Hello,
Everyone's suggestions have worked. I constructed a coil probe (ferrite core) with a value of 57 uH. I am using a 560 pF capacitor and my calculated resonant frequency = 889.950 KHz. My scope tell me that the resonant frequency is 1.25 MHz. Maybe its the capacitor? That is not my question. At resonance, the scope shows:
7.2 v peak to peak, 2.44 v rms, When I put metals such as copper or aluminum at the coil's probe tip there is a drop in voltage as one would expect. The voltage drop is different for each metal. My scope's test points are across the coil, this works fine. I want to use a multimeter to measure volts across the coil, but I get erratic values which are very low. I put a diode to convert to dc and I get very low volts.
This maybe a stupid question, but why is the scope's voltage different from the multimeter?
My goal is to:
convert AC output across coil to DC (use a diode)
Divide voltage if necessary and store in a capacitor for the ADC0831 to measure (I can do this I believe)
Use an SX28 or BSII with the ADC and display the results. (I will use some code from parallax)
Another question:
Do I need to protect my circuit with a current limiting resistor? Series LC circuits in resonance are suppose to have high current, but I cannot verify it experimentally. I will continue to try and test for the current of the series lc circuit.
"This maybe a stupid question, but why is the scope's voltage different from the multimeter?" -· The meter can be loading the resonant circuit more than the scope, therefore lowering the effective 'Q' of your resonant circuit.
"Do I need to protect my circuit with a current limiting resistor? Series LC circuits in resonance are suppose to have high current, but I cannot verify it experimentally." - You are correct, when a coil and capacitor is at it's resonate· frequency they can appear as a short.· A small value resistor 10 Ohms to 100 Ohms is ok, but remember the higher the resistor value the lower the 'Q'.· A lower 'Q' means that your setup will not be as sensitive to small inductive changes.
As a side note: the 7.2V seems very low to me... how are your coils constructed?· I would expect to see almost 7 times the input excite voltage.· What is the load of your scope probes?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Hello Beau,
My best coil is a 36 uH coil with Diameter = Length of Coil. The resistance of the coil = .6 ohms. Its 18 g magnet wire on a 4.3 diameter PVC form. I will use this coil and experiment again. The problem is that I need a small diameter coil to act as a probe of sorts. This coil should have a good "Q". My scope is a HPS10 scope from Velleman. I need a better scope. How do I find out the load on the scope probes? Measure resistance maybe? I am using a function generator with a 50 ohm load. I figured my output voltages should be higher. My excite voltage is 10 volts from the function generator.
Last night I measured the voltage across the capacitor and it was the same as the scope (5.9 VAC peak to peak). The multimeter was accurate I guess.
Hello again,
This is an update. I re-calibrated my scope and then I took the roll of new 26 g (green) magnet wire from radio-shack 3-pack and soldered a wire on each end as is. This gave me a coil with a value of 1.236 mH. I tested with an LCR meter. I used a 560 pF capacitor and sent a 217 kHz signal through it. The scope showed:
Square wave = 257 v peak to peak and 84.8 v RMS at resonance with 10 volts from signal generator.
Sine wave = 221 v peak to peak and 73.2 v RMS at resonance with 10 volts from signal generator.
Beau, does this fits better?
I am still having problems testing with multimeter.
If anyone has any suggestions, I would appreciate it. I am beginner, but I want to learn.
"Beau, does this fits better?" - yes, the 257 v sounds much better, the other coil just doesn't sound like your sending it the proper signal, or the 'Q' is way off somehow, although it looks ok... The PVC will contribute to some parasitic capacitor effects as well as the coil windings themselves which is something you can't really get away from.
"The problem is that I need a small diameter coil to act as a probe of sorts." - lets back track a little... what diameter do you need?· Consider also that the·higher the frequency, the smaller the inductor.· The smaller the inductor the better chance for a better 'Q'.· The better 'Q' the better the sensitivity to inductive changes.
Just something to think about: To some degree, you can also place coils in series where one·coil in the series is a fixed inductor, and the other coil in the series is the sensing element... this way you can off-load some of the "mass" of the sensitive element, but·keep in mind the amount of total inductance affected·will proportionally be smaller depending on how you distribute the inductance.
As far as·measuring with a meter... usually I create a·high impedance voltage divider with something like· 10M on the input and 1M·to GND.· The center connection can go through a diode and then to a capacitor usually in the 100pF to 500pF range.· This also depends on the frequency at hand. ·
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Im about to make a sequencer for an Earths Field Nuclear Magnetic Resonance Spectrometer /NMR
One issue I would like some help with is switching coils adiabatically, or when the
polarizing coil is energized it does so with a ramp up, hold, and ramp down in current - except
the PSU is constant voltage (or constant) current - also the option for a battery is needed.
From what I know about switching coils (from stepper motors) - a pwm process is used to
vary the on time and thus increasing the current, except Im not sure at this point if the
pwm frequency/current ramping would cause major interference in the ramp down phase
when the spin echo signal is present (NMR signal) echo is present.
Does anyone know of any device (external hardware) that can controll current if fed with
a control frequency or digitally set via e.g i2c ?
The sequence goes something like:
Ramp up current in polarizing coil over 200ms, hold it at a set current for 2-5 seconds, ramp current down
transmit an audio pulse at around 2.00 Khz (this is the appropriate lamor frequency)
The receive processing is handled by other hardware, but the prop demo board together with some
optoisolators and fet's will be used to control the polarizing and audio tx coils.
In summary this is the sequencer for an NMR Spectroscope.
Hi,
Harvey Morehouse's articles over at beigebags site are worth reading regarding switch mode modeling, Jim's article is a very good practical build - I noted the dead time / pull down resistors on the mosfets, that was quite interesting. - Just to clarify Im not building a power supply, that exists. The voltage and current can be set from the bench PSU. The problem is how to slowly ramp up the current to a coil (no core, its 95 meters of 1.6mm copper wire wrapped around a tube) - Nearest I could find on the net to what Im attempting can be read from this site: http://www.exstrom.com/magnum/master-Z-H-6.html
Except that coil is switched on/off rapidly (but held on for a few seconds, the off part is the important bit...) e.g ramping the current in, holding and relaxing it (adiabatically) is the preferred method
for NMR, as the 1st PDF page 4265 describes here http://www.magritek.com/earthsfieldscience.html /
- one option of course would be to use the same Kepco PSU. Which I'll try to dig out some info on and see how it works. I'll check if a signal generator driving a 4N35 opto isolator and IRF6215 HEXFET's as used in the 1st URL would work. Although the problem may remain that the switching frequency effects the final SNR disrupting the decay phase.
Its an interesting project for the prop, I suspect the required control instrumentation may be rather more complicated than what the article states. But the sequencing code I wrote in literally 10 minutes, Polarizing coil on, delay, off, wait 100ms, send audio pulse (2.245 Khz) for anything between 10-50ms. switch RX coil on -> Analyse in Baudline / Linux. - Watch for the NMR signal... in theory that is
$WMc%,
Thanks, Jim Stewart's article was great. With Beau's helps and all of you, I am close to success in finishing my eddy current metal tester. I thank everyone. I notice that the Prop is the new thing and is powerful. I have the BSII and the SX28, but should I get the Prop for my experiments with coils and what is a good starter kit for the prop?
Thanks,
Cenlasoft
Comments
Everyone's suggestions have worked. I constructed a coil probe (ferrite core) with a value of 57 uH. I am using a 560 pF capacitor and my calculated resonant frequency = 889.950 KHz. My scope tell me that the resonant frequency is 1.25 MHz. Maybe its the capacitor? That is not my question. At resonance, the scope shows:
7.2 v peak to peak, 2.44 v rms, When I put metals such as copper or aluminum at the coil's probe tip there is a drop in voltage as one would expect. The voltage drop is different for each metal. My scope's test points are across the coil, this works fine. I want to use a multimeter to measure volts across the coil, but I get erratic values which are very low. I put a diode to convert to dc and I get very low volts.
This maybe a stupid question, but why is the scope's voltage different from the multimeter?
My goal is to:
convert AC output across coil to DC (use a diode)
Divide voltage if necessary and store in a capacitor for the ADC0831 to measure (I can do this I believe)
Use an SX28 or BSII with the ADC and display the results. (I will use some code from parallax)
Another question:
Do I need to protect my circuit with a current limiting resistor? Series LC circuits in resonance are suppose to have high current, but I cannot verify it experimentally. I will continue to try and test for the current of the series lc circuit.
Any hints would be appreciated.
Thanks,
Cenlasoft
"This maybe a stupid question, but why is the scope's voltage different from the multimeter?" -· The meter can be loading the resonant circuit more than the scope, therefore lowering the effective 'Q' of your resonant circuit.
"Do I need to protect my circuit with a current limiting resistor? Series LC circuits in resonance are suppose to have high current, but I cannot verify it experimentally." - You are correct, when a coil and capacitor is at it's resonate· frequency they can appear as a short.· A small value resistor 10 Ohms to 100 Ohms is ok, but remember the higher the resistor value the lower the 'Q'.· A lower 'Q' means that your setup will not be as sensitive to small inductive changes.
As a side note: the 7.2V seems very low to me... how are your coils constructed?· I would expect to see almost 7 times the input excite voltage.· What is the load of your scope probes?
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe
IC Layout Engineer
Parallax, Inc.
My best coil is a 36 uH coil with Diameter = Length of Coil. The resistance of the coil = .6 ohms. Its 18 g magnet wire on a 4.3 diameter PVC form. I will use this coil and experiment again. The problem is that I need a small diameter coil to act as a probe of sorts. This coil should have a good "Q". My scope is a HPS10 scope from Velleman. I need a better scope. How do I find out the load on the scope probes? Measure resistance maybe? I am using a function generator with a 50 ohm load. I figured my output voltages should be higher. My excite voltage is 10 volts from the function generator.
Last night I measured the voltage across the capacitor and it was the same as the scope (5.9 VAC peak to peak). The multimeter was accurate I guess.
Thanks for all your help.
Cenlasoft
This is an update. I re-calibrated my scope and then I took the roll of new 26 g (green) magnet wire from radio-shack 3-pack and soldered a wire on each end as is. This gave me a coil with a value of 1.236 mH. I tested with an LCR meter. I used a 560 pF capacitor and sent a 217 kHz signal through it. The scope showed:
Square wave = 257 v peak to peak and 84.8 v RMS at resonance with 10 volts from signal generator.
Sine wave = 221 v peak to peak and 73.2 v RMS at resonance with 10 volts from signal generator.
Beau, does this fits better?
I am still having problems testing with multimeter.
If anyone has any suggestions, I would appreciate it. I am beginner, but I want to learn.
Thanks,
Cenlasoft
"Beau, does this fits better?" - yes, the 257 v sounds much better, the other coil just doesn't sound like your sending it the proper signal, or the 'Q' is way off somehow, although it looks ok... The PVC will contribute to some parasitic capacitor effects as well as the coil windings themselves which is something you can't really get away from.
"The problem is that I need a small diameter coil to act as a probe of sorts." - lets back track a little... what diameter do you need?· Consider also that the·higher the frequency, the smaller the inductor.· The smaller the inductor the better chance for a better 'Q'.· The better 'Q' the better the sensitivity to inductive changes.
Just something to think about:
To some degree, you can also place coils in series where one·coil in the series is a fixed inductor, and the other coil in the series is the sensing element... this way you can off-load some of the "mass" of the sensitive element, but·keep in mind the amount of total inductance affected·will proportionally be smaller depending on how you distribute the inductance.
As far as·measuring with a meter... usually I create a·high impedance voltage divider with something like· 10M on the input and 1M·to GND.· The center connection can go through a diode and then to a capacitor usually in the 100pF to 500pF range.· This also depends on the frequency at hand.
·
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Beau,
I read your article on this subject in Nuts and Volts.· I thought you explained the operation of the sigma-delta ADC quite well.
Thanks!...
Since you made reference to the March 2009 issue of Nuts and Volts, I·think it might be applicable here in this thread.
Here is the correct link to the OBEX software for the article... (hopefully they will fix the link for Part #2 in the April Nuts and Volts issue)
http://obex.parallax.com/objects/416/
... There is some video in the OBEX as well, showing what is supposed to happen with the program.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Somewhat related
Im about to make a sequencer for an Earths Field Nuclear Magnetic Resonance Spectrometer /NMR
One issue I would like some help with is switching coils adiabatically, or when the
polarizing coil is energized it does so with a ramp up, hold, and ramp down in current - except
the PSU is constant voltage (or constant) current - also the option for a battery is needed.
From what I know about switching coils (from stepper motors) - a pwm process is used to
vary the on time and thus increasing the current, except Im not sure at this point if the
pwm frequency/current ramping would cause major interference in the ramp down phase
when the spin echo signal is present (NMR signal) echo is present.
Does anyone know of any device (external hardware) that can controll current if fed with
a control frequency or digitally set via e.g i2c ?
The sequence goes something like:
Ramp up current in polarizing coil over 200ms, hold it at a set current for 2-5 seconds, ramp current down
transmit an audio pulse at around 2.00 Khz (this is the appropriate lamor frequency)
The receive processing is handled by other hardware, but the prop demo board together with some
optoisolators and fet's will be used to control the polarizing and audio tx coils.
In summary this is the sequencer for an NMR Spectroscope.
BR,
Mike
Take A look at Mr.Jim Stewart's Article in the March 09' issue of Nuts/Volts. Pg.46
Mr.Stewart is the Man that rocket scientist look too for advice in the Magnetic World, When they need some explanation in this field!
He explains every thing in great detail.
This is one of the best ( I mean by detail ) Articles that I have read in a long time.
Its after reading this Article, That I see the success and failure's of My past power supply projects.
I think You find the Math formulas useful.
______________$WMc%____________
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
The Truth is out there············································ BoogerWoods, FL. USA
Harvey Morehouse's articles over at beigebags site are worth reading regarding switch mode modeling, Jim's article is a very good practical build - I noted the dead time / pull down resistors on the mosfets, that was quite interesting. - Just to clarify Im not building a power supply, that exists. The voltage and current can be set from the bench PSU. The problem is how to slowly ramp up the current to a coil (no core, its 95 meters of 1.6mm copper wire wrapped around a tube) - Nearest I could find on the net to what Im attempting can be read from this site:
http://www.exstrom.com/magnum/master-Z-H-6.html
Except that coil is switched on/off rapidly (but held on for a few seconds, the off part is the important bit...) e.g ramping the current in, holding and relaxing it (adiabatically) is the preferred method
for NMR, as the 1st PDF page 4265 describes here http://www.magritek.com/earthsfieldscience.html /
- one option of course would be to use the same Kepco PSU. Which I'll try to dig out some info on and see how it works. I'll check if a signal generator driving a 4N35 opto isolator and IRF6215 HEXFET's as used in the 1st URL would work. Although the problem may remain that the switching frequency effects the final SNR disrupting the decay phase.
Its an interesting project for the prop, I suspect the required control instrumentation may be rather more complicated than what the article states. But the sequencing code I wrote in literally 10 minutes, Polarizing coil on, delay, off, wait 100ms, send audio pulse (2.245 Khz) for anything between 10-50ms. switch RX coil on -> Analyse in Baudline / Linux. - Watch for the NMR signal... in theory that is
Cheers,
BR,
Mike.
Thanks, Jim Stewart's article was great. With Beau's helps and all of you, I am close to success in finishing my eddy current metal tester. I thank everyone. I notice that the Prop is the new thing and is powerful. I have the BSII and the SX28, but should I get the Prop for my experiments with coils and what is a good starter kit for the prop?
Thanks,
Cenlasoft
I just ordered the Propeller demo kit. It was a great deal. I'll have fun now,
Cenlasoft