Domestic Hi-Fi Amplifer as Monopolar Coil Driver - See Diagram

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
I would like to power a wire coil to produce EM fields at audio frerquencies for equipment testing. I am looking for advice on the setup as proposed in my diagram attached below. The purpose of the transformer is to remove the DC component to produce a monopolar emitted field.

audio_coil_driver.png

Of particular interest is a recommended value for the ceramic resistors and use of transformer taps since I do not want to overload the amp.

At the moment, I am "playing it safe" by using the standard 600 Ohms, and lowest transformer turns ratio, but I do not want to unnecessarily limit the coil's field intensity. Can someone suggest an accurate method for optimising this part of the design?

Any general advice on using this type of amp to similarly drive resistive end loads would also be greatly appreciated.
 
Last edited:

dendad

Joined Feb 20, 2016
3,332
Why are you driving the transformer via the speaker?
The transformer can connect to the amplifier directly. If you want the speaker as well, hook it to the other channel.
And really, the coil could probably be driven from the amp directly too, without the transformer.
The way you have done it will severly limit the current through the coil.
There should not be a DC output from the amp if it is working correctly.

Have you any info on the coil's electrical characteristics?
 

BobTPH

Joined Jun 5, 2013
2,266
What is a monopolar field? Last I looked, magnetic monopoles are a theoretical object that has not been found in nature.

Bob
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
Good point. I interpreted the term liberally as the TS wanting a uni-directional field rather than an alternating one.
Yes, that is corrent. I want a magnetic field from the coil that does not reverse polarity.

As I understand, the Hi-Fi amp's output will swing +/- around the speaker ground. I refer to that as "bipolar". So my use of the term "monopolar" corresponds to what used to be called a "single ended" signal.

In other words, it is a signal that has no reversals of current from + to - and back again, and therefore would not cause the coil's field to alternate polarity.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
Why are you driving the transformer via the speaker?
The transformer can connect to the amplifier directly. If you want the speaker as well, hook it to the other channel.
And really, the coil could probably be driven from the amp directly too, without the transformer.
The way you have done it will severly limit the current through the coil.
There should not be a DC output from the amp if it is working correctly.

Have you any info on the coil's electrical characteristics?
The speaker is there so I can hear the actual audio signal feeding the coil. It could be driven from the second stereo channel, but I was reserving that for another purpose.

Part of my original question was how to optimise power to the coil, while not endangering the amp. The combined series load of the 8 Ohm speaker and 8 Ohm transformer primary in series shown in my diagram was presumed to provide a margin of safety.

The only purpose of the transformer is to isolate the coil from the current reversal of the amplifier's push-pull output stage, for reasons previously explained. If there is a way to do this without the transformer, please advise.

The coil is a flat spiral wound from 60m of stranded cable. The latter is a single insulated conductor of the type used in shielded microphone cables. I estimate the resistance to be around 2 Ohms. Inductance would be very low.

The 600 Ohm resistance on the transformer's secondary is conservative, being the line-matching standard. As you infer, it will limit current to the coil. I would certainly like to reduce this, but am unsure how to calculate an alternative value.
 

dendad

Joined Feb 20, 2016
3,332
The transformer will ensure you get a reversing field, the opposite of what you want.
A better way to get your non reversing field is to use a variable power supply. A home audio amp is not suitable for your use. They outpus AC, not DC, more or less.
There is no doubt one could be modified to suit, but a power supply is the thing.
It would help is you could specify the current and frequency of the varying DC you want to drive the coil with.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
The transformer will ensure you get a reversing field, the opposite of what you want.
A better way to get your non reversing field is to use a variable power supply. A home audio amp is not suitable for your use. They outpus AC, not DC, more or less.
There is no doubt one could be modified to suit, but a power supply is the thing.
It would help is you could specify the current and frequency of the varying DC you want to drive the coil with.
There is a reason why I prefer to use a hi-fi amp. I am aware of their limitations for my unusual application.

What I need is to convert the amp's AC output signal to one that only swings only positive from ground potential. As you pointed out, I had incorrectly assumed that isolating the transformer's secondary side from the amp's ground return would achieve this.

Is there really no solution using passives?

If I can accept a half-wave signal, would the amp successfully drive the 8 Ohm winding of the transformer and coil through a diode?

In response to your request for clarification, the frequency to the coil is a complex audio signal, not a single frequency. The current should be as much as the 35W per channel amp can safely provide to the coil.
 

dendad

Joined Feb 20, 2016
3,332
You could well try feeding the amp out via a diode and see how it goes.
Do you need a base set magnetic field or just a pulsing one? More info on you actual application will help.
One way is to feed the coil from a power supply, but via the transformer. Then the amp could drive the transformer and modulate the current to the coil. You may have to look into transformer saturation conditions. The power supply voly=tage will determone the set magnetic flux, and the amp out would vary it.
It is really pretty hard as the full details are unknown.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
You could well try feeding the amp out via a diode and see how it goes.
Do you need a base set magnetic field or just a pulsing one? More info on you actual application will help.
One way is to feed the coil from a power supply, but via the transformer. Then the amp could drive the transformer and modulate the current to the coil. You may have to look into transformer saturation conditions. The power supply voly=tage will determone the set magnetic flux, and the amp out would vary it.
It is really pretty hard as the full details are unknown.
Thank you for the suggestions. That gives me a few options to work with.

As I have come to learn, simple sounding concepts in electronics are often not that simple to put into practice.

I did consider an experiment to feed a 20KHz signal into the other channel of the amp, half wave rectify the output and use it as a kind of offset "ground" potential on the centre tap of an isolation transformer with split secondary. The wanted signal from channel one would then be applied to the transfomer's primary and come out the other side as some kind of mix with an offset component of half its amplified voltage.

If that works, it might be OK since I am only interested in the coil emitting detectable frequencies within the lower audio range.
 
I am not absolutely sure of this, but I am pretty certain that once you have sent a signal through the coil into the air it does not matter whether it is 0-10 volts peak to peak or -5 to +5 peak to peak--the receiving coil will get a signal centered on 0 volts.
 

Alec_t

Joined Sep 17, 2013
10,924
You could drive the coil via a full-wave bridge rectifier, using an auxiliary supply to bias the coil current, like this:
CoilDriver.PNG
 

BobTPH

Joined Jun 5, 2013
2,266
Your term EM field is contradictory. If you really mean field, there is no electric field component, so it is a magnetic field. If really you mean EM wave, making the current a single polarity just wastes power without any effect on what is radiated.

So, are you interested in the magnetic field, or radiated waves?

Bob
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
I am not absolutely sure of this, but I am pretty certain that once you have sent a signal through the coil into the air it does not matter whether it is 0-10 volts peak to peak or -5 to +5 peak to peak--the receiving coil will get a signal centered on 0 volts.
That sounds true. But my aplication is not aerial transmission. I am placing plants directly on the coil to test for growth effects at various frequencies and waveforms. The plants will sense the field polarity reversal if the coil is powered by a bipolar signal. That introduces a secondary influence I was hoping to avoid, and makes it impossible to apply a singular north or south magnetic field.
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
You could drive the coil via a full-wave bridge rectifier, using an auxiliary supply to bias the coil current, like this:
View attachment 158493

Your term EM field is contradictory. If you really mean field, there is no electric field component, so it is a magnetic field. If really you mean EM wave, making the current a single polarity just wastes power without any effect on what is radiated.

So, are you interested in the magnetic field, or radiated waves?

Bob
The magnetic field. Please see reply to "whitehaired". Thanks
 

Thread Starter

Sir Kit

Joined Feb 29, 2012
44
You could drive the coil via a full-wave bridge rectifier, using an auxiliary supply to bias the coil current, like this:
View attachment 158493
I was hoping for a solution that would require no voltage adjustment for offset, etc. Makes it difficult for non-technical people to replicate effects. I will certainly keep your diagram for future projects.
 
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