Yes, but many newbies choose DC thinking that it is the simpler easier option. They probably hope that someone else can do the calculations for them (or just tell them the answer, without knowing what the question is).Wavebourn said:
AC wiring produces both magnetic and electric fields. Which causes the most trouble depends on context.
SMPS vs Transformers.
I feel your position.
It is quite something to disconnect (disassociate?) the relationship between that Great Magnetic Conundrum (the power transformer) with all its fussy windings and limit values, and just use switching mode power supplies as functional blocks. Since I'm about as old as dirt, my question to you is how are the SMPS's at filtering out their internal switching noise; on a similar vein, how are they at encapsulating their RFI equivalent noise?
Back when copper was cheap and iron essentially free, the Big Power Transformer accomplished a rather otherwise-hard-in-the-vacuum-tube-era conversion of line power to the various internal needs of “a box”, be that a LoFi radio, an equally lo-fi television, a color TV or a fancy-schmancy HiFi integrated amplifier. Primary, many secondaries.
Technically, rectification is a “hard load” for a transformer. The larger C1 (if C-loaded, not L-loaded) the more “peaky” the transformer current-over-time demand. Might be a hundred or two milliamps in DC draw, but 10× to 25× that in peak rectification amperage. Since all losses in the final analysis are ohmic, its the comparison of "10 amps for 1 second vs 1 amp for 10 seconds".
So therefore
Demonstrating how peaky rectification can lead to a whole lot more in-transformer wiring ohmic heating, than the steady state or RMS (sinusoidal) resistance load case.
It is one of the reasons why the Old Masters used relatively small C1 capacitors for their capacitor-load FWB supplies. Its also why they liked modest sized chokes after C1 instead of a resistor-filter. Lower DC resistance for a substantial amount of A/C ripple filtering.
Just saying,
GoatGuy
_______
PS: Is it against forum rules to ask what brand of SMPS you like using? I'm eager to dig into some spec sheets.
I feel your position.
It is quite something to disconnect (disassociate?) the relationship between that Great Magnetic Conundrum (the power transformer) with all its fussy windings and limit values, and just use switching mode power supplies as functional blocks. Since I'm about as old as dirt, my question to you is how are the SMPS's at filtering out their internal switching noise; on a similar vein, how are they at encapsulating their RFI equivalent noise?
Back when copper was cheap and iron essentially free, the Big Power Transformer accomplished a rather otherwise-hard-in-the-vacuum-tube-era conversion of line power to the various internal needs of “a box”, be that a LoFi radio, an equally lo-fi television, a color TV or a fancy-schmancy HiFi integrated amplifier. Primary, many secondaries.
Technically, rectification is a “hard load” for a transformer. The larger C1 (if C-loaded, not L-loaded) the more “peaky” the transformer current-over-time demand. Might be a hundred or two milliamps in DC draw, but 10× to 25× that in peak rectification amperage. Since all losses in the final analysis are ohmic, its the comparison of "10 amps for 1 second vs 1 amp for 10 seconds".
E = IR
P = IE … ≡ I²R ≡ E²/R and when time is involved
W = Pt
W = I²Rt
P = IE … ≡ I²R ≡ E²/R and when time is involved
W = Pt
W = I²Rt
So therefore
W = 10² amp × 1 Ω × 1 sec → 100 joules … vs …
W = 1² amp × 1 Ω × 10 sec → 10 joules
W = 1² amp × 1 Ω × 10 sec → 10 joules
Demonstrating how peaky rectification can lead to a whole lot more in-transformer wiring ohmic heating, than the steady state or RMS (sinusoidal) resistance load case.
It is one of the reasons why the Old Masters used relatively small C1 capacitors for their capacitor-load FWB supplies. Its also why they liked modest sized chokes after C1 instead of a resistor-filter. Lower DC resistance for a substantial amount of A/C ripple filtering.
Just saying,
GoatGuy
_______
PS: Is it against forum rules to ask what brand of SMPS you like using? I'm eager to dig into some spec sheets.
Maybe with a relay, controlled with a low noise logic as CMOS and a 24Cxx EEPROM to memorize the last state.
The DC magnetic field due to a single loop of DC current is
Hdc = 4 π i(DC) / 9 l ... (*)
Then, the possibility of magnetization increases with DC current and hence the circuit to change the polarity is only needed with high current filaments.
If filaments are made as a double helix, there is a cancellation mechanism, which is not perfect, and yes the resultant magnetic field is parallel to the axe of the double helix, but this is only a mental representation, the magnetic field follows the corkscrew rule as on any current line.
I repeat, for a single loop (N=1) if you apply a time varying voltage Uac, the magnetic field is
Bac = (Uac x 10⁸) / [√2 π f S] ... (**)
It has nothing to do with AC current, however there is an analogous of equation (*) for the Hac magnetic field, but, the constitutive relation
B = μ H
In vacuum, μ=1 and then
B = H
The apparent contradiction is due to you must take the analogous of equation (*) as an approximation and some parameters are arbitrary (the magnetic circuit length, l, par example)
In transformers it is easy to assign the right parameters, in a current loop don't.
The relation between the electric field E and the magnetic field B is given by Maxwell equation
∇ x E + (1/c) ∂B/∂t = 0
Using Stokes theorem, Leibniz theorem for integrals and the definition of electro motive force, you obtain the equation (**)
I 'bet' (nothing) that the massive DHT heaters have a lower temperature at DC 5V than at AC 5V in DHT due to self inductance heat dissipation at AC voltages...
Anyways, Poplin, at 12.6V drop across the heater the ac current in the heather is half down from 6.3V. magnetic fields only exist due to current, voltage is only a potential and cannot cause any magnetic field.
The only way magnetic field propagate is by mutual inductance... there is also a shield in the tubes which can be used in many ways to reduce M...
Raising heater voltage has many beneficial effects, it is easier on the transformer, less current, and reduce the cathode/element potential.
Remember, tubes were designed for AC elements from AtoZ. If using DC expect many surprises, I am sure using a CLRC filter you can get good results with a DHT if you soft start the voltage (remember the element has no means of protection against over current at cold start with NO self-inductance for DC.
Anyways, Poplin, at 12.6V drop across the heater the ac current in the heather is half down from 6.3V. magnetic fields only exist due to current, voltage is only a potential and cannot cause any magnetic field.
The only way magnetic field propagate is by mutual inductance... there is also a shield in the tubes which can be used in many ways to reduce M...
Raising heater voltage has many beneficial effects, it is easier on the transformer, less current, and reduce the cathode/element potential.
Remember, tubes were designed for AC elements from AtoZ. If using DC expect many surprises, I am sure using a CLRC filter you can get good results with a DHT if you soft start the voltage (remember the element has no means of protection against over current at cold start with NO self-inductance for DC.
Exactly (in bold). It takes moving charges to induce the 'magnetism' part of electromagnetism. Charge tension AKA voltage is highly likely to force conduction and thus magnetic field genesis, directly in production of both ohmic conduction and inductive/capacitive 'reactance' conduction.
GoatGuy
Higher heater voltages are definitely a great way to build when doing AC heaters in spaces where one may expect compromises to be made, less current through the wires will reduce the field radiated. For some instances (parallel operation of many output pentodes, like sweeps with hungry heaters) going to a 12, 25, or 36 volt filament can be a Good Idea. Like in a big OTL, or parallel push pull, or some ungodly large voltage regulator.
Yes, except Zener. And if you have 2 of 6.3V windings, better make 2-diode rectifier, for lower voltage drop.
From Maxwell's equations, a time varying voltage Uac will produce a time varying magnetic field Bac, and a time varying current Iac will produce a time varying magnetic field Hac.
For the simple case of an infinite straight line of current you can find a simple solution for the magnetic field Hac, if the line of current is more complex, to find a solution for Hac is a nightmare, and you will have the false understanding that more curren gives more magnetic field Hac.
Fortunately, Maxwell's equations are consistent, and if you can find a solution starting from voltage, the solution starting from current must be consistent, i.e. B = µ H
It seems to me that you have never designed a transformer, Maxwell's equations are from about 1861, and the first transformer is from about 1885, people who design transformers calculate the magnetic field Bac with the ubiquitous
Bac = (Uac x 10⁸) / [√2 π f S N]
Did you know that in a power transformer, the magnetic field Bac reaches a maximum at no load -minimum current-? Crazy huh?
Maxwell's equations are a set of consistent equations, and does not exists a "magnetism" part of electromagnetism, both phenoms, electric and magnetic are interrelated. Moving charges can also produce electromagnetic waves.
Does not exists in physics a thing like "charge tension" voltage is defined as a potential difference.
Wire and heater are two different systems, if you reduce the AC current the wire will reduce the radiated magnetic field, but the increased AC voltage will increase the radiated magnetic field by the heater, and the grid is close...
DC has less compromises, but AC intermodulation sounds warmer, or less efforted, or something alike...
AC and DC heater have the same electromagnetic field strength in a 'straight' wire. The difference is that 60hz is fully amplified and audible noise versus the DC which induce only a DC, filtered, not amplified, with a small residual AC noise and 120hz from the diodes.
In theory, one should not be able to tell the difference with an indirect heating tube.
In theory, one should not be able to tell the difference with an indirect heating tube.
https://www.electronics-tutorials.ws/electromagnetism/electromagnetic-induction.html
This is explaining how induction works with DC.
Induction, I understand, is when a current flows through a conductor. The current can be AC or DC or a multiple/combination.
This is explaining how induction works with DC.
Induction, I understand, is when a current flows through a conductor. The current can be AC or DC or a multiple/combination.
That is not induction, this is induction
If you don't take my word, see here
Electromagnetic induction - Wikipedia
For DC
∂B/∂t=0
Then
∇ x E = 0
And no induced EMF, i.e. no induction
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You are right, to induce electricity you need AC, DC only cause magnetization, an electromagnetic field which polarizes magnetic objects.
To have electrical induction you need AC absolutely.
edit: this changes nothing to the fact that the greater the current the greater the AC inductive field.
To have electrical induction you need AC absolutely.
edit: this changes nothing to the fact that the greater the current the greater the AC inductive field.
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Well yes as a matter of fact in the amp thd analysis this is what the final product of the heaters looks like.
It is fascinating, out of context, the ramp for low frequencies caused by the power supply impedance.
I found an article on the DHT not built for DC heaters and indeed the heater elements are actually drawing less amps, less hot! this means that probably you have to raise a little the voltage for DC supplies to get the same performance than AC, but this is only trivial, I suspect this a non-issue.
DC heaters reduce 2 to 10 x the life of the filament in x-ray tubes!!!
Common X-ray Tube Failure Modes
Why series tube heaters is running with scissors and a study on the constant current model of the heaters, the most complex parts of the tubes.
The constant current user inside vacuum tube heaters,
Expecting a drop of anode current of at least 20% for non DC designed DHT!
http://www.roehrentest.de/Gleich-Wechselstromheizung_EN.pdf
http://www.roehrentest.de/Gleich-Wechselstromheizung_EN.pdf
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