Can a isolation transformer that's fused on the secondary give voltage spikes? - Page 1
Forum section: electronics / beginners

author: Eric_S
author_rank: Regular Contributor
timestamp: June 14, 2023, 09:20:32 am
content: Random musing: Can a isolation transformer that's fused on the secondary potentially give voltage spikes?Say that one of the fuses on the secondary decides to give up the ghost.Will the charged coil on the secondary side feed back the energy to the primary (as the voltage rises), or will it potentially start to fry things on the secondary, if the current can find its way to ground somehow?I'm guessing not as the core is shared. But if you have any inductors on the secondary, that might give you issues if they don't have a snubber diode or such?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 14, 2023, 10:19:47 am
content: Anything connected prior to the fuse could experience a spike - the fuse blowing should disconect anything after it.

author: Eric_S
author_rank: Regular Contributor
timestamp: June 14, 2023, 10:52:21 am
content: Thank you for the reply!There are two fuses, one for each wire.But I interpret you, mikeselectricstuff, as this not mattering?

author: Whales
author_rank: Super Contributor
timestamp: June 14, 2023, 12:32:53 pm
content: EDIT: This is just a rewrite of the OP's postI am very tired.Can a transformer secondary ever act like a flyback whilst the primary is still connected to the wall? I presume any such effects would be massively dampened (to almost nothing) because the primary is shorted into a voltage source and the primary-secondary coupling is very high.(Otherwise: potentially a blown fuse could allow a sudden flyback effect, which then ionises/jumps the fuse and pushed a high voltage onto the second side?)

author: Jwillis
author_rank: Super Contributor
timestamp: June 15, 2023, 02:29:48 am
content: Quote from: Eric_S on June 14, 2023, 09:20:32 amIf the fuse on the secondary side of a transformer "blows" the energy in the secondary coil will not feed back into the primary because the primary and secondary coils remain charged. Its the same as turning off a switch on the secondary side.If there is a coil attached to the secondary side of a transformer and a fuse "blows" the energy in the coil will never become greater than the energy that was applied to it. So although a voltage spike will occur. The total energy will be the same .This is no enough to damage a primary or secondary winding of a transformer The energy will simple dissipate as the field collapses just the same again like flipping a switch.Fly back diodes are applied to coils because the coil discharges in the opposite direction than when being charged. This reverse voltage is what can damage sensitive semiconductors.What does cause problems with coils is Inrush current. This is the moment when the Field is at the minimum and the coil acts like a dead short. This becomes worse at the peak of an AC cycle on the primary side of a transformer.This also leads to a voltage spike until the field become fully charged.Inrush current is also a problem from the charging of capacitors.Any excessive inrush current can damage components in circuit and the coil on the secondary side. The secondary coil is damage because the inrush current is greater than what the coil is capable of.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2023, 02:50:25 pm
content: Quote from: Jwillis on June 15, 2023, 02:29:48 amThis is quite misleading.Energy is not stored in any of the coils, but it's stored in the magnetic field in the core of the transformer.When a current (energy) is pushed through a (usually primary) coil, then energy is added to the magnetic field in the core, and when the amplitude of the magnetic field changes, then it induces a voltage in every coil of the transformer (proportional to the winding ratio).When a fuse in one of the secondary windings blows, then that winding will no longer substract energy from the core, and this changes the rate of energy transfer to & from the magnetic core. Normally a power transformer is being driven from a low impedance source, which has a quite "hard" voltage, and this prevents other transformer windings from generating voltage spikes.Note this is very different from an inductor which has just a single coil of wire. If you attempt to stop the current through that coil at the moment there is energy stored in the magnetic core, then that energy will come out of the core in some way. And usually this happens by outputting enough voltage so it sparks somewhere. Usually it sparks over the connection (fuse, switch) you are trying to open, but it can also be internal such as isolation breaking down, or a circuit designed to absorb that energy (Varistor, TVS, capacitor, etc).

author: Jwillis
author_rank: Super Contributor
timestamp: June 17, 2023, 02:26:50 pm
content: Quote from: Doctorandus_P on June 16, 2023, 02:50:25 pmWhen it is said that a coil is charged it is understood that a magnetic field is present. It's just easier to say that a coil is charged instead of going through the redundancy of reexplaining the how by getting over technical. So to say that a coil is being charged is not misleading at all.What is misleading is that Current is "pushed". At no time can current be "pushed " Current can only move from high potential to low potential as defined by the resistance/impedance of a closed circuit.The field in a transformer only transfers energy by induction, it does not carry energy . The field is a product of electron motion. Electrons carry the charge not the magnetic field. The field of the primary side of a transformer induces electron motion on the secondary side. This creates a secondary field that is at the the same energy potential as the primary side. 1W of energy on the primary side will induce 1W of energy on the secondary side regardless of the coil ratio. The coil ratio only defines the Voltage and Current. But the energy potential on both sides remains the same.When the current path of the secondary coil is severed from a load , both primary and secondary remain at the same energy potential ,so no energy can be induced back into the primary from the secondary.The energy of a coil is dissipated as heat when the circuit is opened. A spark is the high potential energy moving to low potential energy across the dielectric of the air. This is also in the form of heat. When the energy level is reduced to a point where the dielectric strength of the air can no longer be crossed the remaining energy within the coil continues to dissipate as heat.TVS diode and varisters don't absorb energy but convert energy into heat. This heat is dissipated and reabsorbed in the surrounding air or materials.We could do this all day and both still end up with exactly the same conclusion but stated in a different way. So how technical and over complicated do we really need to get ?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 19, 2023, 07:35:24 am
content: Quote from: Jwillis on June 17, 2023, 02:26:50 pmI have never heard of a "coil being charged", the term is meaningless to me. I do agree that writing "pushing current" is ... unconventional, but when working with inductors, voltages do not mean much. In the end, the terminology used is influenced by background, language and which books you read.Quote from: Jwillis on June 17, 2023, 02:26:50 pmThis is completely wrong. You can store energy in an electric field (capacitor) or in an magnetic field (inductor). It is one of the core principles of electricity.https://en.wikipedia.org/wiki/Magnetic_energyIn more detail, have a look at Maxwell's equations, which deal with electromagnetic fields and their energies. (But Maxwell is not a good fit for beginners, so many simplifications and analogs are used throughout literature).In the rest of your description:Quote from: Jwillis on June 17, 2023, 02:26:50 pmThe energy does not go directly from the "primary" to the "secondary" side. The energy goes from the input to the magnetic core, and then from the magnetic core to any of the coils around that core. This is not an "redundant technical detail", it is a fundamental principle of how transformers work. Transformers do not work without a magnetic core to transfer the energy from one coil to another.Quote from: Jwillis on June 17, 2023, 02:26:50 pmPartly it is indeed semantics, and no need for nitpicking about details, except for the magnetic energy. You simply can not understand transformers by skipping that part. The simplification of "energy stored in a charged coil" is good enough for a single inductor, but it does not work anymore in a transformer with multiple coils.Going back to the original post:Quote from: Eric_S"Charged coil" is also used, but the interaction with "the other coils" in a transformer (such as which of the coils would need a snubber) becomes easier to understand once you realize that it is the core itself where the energy is stored, and not in any of the coils.

author: Jwillis
author_rank: Super Contributor
timestamp: June 19, 2023, 05:23:06 pm
content: The core of as transformer only confines and directs the magnetic field. This improves the efficentcy of the field by keeping the lines of flux close together and pointed in the right direction which maximizes magnetic induction. But you do not need a core to make a transformer. There are coreless transformers and use the same principle as I described. Even if you had 2 wires side by side in a vacuum, the chargeing of one wire will induce a charge in the second wire. No core is required to transfer energy from one wire coil to the next.Its a Conversion of energy. Electromagnetic fields are generated by charges, currents, and changes of the fields.And a electromagnetic field can generate a charge, current, and changes in a magnetic field. Does the primary coil have energy? Yes! Does the field have "energy"? Yes! But different. It's a electromotive force which is the force that causes electrons to flow through a conductor. Does the field induce an electromotive force into the secondary? Yes! Are the energy potentials the same across all stages of energy conversion in a transformer? YES! And at no time can the energy potential in any stage of conversion become greater than what is applied. At no time does a charge exist in the field.The wire in the coil contains the charge. That's the part we want to know about.Its unnecessary to to repeat all the steps of how a transformer works.To say that the primary induces a charge in the secondary by a alternating magnetic field is good enough.If a person would like to delve deeper into the conversions of energy in a transformer, then all the power to them.No pun intended. I'm no physicist an I don't expect others be one either.And I admit that composition and proper English has never been my strongest subject.So basic knowledge on how things work in electronics is good enough for the novice.If someone wants to add or clarify or needs clarification, then that's fine.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 19, 2023, 06:28:18 pm
content: Please stop making a fool of yourself by writing half truths.Quote from: Jwillis on June 19, 2023, 05:23:06 pmEven if you have two straight wires in vacuum, you still have an "air core" (vacuum core, the difference is negligible) and the coupling is defined by the length and distances between the wires.So, again, this is just plain wrong:Quote from: Jwillis on June 19, 2023, 05:23:06 pmI don't know what you consider a "charge" in this context. May I call it "energy"?Current flows through a wire, but the electromagnetic energy is contained in the electromagnetic field around the wire, and not in the wire.https://www.youtube.com/results?search_query=does+energy+flow+through+a+wireQuote from: Jwillis on June 19, 2023, 05:23:06 pmNo, it's not (always) "good enough".It already failed with OP's question of where the "voltage spikes" are induced.An example where it fails is with magnetic core memory, where magnetic energy is stored for an indefinite time in a piece of ferrite.Another example to make it clear is with a transformer with a magnetic shunt. For example a microwave transformer, or even a welding transformer with an adjustable magnetic shunt.Yet another example is a ferrite core antenna.That the electromagnetic energy is contained in the field around a wire is also extremely important to realize with transmission lines, differential pairs on PCB design, and in any PCB design that works with higher frequency signals. Above approximately a few kHz the "loop area" and it's impedance dominates the DC resistance of a copper track, and therefore also with the GND plane in PCB design.There simply are too many shortcuts you take by using your oversimplified model. Sure, sometimes it's "good enough", but you also have to know it's limitations, and go to a more detailed model when it's needed.

author: Jwillis
author_rank: Super Contributor
timestamp: June 19, 2023, 08:12:07 pm
content: All you do is cherry pick what you want to read completely out of context just to create an argument. I won't waste my time with you any further.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 19, 2023, 10:59:38 pm
content: I also already guessed we would never become friends.It's quite off-topic, but I am a bit curious whether a 3rd party would care to comment.Am I being to dense, or is Jwillis too stubborn?