PI: Effects of pulse shape and transmit time.

[Qiaozhi]

Aha! ... got it to work!
I was making it more complicated than it needed to be. It's bleedin' obvious once you know how to do it.

[Davor]

I think I may have my contender for a CCPI solution up my sleeve. I'm tinkering an old solution of mine to short a coil at a zero crossing and thus obtain a perfect CCPI with energy recycling.
See http://www.geotech1.com/forums/showt...135#post145135
(replace D with MUR460 or any other fast recovery diode)

[green]

Trying to do constant current Tx and target response with spice. Adjusted time on upper circuit for 1 amp peak current. Adjusted time on lower CC circuit (1 amp peak) for equal target response with a 3 usec TC target. Increased target time constant from 3 usec to 100 usec. The target response was 36 percent less for the CC circuit compared to the the upper circuit for a 100 usec TC target. Not a big drop, Davor's reply above.

[green]

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.

[green]

[Essentially (for those wondering what the ternary operator does) it's basically an IF-THEN-ELSE statement. In this case: If the coil voltage is greater than or equal to zero then R=100k, else it equals the product of the absolute value of the coil voltage and 1.7, plus 1. This produces a resistor that changes its value, when the coil voltage is negative, such that it sinks a constant current of approximately 600mA. If the coil voltage is zero or positive, the value is fixed to 100k]

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.

[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.

[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.

[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.

[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?

[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)

[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.

[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.

[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]

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?

[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.