, but found NO increase of target detection distance.

Why is this?

Possibly due to having too short of a TX pulse time (this was done with a 100us pulse) and also possibly due to either or both the MOSFET avalanche and longer pre-amp recovery.

[waltr]

Coil settling time is independent of the TX voltage, it only depends on the coil tau which is L*Rs, where Rs is the total series resistance.

Carl,
Isn't the Coil Tau = L/R. Which is why increasing the series R decreases the Coil Tau.

[bbsailor]

Coil Time Constants- Here is a good mental model.


The coil L/R time constant tells you where on your coil's TC graph the current turns off with a specific length TX pulse. A 300uh coil inductance that has a total of 6 ohms resistance, including the coil wire, MOSFET on-resistance, cable length and any series resistor values has a 300/6 or a 50 uS TC. Each length of 50US pulse width rises in a typical TC curve in the following way for this coil's characteristics.

1TC-63.2% 50uS
2TC-86.5% 100uS
3TC-95.1% 150uS
4TC-98.2% 200uS
5TC-99.3% 250uS

These numbers are rounded off but represent what percentage of the potential maximum current the coil current has risen to with any particular length of TX pulse. Here are two mental models to keep in mind.

1. Where is the best place on the curve to turn off the current?
Select a place where the curve is more horizontal then vertical to prevent cancelling some of your already spent TX energy. Typically this is around the 3TC point or 95.1% of maximum current.

2. Why are the numbers spaced the way they are?
Each TC represents the same amount of energy increase in the remaining current rise. At 1TC you are at 63.2% of maximum current with 36.8% remaining or 100 - 63.2 =36.8. Now, multiply 36.8 times 63.2% (.632) and get 23.2576. Now just add 23.2576 to 63.2 or one TC to get the value of 2TC or 86.4576 rounded off to 86.5%. Do this again with the remaining current times 63.2 percent and get the next TC.

At 5TCs you are at 99.3 percent of max current and the curve is almost horizontal. Even if you added another TC of TX time, the current would only rise to .7 times 63.2% or another .4424 or a current of 99.3 plus .4424 or 99.7424%.

Based on the size and thickness of your desired targets, you should choose a TX length to fully saturate the desired target to have the longest lasting target discharge eddy currents left to detect, as in 5 target TC length of time, the induced signal in the target will have decayed into the noise level in 5TCs in the RX mode. This theory works with large underwater targets like cannons, cannon balls and large metal boat parts with large underwater search coils and Time Constants measured in milliseconds. It gets more challenging when you want to detect small gold targets with a TC near 1 or 2 uS. But, always strive to turn off the TX pulse at about 3 TCs or 95.1% of maximum current. You can go up to 5TCs for very large and thick metal targets.

Having an easy to understand mental TC models is very valuable when experimenting with coils, TX power, delay time and desired RX target sensitivity.

Joseph J. Rogowski

[green]

My bench circuit has two modes. Constant current, ramps to .5A in 20us then stays at .5A for rest of Tx on time. Constant rate, ramps to 1A in Tx on time. Both modes same average current for Tx on. Constant current, Tx on times 3 an4 target TC's not much different than 2TC. If I understand the theory correctly, at 1TC subtract 36.8% at 2TCsubtract 13.5% at 3TC subtract 4.9% at 4TC subtract 1.8%. 4TC on time twice the average current of 2TC on time for a 11.7% increase in signal. Don't know if could detect a difference in detection distance in a blind test switching between 2 target TC or 4 target TC constant current Tx on times. See a difference between 1, 2 or 3 target TC with constant rate, still the increase in signal is less than increase in average current. Including a test with a nickel, quarter and a IKE dollar varying Tx on time 1,2,3 and 4 quarter TC's. Think 2 target TC on time is enough. 300us for the quarter but 300us is less than 1TC for the IKE dollar. Looks like desired average current effects in Tx on time. One division is 26%increase in signal for the test charts.

Tried a spice simulation also. Series resistance 8ohms, some increase from 1 to 2 target TC. Series resistance 1ohm, some increase from 1 to 2 target TC and some from 2 to 3 target TC maybe due to increase in peak current.