A resistor that never gets hot?

[John Heath]

I am going to take a cd4016, quad FET switch and tie 2 switches to one side of  a 1 u Henry coil and the other 2 to the other side. I will then drive the cd4016 at 100KHz in such a way that the 1 u Henry coil is changing polarity 100,000 times a second. A DC current will always see this 100,000 per second polarity switching 1 u Henry coil as reactive limiting the flow of DC current. As it is reactive there is no heat. A kind of endless perpetual counter EMF. Would this opposition to DC current be a resistor that never gets hot?

[xavier60]

When an AC voltage is applied to an inductor, the inductor alternately stores energy from the voltage source, then returns it back to the source, less losses. Twice per cycle.

[AG6QR]

Would this opposition to DC current be a resistor that never gets hot?

No.

You mention switching the polarity at 100,000 Hz, so you're not talking about DC anymore.

Reactance is not the same as resistance.  A resistor always has its voltage and current in phase.  A reactance does not.  Ideal reactance doesn't dissipate energy, and doesn't get hot, but its impedance is non-resistive, so it's not a "resistor that doesn't get hot".

In addition to the non-resistive reactance, there will, of course, be at least some parasitic resistance, which will dissipate energy and cause the resistor to get warm.

The only resistor that doesn't get warmed up is one that has no voltage across it, and no current passing through it (one of those conditions implies the other).

[AndyC_772]

This doesn't sound like a question about parasitic components, but about pure theory.

John, can you please post a rough schematic of what you have in mind? How exactly is your coil connected, and where does the current flow?

Your assertion that "A DC current will always see this 100,000 per second polarity switching 1 u Henry coil as reactive limiting the flow of DC current" doesn't make any sense to me. A DC current flows through a pure inductor just like it would through a straight wire; there's no resistance at all.

[John Heath]

Would this opposition to DC current be a resistor that never gets hot?

No.

You mention switching the polarity at 100,000 Hz, so you're not talking about DC anymore.

Reactance is not the same as resistance.  A resistor always has its voltage and current in phase.  A reactance does not.  Ideal reactance doesn't dissipate energy, and doesn't get hot, but its impedance is non-resistive, so it's not a "resistor that doesn't get hot".

In addition to the non-resistive reactance, there will, of course, be at least some parasitic resistance, which will dissipate energy and cause the resistor to get warm.

The only resistor that doesn't get warmed up is one that has no voltage across it, and no current passing through it (one of those conditions implies the other).

I was not as clear as I could have been when describing this circuit. The applied power to this circuit xould be a simple DC 5 volt source from a 7805. The only thing that is switching at 100,000 times persecond is the 1 u Henry coil. You may assume idea condition of no capacity and no resistance.

Another way to describe this circuit would be the exact opposite of a class D amplifier. A high frequency square wave can drive a inductor without high current as the reactance of the inductor prevents it. In this case it is the opposite with a DC source that is driving it with the only high frequency AC being the inductor switching it polarity using a cd4015 quad FET to accomplish this.

[John Heath]

This doesn't sound like a question about parasitic components, but about pure theory.

John, can you please post a rough schematic of what you have in mind? How exactly is your coil connected, and where does the current flow?

Your assertion that "A DC current will always see this 100,000 per second polarity switching 1 u Henry coil as reactive limiting the flow of DC current" doesn't make any sense to me. A DC current flows through a pure inductor just like it would through a straight wire; there's no resistance at all.

Can not give a diagram as I have not built it. It is a thought experiment only. It does not have to be a cd4016 quad FET switch. Anything that flips the polarity of the inductor 100,000 times a second will do.

Would this be a resistor that never gets hot?

[John Heath]

A coil has an inductance, but it also has a capacitance and resistance. When you are switching it, assumimg your waveform is ideal (no DC bias), you will still have a reactive current, and that current across the parasitic resistance will generate heat.
Also, the capacitance also generates heat due to capacitive loss during voltage commutation.

Yes there would be efficiency issues in the real world where switches are not perfect nor are inductors. Too soon for this. First does it work as a heatless resistor in theory assuming idea condition. If so then worry about building it in the real world.

[wraper]

A coil has an inductance, but it also has a capacitance and resistance. When you are switching it, assumimg your waveform is ideal (no DC bias), you will still have a reactive current, and that current across the parasitic resistance will generate heat.
Also, the capacitance also generates heat due to capacitive loss during voltage commutation.

Yes there would be efficiency issues in the real world where switches are not perfect nor are inductors. Too soon for this. First does it work as a heatless resistor in theory assuming idea condition. If so then worry about building it in the real world.

It does not work as resistor. Energy that is apperantly consumed at one moment of time is returned back at another. If you take an ideal inductor without losses, net energy consumed will be 0.

[westfw]

This is essentially how fluorescent ballasts work.   The Tube itself, once running, would happy pass enough current at normal mains voltage to damage itself, and needs a current limit (like an LED.)  A "magnetic ballast" is essentially a huge inductor that appropriately limits the current at the 60Hz line frequency.  An "Electronic Ballast" (also as used in CFLs) steps up the frequency and can thus use a smaller inductor to produce the same AC current limit.
(A ballast also contains some magic to handle startup, when the tube needs the filament heated, and/or a temporary higher-voltage supply to start the tube discharging.  But once operating, the ballast is essentially an "low-loss AC resistor"...)

[retrolefty]

This doesn't sound like a question about parasitic components, but about pure theory.

John, can you please post a rough schematic of what you have in mind? How exactly is your coil connected, and where does the current flow?

Your assertion that "A DC current will always see this 100,000 per second polarity switching 1 u Henry coil as reactive limiting the flow of DC current" doesn't make any sense to me. A DC current flows through a pure inductor just like it would through a straight wire; there's no resistance at all.

Can not give a diagram as I have not built it. It is a thought experiment only. It does not have to be a cd4016 quad FET switch. Anything that flips the polarity of the inductor 100,000 times a second will do.

Would this be a resistor that never gets hot?

 Generally it works better if you design and draw a schematic before you attempt to build a circuit.
 
I really don't understand clearly what you are proposing. The reason that schematics work so well
in electronics is that's it's true that one schematic in worth a thousand of your typed words.

[DaJMasta]

I don't see how you'd pass current through a resistor and prevent it from heating up - if you were oscillating both ends in phase, then you'd never have a potential across it, no current would flow, and you'd have effectively no loss, but if you've got current - AC or DC, positive or negative - you're going to have power dissipation in the resistor.


Just because the average voltage is zero doesn't mean the resistor won't act as a load - the potential would have to be the same on both sides all the time for no power to be lost, but then no current would be flowing through it, and why would you bother putting the resistor in, it would have no effect (in the limited scope of the exercise, of course it could be used for abnormal conditions or fixed impedance at different frequencies or what have you).

[Marco]

Just run it through a simulator with ideal components, your impedance will approach infinity.

If you want to emulate a resistor with a switching circuit you have to dump the energy somewhere,  if you want to avoid generating heat the most likely suspects are stored charge in a capacitor or chemicals in a battery.

[John Heath]

This is essentially how fluorescent ballasts work.   The Tube itself, once running, would happy pass enough current at normal mains voltage to damage itself, and needs a current limit (like an LED.)  A "magnetic ballast" is essentially a huge inductor that appropriately limits the current at the 60Hz line frequency.  An "Electronic Ballast" (also as used in CFLs) steps up the frequency and can thus use a smaller inductor to produce the same AC current limit.
(A ballast also contains some magic to handle startup, when the tube needs the filament heated, and/or a temporary higher-voltage supply to start the tube discharging.  But once operating, the ballast is essentially an "low-loss AC resistor"...)

Interesting. Yes I see your point. I put a small neon bulb on a variac one day then turned the variac up until the the neon voltage break down was met , around 70 volts. When this happened the neon bulb blew a 2 amp fuse on the variac. The little neon bulb never worked the same after that. It was a little blackish inside. I can conclude from this that a larger fluorescent tube is using the inductive " magnetic ballast" as it's current limiting resistor.

This could be considered proof of concept that a heatless type of resistor is possible. However one would have to do it the opposite way to know for sure. By opposite I mean 150 volts DC supply for a standard fluoresent tube then switch the polarity of a standard ballast transformer 60 times a second. Is alternating polarity of the basllast transformer with a 150 volt DC power source the same as alternating alternating the power source ,  AC power , and not changing the polarity of the ballast transformer? If so then a general heatless type of resistor should be possible with a polarity switching inductor. 

[John Heath]

This doesn't sound like a question about parasitic components, but about pure theory.

John, can you please post a rough schematic of what you have in mind? How exactly is your coil connected, and where does the current flow?

Your assertion that "A DC current will always see this 100,000 per second polarity switching 1 u Henry coil as reactive limiting the flow of DC current" doesn't make any sense to me. A DC current flows through a pure inductor just like it would through a straight wire; there's no resistance at all.

Can not give a diagram as I have not built it. It is a thought experiment only. It does not have to be a cd4016 quad FET switch. Anything that flips the polarity of the inductor 100,000 times a second will do.

Would this be a resistor that never gets hot?

 Generally it works better if you design and draw a schematic before you attempt to build a circuit.
 
I really don't understand clearly what you are proposing. The reason that schematics work so well
in electronics is that's it's true that one schematic in worth a thousand of your typed words.

I understand your point of no diagram. A 5 volt reg 7805 with the 5 volts going to a 1 u Henry inductor then a 5 ohm resistor to ground.  That's it. Turn it on and the resistor should have 5 volts at 1 amp , 5 watts of heat as the inductor has 0 ohms resistance to DC current. Now switch the polarity of the inductor back and forth. The faster you switch the less current to the resistor as inductance is like a flywheel resisting a change in direction. The frequency of the 1 u Henry polarity switching should set the effective resistance of the inductor. A heatless resistor of sorts.   

[Conrad Hoffman]

A pure reactance, capacitive or inductive, will have no losses. Unfortunately, there is no way to supply and switch the current with zero ohms in series. Look up the 2-capacitor problem. Now, there are other ways to do things. See https://www.av8n.com/physics/capacitor-transfer.htm

[T3sl4co1l]

Reactive power at DC can be a useful concept, but strictly by definition, that means nonsense.

Reactive power is most commonly introduced as an AC steady state property.  At 0Hz, the average value of the imaginary component is always and forever zero.  So, there cannot be reactive DC power.

Let's take a more general definition:

Instantaneous power is p(t) = v(t) * i(t).

Real power is the time average of p(t).

Reactive power is the averaged difference between p(t) and the average.  (I forget exactly how; RMS I think?)

When only DC is present, real power is simply the product of V and I, whether you take the average or RMS figures.  (Same thing.)

When AC and DC are present, you must take account of them separately.  You can have real DC power, but AC reactive power -- like in a switching supply, where the AC power is ~zero (because the ripple voltage (for a CV supply) is negligible) but real reactive power is being cycled back and forth (the inductor in a typical converter handles reactive power about 1/4th the DC power converted).

Tim

[AG6QR]

Would this opposition to DC current be a resistor that never gets hot?

No.

You mention switching the polarity at 100,000 Hz, so you're not talking about DC anymore.

Reactance is not the same as resistance.  A resistor always has its voltage and current in phase.  A reactance does not.  Ideal reactance doesn't dissipate energy, and doesn't get hot, but its impedance is non-resistive, so it's not a "resistor that doesn't get hot".

In addition to the non-resistive reactance, there will, of course, be at least some parasitic resistance, which will dissipate energy and cause the resistor to get warm.

The only resistor that doesn't get warmed up is one that has no voltage across it, and no current passing through it (one of those conditions implies the other).

I was not as clear as I could have been when describing this circuit. The applied power to this circuit xould be a simple DC 5 volt source from a 7805. The only thing that is switching at 100,000 times persecond is the 1 u Henry coil. You may assume idea condition of no capacity and no resistance.

I think you were quite clear.  But what's important isn't the nature of the original power source, what's important is the nature of the power presented to the load.  AC can be converted to DC, and DC can be converted to AC.  If you switch the polarity of electricity presented to the coil, no matter how you do it, the coil is not seeing DC anymore, even if the original power source was a battery.

Putting aside parasitics for the moment, an inductor is not a resistor.  The voltage across a resistor is proportional to the current.  The voltage across an inductor is proportional to the derivative with respect to time of the current (that derivative, and thus the voltage across an inductor, is always precisely zero by definition under DC conditions).  These two are not the same, and understanding the difference is fundamental to understanding general electronics and AC circuit theory.

This inductor is not at all resistive, so how could you consider it a resistor that doesn't get hot?

[John Heath]

A pure reactance, capacitive or inductive, will have no losses. Unfortunately, there is no way to supply and switch the current with zero ohms in series. Look up the 2-capacitor problem. Now, there are other ways to do things. See https://www.av8n.com/physics/capacitor-transfer.htm

Cool link. There is a less complicated way to transfer 99 % of the energy from one condenser to another. Let us start with 100 uf condenser "A" charged to 150 volts and condenser "B" 100 uf 0 volts. One diode in series with a inductor from conderser A to B. The inductor avoids the infinity current problem of just shorting the condensers together. The diode prevents the current from going into resonance from A to B then B to A.With all this in place 99 % of the energy from condenser A can be moved to condenser B.

[John Heath]

Would this opposition to DC current be a resistor that never gets hot?

No.

You mention switching the polarity at 100,000 Hz, so you're not talking about DC anymore.

Reactance is not the same as resistance.  A resistor always has its voltage and current in phase.  A reactance does not.  Ideal reactance doesn't dissipate energy, and doesn't get hot, but its impedance is non-resistive, so it's not a "resistor that doesn't get hot".

In addition to the non-resistive reactance, there will, of course, be at least some parasitic resistance, which will dissipate energy and cause the resistor to get warm.

The only resistor that doesn't get warmed up is one that has no voltage across it, and no current passing through it (one of those conditions implies the other).

I was not as clear as I could have been when describing this circuit. The applied power to this circuit xould be a simple DC 5 volt source from a 7805. The only thing that is switching at 100,000 times persecond is the 1 u Henry coil. You may assume idea condition of no capacity and no resistance.

I think you were quite clear.  But what's important isn't the nature of the original power source, what's important is the nature of the power presented to the load.  AC can be converted to DC, and DC can be converted to AC.  If you switch the polarity of electricity presented to the coil, no matter how you do it, the coil is not seeing DC anymore, even if the original power source was a battery.

Putting aside parasitics for the moment, an inductor is not a resistor.  The voltage across a resistor is proportional to the current.  The voltage across an inductor is proportional to the derivative with respect to time of the current (that derivative, and thus the voltage across an inductor, is always precisely zero by definition under DC conditions).  These two are not the same, and understanding the difference is fundamental to understanding general electronics and AC circuit theory.

This inductor is not at all resistive, so how could you consider it a resistor that doesn't get hot?

You are correct. A "resistor" that does not get hot was a poor choice of words on my part. I will rephrase. A dynamically controlled reactive inductor as a replacement for a resistor to reduce heat loss.

[kalel]

Is not that in some way what e.g. a step down converter does (I could be wrong, still newbie)?

[John Heath]

Just run it through a simulator with ideal components, your impedance will approach infinity.

If you want to emulate a resistor with a switching circuit you have to dump the energy somewhere,  if you want to avoid generating heat the most likely suspects are stored charge in a capacitor or chemicals in a battery.

Do you have a link to a freeware simulator?

[John Heath]

Is not that in some way what e.g. a step down converter does (I could be wrong, still newbie)?

I think you are right on the money. The principle is  the same with a twist of flipping the inductive polarity around instead of the current. I guess you could say a mirror image of a inductive voltage down converter.

[timb]

Just run it through a simulator with ideal components, your impedance will approach infinity.

If you want to emulate a resistor with a switching circuit you have to dump the energy somewhere,  if you want to avoid generating heat the most likely suspects are stored charge in a capacitor or chemicals in a battery.

Do you have a link to a freeware simulator?

LTspice

[John Heath]

Just run it through a simulator with ideal components, your impedance will approach infinity.

If you want to emulate a resistor with a switching circuit you have to dump the energy somewhere,  if you want to avoid generating heat the most likely suspects are stored charge in a capacitor or chemicals in a battery.

Do you have a link to a freeware simulator?

LTspice

  thanks

[John Heath]

A coil has an inductance, but it also has a capacitance and resistance. When you are switching it, assumimg your waveform is ideal (no DC bias), you will still have a reactive current, and that current across the parasitic resistance will generate heat.
Also, the capacitance also generates heat due to capacitive loss during voltage commutation.

Yes yes very interesting blah blah blah.

There is no problem with multi tasking a different subject within a thread especially if it is welcomed by the one that started the thread , myself. With this in mind I and I suspect many other members would like to know how you manage to build your own home brew mass spectrometer out of off the shelf parts? How could you miss with americium 241 ? How does this business of a ion breeze work. The questions are endless. If you happen to come back and read this please knock yourself out sparing no details on the home brew mass spectrometer.