Measuring the power of sine and square waves.

Gramps

Senior Member
Unless the frequency is changing rapidly, a simple RC filter, droppng resistor, and two ADC readings should get what you want from any Picaxe.
Thanks. rq3!
The frequency is not changing at all so this "should" work.
Here's the story behind the magnetic bed.
My wife's sister discovered a man on the east coast of Florida who invented a magnetic bed that, he said, could cure many diseases. She traveled from South Carolina to visit this guy and asked my wife to join her to get some "treatments".
Unimpressed by his claims but curious, we chauffeured them over to a hole-in-the-wall clinic about 120 miles from our home.
He said he didn't charge but only took donations to keep out of trouble with the law.
The "bed" was a leather couch about 3 inches thick connected by a cable to a control box. After my wife was comfortable, he energized the machine. Handing me a neodymium magnet, he instructed me to hold it in my palm and and pass it over my wife's body. Even at 18 inches above the bed surface I could still feel the magnet vibrating! After a 45-minute treatment we headed for home. My wife felt that the bed did her some good, so we returned the next week and stayed overnight for two more treatments.
Motels, meals, fuel, and a donation was getting expensive.
Time to head to the lab and do some experimenting.
Pumping 5 amps through an auto relay got our Harbor Freight neodymium magnet vibrating nicely 12 inches away, for perhaps 20 seconds, before the Magic Smoke appeared! Success!!
Needing bigger coils that could handle the current, we constructed the ones we are now using.
We're saving a bundle not driving to the East Coast every week and now I've become the family hero!
 

AllyCat

Senior Member
Hi,
No we are not transmitting energy but we are developing a PEMF therapy bed.
(Note:The same precautions taken with MRI machines )
It seems you are trying to transmit energy (in the form of a changing magnetic field into the human body); you are just not attempting to "harvest" that energy (hence the NMI precautions). A possible concern about your "apparatus" is that the frame of the chair might be acting as a "Short-Circuit Turn", disrupting any changes is the magnet field?

However, it seems you should be using a (pulse) frequency below 50 Hz, if the treatment caused a magnet to vibrate subjectively! Fortunately, you can probably use your Class D Power Amplifier to drive the coils down to "d.c" (i.e. pulses of one or both polarities), if you eliminate the a.c. coupled Pre-Amplifier and drive the Power Amp. directly.

..these very low inductance coils. At 1200 Hertz you can basically just consider them to be pure resistors,
Have I made an error in my calculations? To me, a reactance of about 6 ohms in series with a resistance of 0.9 ohm is NOT a pure resistor. Also, a "pure resistor" can't generate a magnetic field (only heat), so the OP would appear to be wasting his time. Of course the resistive element of the coils will cause them to get warm, but if that is an aim, then it's most efficiently done with d.c..

Certainly the OP can use two peak detectors (with low pass filtering) into two ADCs to measure the peaks of the current and voltage through/across the coils (and then calculate the RMS value IF it's a Sine Wave). The current may indicate the (relative) magnitude of the magnetic field created, and the voltage whether the amplifier is running close to its maximum output swing (i.e. overload and/or distortion of sine waves). But they cannot (necessarily) be used to calculate the "Power", because the V and I (sine waves) are not in phase with each other (so cannot be simply multiplied).

Cheers, Alan.
 

Gramps

Senior Member
). A possible concern about your "apparatus" is that the frame of the chair might be acting as a "Short-Circuit Turn", disrupting any changes is the magnet field?
The aluminum frame of the lawn chair does not seem to change the field, at least using a rough mapping with the magnet. Accurately "seeing" the field is one later objective.

However, it seems you should be using a (pulse) frequency below 50 Hz
The literature says they used 40 hurtz for depression.
I was able to snap a photo of the control box on the bed on the east coast. It was set on 62 hurtz square wave.
 

Gramps

Senior Member
Certainly the OP can use two peak detectors (with low pass filtering) into two ADCs to measure the peaks of the current and voltage through/across the coils (and then calculate the RMS value IF it's a Sine Wave).
Peak detectors?
Not sure how to do that
What if we note the current the amplifier is drawing and divide by 2, then measure the voltage output of each channel to get IV?
 

hippy

Technical Support
Staff member
I am all on-board with "push AC through some coils and make people feel better" but ... why are we needing to measure power in any form ?
 

tmfkam

Senior Member
Is that the "round thing?" I could not get Google to find it.
Big chunky box you can just see under the desk. Try "AVS7000 power conditioner" as your search term. Turned up for me as AVS7000, though I did use Google Images to help...
 

julianE

Senior Member
would it make sense to measure the magnetic field using one of many magnetic sensors. even a hall effect sensor should work.
PEMF devices used to be very expensive i did a quick look on amazon and the prices have dropped a lot. i remember years back there were products to measure stray magnetic fields, it was claimed that emf waves were very harmful to the body. the wheels keep on turning.
 

Gramps

Senior Member
measure the magnetic field using one of many magnetic sensors. even a hall effect sensor should work.
Yes, thought of that. That would require 2 more leads to each half of the bed. We had hoped to do this measurement without the extra leads.

What if we note the current the amplifier is drawing and divide by 2, then measure the voltage output of each channel to get IV?
This idea from post 46 seems the most practical. We are moving from Florida to south Carolina this month, so we've already packed up the bed. More testing later in May??? and will report back!
emf waves were very harmful
We are reproducing the same kind of emf- magnetic energy used in MRI machines everyday which are 1 to 3 tesla's more powerful than our equipment!😉
Gramps
 
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