Power op amp suggestions for a coil

[HWguru]

Hi ALL,

I am not an expert in analogue design , and i need to design a power amplifier to derive a air-core coil made up of 40 awg wire to create a strong magnetic field , its total impedance is 5ohm.

I have tried LM1875 , my input frequency is from 22hz to 32Hz square wave , but for some reason it gets hot and there is no output coming from it . The input oscillations are being generated from a 555 , but in future i intend to connect the LM1875 to a pure sinusoidal oscillator .

I need suggestions as to which op-amp are out there with some decent application notes and easy to use , because with LM1875 the TI datasheet dosent give reader with the BOM list , i couldnt figure the voltage rating and type of capacitor to use with it .

Thanks

 

[Audioguru]

A coil with an air core is used for frequencies that are many millions of Hz. Your coil has almost no inductance (how did you calculate an impedance of only 5 ohms?) so it is simply a piece of wire that is shorting the output of the amplifier. For your very low frequency you need a laminated iron core.

A strong magnetic field occurs when both ends of the core are close together so it is in a U shape.

Maybe you can use the U-shaped laminated iron core electromagnet from an old doorbell:

 

[crutschow]

I don't know what a "strong magnetic field" is.
How much current do you want through this 5 ohm coil?

 

[HWguru]

Hi ,

I am not sure what should be the approach , i am trying to make a electromagnetic pulsing PIG or Inspection tool as a small project , these are used to inspect oil or gas pipelines and they travel inside the pipe either by maintaining a strong magnetic field or a pulsating one .

i am not sure they should be air core or have metal core , i was sent these pictures and i cantt figure out their core .
See pictures attached .

Any suggestions ?

 

[FvM]

To start the project, you should have specifications in terms of field strength and field geometry. Then design the coil according to the specification. Finally the driver circuit.

I already stumbled upon the 40 AWG specification in your first post, that's a really tiny magnet wire.

 

[schmitt trigger]

"you should have specifications in terms of field strength and field geometry. Then design the coil according to the specification"

This is absolutely correct. And the most important thing here, will be the type of core magnetic material and its actual shape. This, by far, will be the most challenging part of your project.
For experimentation purposes, I would start with Audioguru's doorbell suggestion.