PI: Effects of pulse shape and transmit time.

[Chet]

Added a CC sink to circuit. Doesn't go to zero volts, but low enough to show the effect on target signal. The 3 usec target comparison, critical damped vs CC discharge the CC signal was just over half. With the coil shunt capacitor 0 pf the signal dropped another half. The peak voltage is critical to R2 wich sets the CC. The target signal does change with CC discharge with spice, but I think the wave form not being perfect and shunt capacitance has something to do with it

Green great work as usual. Your chart indicates a very improved fall time (less than 0.33us). If this can be achieved in real circuity it will allow for a very short delay time for detecting gold nuggets. Do you think the simulation is accurate in simulating added capacitance and loading from the added circuity? Is L2 a single turn with a ratio to L1 of approximately 1 to 30 turns? I am not versed in Spice, what does the Pulse and .tran codes translate to in values? It would be nice to get a circuit like this breadboarded and measured with an oscilloscope.

[green]

[The value of the current at the start and ends of the period are different, producing a rate-of-change of current over time (di/dt).]

I guess I'm still confused, The coil current gets discharged thru R or the constant current sink if the voltage doesn't exceed the fet clamp voltage. I thought you were adjusting R so that the discharge current was the same at the start and the end of discharge (600 ma). I'll think about it. You are right about the terminology being some of the problem, I'm not speaking the right language yet. I'll try to work on that.

[green]

Added a CC sink to circuit. Doesn't go to zero volts, but low enough to show the effect on target signal. The 3 usec target comparison, critical damped vs CC discharge the CC signal was just over half. With the coil shunt capacitor 0 pf the signal dropped another half. The peak voltage is critical to R2 wich sets the CC. The target signal does change with CC discharge with spice, but I think the wave form not being perfect and shunt capacitance has something to do with it

[Qiaozhi]

Thanks for the reply My spice model shows a target signal increase when the coil current is changing and a target signal decay when the current is constant. The bottom CC circuit shows a signal over 30 mv during the time the CC pulse is building up (changing), close to what it shows on turn off. The model doesn't show an increase when the current is constant. Does a CC coil discharge kick the target? My suggestion was to turn the fast on ramp off before it flat-topped while there is still a signal. It only works if the CC discharge doesn't change the target. Is my spice model showing things that aren't true? If a CC discharge doesn't kick the target it seems like it couldn't be used with a normal PI circuit. Still trying to add a CC discharge to my spice circuit to see what it would show.

It's probably the terminology you're using that's causing some confusion.
To make it clearer ... eddy currents are only stimulated in the target during the time(s) when the coil current is changing. That is, during the time when the mosfet is initially switched on (and up until the current starts to level off), and also during the the time when the mosfet is initially switched off (and up until the time the current has decayed to zero). If the coil current is constant (i.e. not changing) then the target is not being stimulated. There is a difference between a coil current that is constant, and a constant current ramp. In the former, the coil current is indeed constant. Having a current of 1A flowing in the coil for a period of 100us (for example) does not produce any rate-of-change of current over time, and therefore no changing magnetic field. The current is 1A at the start of the period, and it is still 1A after 100us. But ... a constant current source being used to energize a coil (or a constant current load being used to de-energize the coil) will produce a rate-of-change of current, and a corresponding change in the magnetic field (the magnetic field is either expanding or contracting depending on whether it's due to a constant current source of load, causing flux cutting of the target). The value of the current at the start and ends of the period are different, producing a rate-of-change of current over time (di/dt).

[green]

This is a popular misconception that has been discussed many times in the forum.
The target is not charged during the coil on-time, and discharged during the off-time. This is a fallacy.

During the on-time, a magnetic field is established around the coil. Although it is true that eddy currents are generated in the target during the ramping up of the current, these will have substantially died away if the current is allowed to flat-top. When the mosfet is switched off, the magnetic field of the coil collapses, and it is the rapidly changing current in the coil that kicks the target. This rate of change of current over time (di/dt) causes the magnetic flux lines to cut through the target, and generate eddy currents in the material.

Thanks for the reply My spice model shows a target signal increase when the coil current is changing and a target signal decay when the current is constant. The bottom CC circuit shows a signal over 30 mv during the time the CC pulse is building up (changing), close to what it shows on turn off. The model doesn't show an increase when the current is constant. Does a CC coil discharge kick the target? My suggestion was to turn the fast on ramp off before it flat-topped while there is still a signal. It only works if the CC discharge doesn't change the target. Is my spice model showing things that aren't true? If a CC discharge doesn't kick the target it seems like it couldn't be used with a normal PI circuit. Still trying to add a CC discharge to my spice circuit to see what it would show.

[Qiaozhi]

I'm going to show my ignorance with metal detectors and ask a stupid question. If the target is charged with a change in current over time and we discharge the coil with a constant current over time do we change the charge on the target? If not, can we charge the coil with a fast ramp, turn the charge off before the current flattens out, discharge with a constant current and read the coil on charge signal?

This is a popular misconception that has been discussed many times in the forum.
The target is not charged during the coil on-time, and discharged during the off-time. This is a fallacy.

During the on-time, a magnetic field is established around the coil. Although it is true that eddy currents are generated in the target during the ramping up of the current, these will have substantially died away if the current is allowed to flat-top. When the mosfet is switched off, the magnetic field of the coil collapses, and it is the rapidly changing current in the coil that kicks the target. This rate of change of current over time (di/dt) causes the magnetic flux lines to cut through the target, and generate eddy currents in the material.

[green]

I'm going to show my ignorance with metal detectors and ask a stupid question. If the target is charged with a change in current over time and we discharge the coil with a constant current over time do we change the charge on the target? If not, can we charge the coil with a fast ramp, turn the charge off before the current flattens out, discharge with a constant current and read the coil on charge signal?

[Teleno]

I was thinking we were comparing difference in amplitude between a CC circuit and a normal ramp circuit with the same peak current, maybe not.

Not what I had in mind, green. I was looking at short vs. long square pulse of same current.

The advantage of CC is that for a given peak current (i.e. target magnetization) you can achieve a much shorter pulse than a ramp can. For a 300u coil at 12V and 1A the minimum pulse is 25us. By using CC you can get into single digit magnetization times at full power.

That' s why I wanted to get rid of the ramp.

[green]

Interesting simulation, green. But you have to compare the CC amplitude of a 3us target with the CC amplitude of a 100us target for let's say a 20us pulse. The ratio should be around 25:1 (14dB)

I was thinking we were comparing difference in amplitude between a CC circuit and a normal ramp circuit with the same peak current, maybe not.

[Qiaozhi]

What was or is confusing me. Decay TC equals L/R. The lower the R the longer it takes to decay. I think what I had forgotten is a constant current sink is high resistance. I'm a novice with spice and haven't gotten the simulation to work yet. Is the 1.7 multiplier or 600 ma magical or does it work with any constant current?

There's nothing magical about the numbers. In my conversion of Teleno's simulation to LTSpice, I simply used the same constants in order to get the same results. My interest was in finding a means of replicating the ternary function, as I needed to use something similar in a completely different and unrelated electronics project.

[Teleno]

I used 9 usec to give equal response for a low TC target because I understood Teleno was looking for gold and wanted to reject high TC targets. If I make coil on time 50 usec for both circuits The CC circuit target amplitude is 1.1 times higher for the 3 usec TC target and 1.8 times higher for the 100 usec TC target. Not saying which is best, just wondering if the spice simulation makes sense. If it does it might help decide which is best for the application.

Interesting simulation, green. But you have to compare the CC amplitude of a 3us target with the CC amplitude of a 100us target for let's say a 20us pulse. The ratio should be around 25:1 (14dB)

[green]

The smaller the value of the damping resistor, the faster the decay. Too high a resistance will make the coil underdamped, and produce ringing. Too low a resistance will overdamp the coil, and reduce sensitivity. In a standard PI, you need to find the "Goldilocks" value that is just right for critical damping.

For the CC method, the resistor value varies to maintain a constant current. At (for example) 400V, the resistance becomes 681R, but at 50V it's 86R. In both cases the current flowing in the damping resistor will be just under 600mA. Once the coil voltage reaches zero, or swings positive, the resistance defaults to 100k.

What was or is confusing me. Decay TC equals L/R. The lower the R the longer it takes to decay. I think what I had forgotten is a constant current sink is high resistance. I'm a novice with spice and haven't gotten the simulation to work yet. Is the 1.7 multiplier or 600 ma magical or does it work with any constant current?

[green]

What you have here is completely reversed to situations we are talking about. The idea of constant current drive is to separate initial di/dt from a flyback, so that the initial di/dt subsides as much as possible. The charging period is not the same in upper and lower situation. You do have a constant current drive in a lover one, but its duration is only 9us. Change it to 50us or more (which is the very idea here) and see what happens.

I used 9 usec to give equal response for a low TC target because I understood Teleno was looking for gold and wanted to reject high TC targets. If I make coil on time 50 usec for both circuits The CC circuit target amplitude is 1.1 times higher for the 3 usec TC target and 1.8 times higher for the 100 usec TC target. Not saying which is best, just wondering if the spice simulation makes sense. If it does it might help decide which is best for the application.

[Qiaozhi]

[I'm confused. Based on your formula the calculated resistor is going to be less than a fixed resistor for critical damping most of the decay curve. I was thinking the higher the resistance the faster the coil would decay to zero current.

The smaller the value of the damping resistor, the faster the decay. Too high a resistance will make the coil underdamped, and produce ringing. Too low a resistance will overdamp the coil, and reduce sensitivity. In a standard PI, you need to find the "Goldilocks" value that is just right for critical damping.

For the CC method, the resistor value varies to maintain a constant current. At (for example) 400V, the resistance becomes 681R, but at 50V it's 86R. In both cases the current flowing in the damping resistor will be just under 600mA. Once the coil voltage reaches zero, or swings positive, the resistance defaults to 100k.

[Davor]

Are there reasons to use active damping other than possibly sampling sooner? In reply #18 I said 36 percent less, should have said the CC target response was 36 percent of the upper circuit response.

What you have here is completely reversed to situations we are talking about. The idea of constant current drive is to separate initial di/dt from a flyback, so that the initial di/dt subsides as much as possible. The charging period is not the same in upper and lower situation. You do have a constant current drive in a lover one, but its duration is only 9us. Change it to 50us or more (which is the very idea here) and see what happens.