Strength of a pulsed electromagnetic field

coilelectromagneticresistance

Forgive me if my question sounds a bit naive:

I have built a coil (no solid core) out of an insulated copper wire, and I calculated that it has 8 ohms of resistance. I tried it and it gave me the result I expected. I am using a professional (meaning – it can stand a lot of abuse) audio amplifier as a source of (pulsed) power. The amplifier automatically adjusts to different resistance – from 2 to 8 ohms. Currently it is running at 8 ohms.
Now, if I built another coil using exact same specs, EXCEPT 50% shorter wire, the coil would end up having 4 ohms of resistance right? That would further mean that it should give me a more powerful magnetic field I hope.
If I then connected this new coil to my audio amplifier, the amp would adjust itself to the new resistance, and hopefully would be able to deliver more power. This amp is rated at around 800w in bridged mode, at 8 ohms and much more (1200 W I think) at 4 ohms.

Am I right to believe that lower resistance coil will give me a stronger magnetic field all other things being equal?

Any input is appreciated!

Best Answer

If you are trying to attract an object magnetically, the magnetic force from an electro-magnet is: -

Force \$= \dfrac{(F_M)^2\mu_0\cdot A}{2 g^2}\$

Where

Fm is the product of amps and turns
A is the cross sectional area of the solenoid
\$\mu_0\$ is \$4\pi\times 10^{-7}\$
g is the gap to the magnetizable object you wish to attract

Note that Fm is governed by amps and turns.

Related Solutions

Electronic – Detecting a low pulse from an electromagnet induction

There is a little amplifier I am very fond of which I have used with dynamic mics. Since your coil situation is much like the one faced by someone trying to amplify a dynamic mic, I encourage you to try this circuit and see if produces a usable signal. Just replace the "microphone" (speaker) with your coil.

http://electrosuite.com/audio/bc547-dynamic-microphone-amplifier.html

The lower transistor acts as a common base amplifier, which is fairly unusual. It has a current gain of less than 1, but it does provide significant voltage gain. The upper transistor is in a common collector configuration, so it has essentially unity voltage gain, but the current gain is good. In short, this first amplifies the voltage, then the current, to give you a strong output signal. Although you will likely need a second stage after this, that should be much easier.

EDIT:

Concerning your noise issue, do you know where the noise is coming from? Sometimes I just have a power supply that's super noisy and ruining everything. I like low budget solutions, because of, well, my budget, so I usually test this by running the circuit on batteries. If things start magically working, then it's time to work on the power supply.

Using audio amplifier speaker output to drive an electromagnet

In this case, you are being led astray by what you don't know. A speaker coil has a (more or less) constant impedance because it drives a cone against air resistance. In your case, with no speaker attached, it behaves as an inductor, not a resistor. From the numbers you provide, it will have an inductance of ~ 5 milliHenrys (.005 H). For an inductor, the current (and the magnetic field) will go down as frequency goes up. The relevant equation deals with impedance, not resistance, represented by Z.

                    Zl = 2 x pi x L x f

and this will equal 8 ohms at f = 250 Hz (about). So, for frequencies below about 250 Hz, the total impedance (sqrt(Zl^2 + R^2)) will increase with frequency, until at 250 Hz your current amplitude will be about 70% of its DC value. Further increases in frequency will continue to increase impedance and current will drop further as you increase the frequency. You probably don't need to hear about phase shifts.

The effect of all of this is to reduce power dissipation in your coil as frequency increases. But particularly at low frequencies you do need to keep a very close watch on your temperatures. Dissipating 800 watts in a coil the size you're considering stands a pretty good chance of melting the interior layers. It certainly nay cause the bobbin to melt or catch fire.

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