AC and DC Magnetic Fields

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***Reply to a post from another forum that you might find useful...

" Well Gentlemen, good and sound concepts you present but in actuality

that is not what happens with respect to my DC pulser. Let me explain

in my opinion of what is happening...

Let's start with basic inductor theory. Take a 9 Volt battery and

connect it to a coil through a pushbutton switch. Close the switch

and the current rushes into the inductor and creates a magnetic

field. (The maximum current will be governed by the resistance of the

coil/wire combo (DCR) and the inducative reactance of the coil which

tends to oppose the primary current.) Keep the switch closed and you

will get a high current flowing into the coil, which will keep it

energized until either the battry runs flat or the switch is opened.

Measure the Gauss and you will get a reading. It will be a DC reading

since the coil has remained energized and current is flowing in one

direction as dictated by the polarity of the battery. You have just

created an electromagnet! Same type is used at a junk yard to pick up

scrap metal. Does the magnetic field ever reverse polarity while the

battery is still hooked up to it? No. Never. At the instant the Gauss

field is energized and rises from zero to maximum (dI/dT or change in

Current over a change in Time) you will have the poetntial to INDUCE

a current if there happens to be a conductor lying across the coil

face...like a loop of wire in series with a current meter. You will

ONLY see an induced current at the moment the coil gets energized.

There will be ZERO induction once the coil has reached a maximum

storage of current. In fact the coil will behave identical to a

permanent magnet. If you lay a coil of wire over a permanent magnet,

you will not induce any current into the wire whatosever. It is only

when you move the magnet, and the lines of flux cut the conductor

will you then induce current. Back to the inductor. Once the inductor

is filled with electrons and the battery remains tied to it, the

dI/dT component disappears and induction stops. The magnetic field in

the coil will be a certain polarity depending upon the winding

direction fo the coil and the polarity of the applied current. Since

the current source is a DC battery, North will be at one side of the

coil. Now let's open the switch. That's where the fun begins.

As soon as the switch is opened, the magnetic field collapses (at the

same rate as it was created when the switch was closed. dI/dT will

once again dominate.) When the field collapses, an equal and opposite

current will begin to flow out of the coil. The magnetic polarity

will reverse as a consequence of the reversing current flow. Now,

with the switch OPEN where does all this reverse current flow to?

It's got to go somewhere. This high current immediately creates a

large ARC across the contacts of the switch. That is why the battery

operated MPGs get burned out solenoid contacts. If one where to place

a DIODE in reverse connection across the contacts of the switch the

reverse current will flow into the diode instead of across the

switch. This would greatly reduce the acring effects of the current

and save the switch contacts. And, that is exactly why you see CLAMP

DIODES connected across the coils of solenoids and relays.

As the magnetic field collapses, you get a second induction of

current into the coil of wire. Repeat this process over and over and

you will mimic an AC generator. The faster you can open and close the

switch, this higher the frequency of the AC induced current. Do it 60

times per second and you just created 60 Hertz!

Now, instead of using a battery to charge the inductor use a non-

polarized capacitor. Now, if you first charge the capacitor with you

9 volt battery, then use the same switch to energize the coil you

will get an interesting occurence. Since the capacitor is non-

polarized it can accept reverse currents. At the moment you close the

switch the current stored in the capacitor will rush into the " empty "

inductor, charging it. As soon as the current fills up the inductor,

the capacitor will become " emptied " . The inductor now dumps its store

into the capacitor. The capacitor gets filled, and then dumps its

charge into the indcutor. This process will continue until the losses

prevent any more current flow. The capacitor and inductor will " ring "

with decaying intensity until the current runs out. This is a simple

parallel resonant circuit. The ringing frequency will be a direct

function of the inductive and capacitive reactance, or XL and XC. XL

= 2*PI*F*L and XC = 1/2*PI*F*C

PI=3.14, F=Freqency in Hertz, L=Inductance in Henries, C=Capacitance

in Farads. When XL=XC ringing will occur at F frequency.

Now, what would happen if you used a polarized capacitor instead of a

non-poarized capacitor? When the current begins to reverse and flow

from the inductor to the capacitor, this will reverse bias the

capacitor and either blow it up or shorten it's life considerably.

But hey, we have a way to prevent this " kick-back " current from

reverse-biasing capacitor...when can use a clamp diode! Indeed, that

is precisely what I use on my Mag Pulser to ensure I never reverse-

bias the main capacitor. But there's more!

I use an SCR (Silicon Controlled Rectifier) to dump the charge from

the main capacitor into the coil. Being a rectifier, it acts like a

one-way switch. This helps to ensure the coil gets isolated from the

capacitor after the charge is dumped from capacitor into the coil.

Here is where I get my DC magnetic field being generated. After the

SCR triggers, current from the capacitor will flow in one direction

only into the inductor until the current reaches a minimum level. At

this point the SCR closes and the current stops. At this precise

moment, the inductor wants to reverse the current flow, but the SCR

prevents this since it is a one-way " valve " . The inductor's magnetic

field simply dissipates. Any cross-conduction current from the time

lag of the SCR closing is totally clamped by the diode. You might see

an iddy-bitty reversal of current, but for all intents of purposes it

is non existent.

That is why SOTA's MPG is a purely DC generated magnetic field.

Some manufacturers deliberately use a non-polarized capacitor in

parallel with the coil to actually create a ringing, resonant action.

The result is a MPG like the 5K pulser which has a decaying ringing

frequency of 5,000 Hertz. Raise the capacitor and you lower the

frequency. Lower the capacitor to raise the frequency.

I hope this is helpful.

Cheers!

Russ :>)

SOTA Instruments Inc.