# Is it possible to reduce the resistance of a voltage source (coil)? If yes, how can I do that? [closed]

First, I should inform you that I am a mechanical engineer, so this is an adventure to me. What can sound as basic to you, is like a new discovery for me.

Second, I would like to introduce you the context I am going to use this: I have set up an experiment in which I have a beam with a fixed end and a free end. The free end has a magnet coupled, which moves inside a coil, in order to transfer energy from my mechanical domain (beam) to an electrical circuit.

Due to my experimental restrictions, I can't attach the coil to the beam and build a proper magnet circuit around it, so in order to improve my electromechanical coupling I have built a coil with 27k turns (using AWG 34), which gave me a 3.3 KOhms resistance and 10.8 H inductance.

Since I am investigating the damping provided by the electrical domain to my beam, I don't care about the inductance, but the internal resistance of my coil is an issue. Finally I come to my electronics question: is it possible to reduce the resistance of a voltage source (coil)? If yes, how can I do that?

I have tried building a Negative Impedance Converter (NIC) so I could achieve a static negative resistance. Apparently, this NIC only works to improve the impedance of current sources, but it is not practical to use it to improve the impedance of voltage sources. I have studied a bit of Norton's theorem to get my equivalent current source and I have tested it with the INIC, but the results were just equivalent to having my coil short circuited, which means the equivalent resistance was the internal resistance (Rs) of my coil (or near it).

So is it even possible to reduce my coil's internal resistance without changing it, just using electronics? I have seen there are another options like gyrators, but I would not like to lose more time to just discover it won't work.

Hopefully you will be able to help me and if you think I should improve my question just ask for more details.

Thanks!

• Wow, that's about 13,000 feet of wire! How did you come up with that particular design in the first place? Obviously, you cannot change the actual resistance of your coil without physically modifying it. But you can mitigate the effects of that resistance by designing an appropriate driving circuit. But you'd have to tell us something about what kind of waveforms (current? voltage?) you want to apply to the coil. But the better answer may well be to build a different coil that has more appropriate characteristics to begin with. – Dave Tweed Jul 24 '19 at 13:12
• You can't change a physical characteristic that is inherent within the materials used in the design. The wire you chose at the size you chose at the length you chose, has a defined physical electrical resistance. It would be better to tell us what issues this resistance is causing you so that we can help with that. – Jarrod Christman Jul 24 '19 at 13:16
• conceptually the simplest way to increase damping beyond short-circuiting the coil is to use a second magnet/coil whose voltage output measures the velocity of the tip of the beam, and use that to force a current through the first coil, at a ratio (effective resistance) you choose with the gain of the amplifier. If that doesn't meet what you're trying to do, perhaps you could tell us what you're actually trying to do. 'Reduce resistance' is a solution, but it might not be the best one for your actual problem. – Neil_UK Jul 24 '19 at 13:20
• Obviously, you've gone too far in the other direction, which is what I was hinting at in the last sentence of my first comment. One way to work around this might be to use a transformer, as @brhans suggests. But we still don't know what your requirements are. – Dave Tweed Jul 24 '19 at 13:31
• You could use a copper pipe instead. Lenz's law and eddy currents will do the job. – Kripacharya Jul 24 '19 at 13:50

Caveat: I think you would be best off using a different design of coil, but if you want a direct answer to the question you asked:

Immerse the coil in liquid nitrogen.

This will reduce its resistance by a factor of 70 or so. The coil will also stay cold and low resistance for a few minutes after draining away the nitrogen, if the presence of the liquid would interfere with your experiment.

This is very much a one-off solution, you wouldn't want to build a product with it, but it sounds like this is a one off experiment, so it might work for you.

Note that liquid nitrogen is a somewhat dangerous substance. It can cause severe burns, it can cause remarkably large explosions if stored in sealed vessels, and if you use/spill a lot at once you can drown in the resulting nitrogen gas. Find someone with experience handling it to show you the ropes.

• +1 for actually warning of the dangers of liquid nitrogen. It's easy to overlook the asphyxiation hazard. – Hearth Jul 24 '19 at 14:23
• +1 from me as well, as I often handle large quantities of LN2 for HALT testing purposes - anywhere from 240 to 720L, at 50PSI - more than enough to be fatal to myself and anyone in the vicinity! The risks are real. – Adam Lawrence Jul 24 '19 at 15:35

Is it possible to reduce the resistance of an existing voltage source (coil)?

Your question however seems to be more than a little confused, you write:

1. The free end has a magnet coupled, which moves inside a coil, in order to transfer energy from my mechanical domain (beam) to an electrical circuit.
This implies you are using the coil as a sensor and reading the voltage produced by the magnet movement in the coil. The impedance (and inductance) of the coil has almost no relevance here providing you use a high impedance amplifier to provide the required gain you need.

2. I am investigating the damping provided by the electrical domain to my beam, I don't care about the inductance, but the internal resistance of my coil is an issue.
This implies you are using the coil as a transducer, and driving the coil with a voltage (power) to damp the mechanical vibrations. Here the coil impedance (both resistance AND inductance) are critically important and should be very low ....allowing more power to be delivered to the mechanical system.
As an example, I've used these to create glass speakers that are quite effective, though the maximum deflection is only about 0.15". The transducers are driven just like a speaker and support around 3-5W of driving power. You could however modify a large base speaker to get voice coil deflections approaching 1" if you are that way inclined, or you could purchase professional Voice coil motors of adequate power and deflection length.
A vpoice coil motor could also apply damping passively (without driving the coil). If you short the transducer terminals, you will apply the maximum damping retardation possible due to recirculating current in the coil (you want the coil to be fractions of an Ohm here). Varying an external resistor applied across the coil would vary the damping factor.

The requirements for the two goals are quite different ....so which one is actually what you want to do?

The coil you have made is fine for #1, but is completely unsuitable if #2 is your goal since you'd need immense voltages to drive the coil.