How does large over currents passes through a device during turn-on - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: December 30, 2018, 09:56:16 pm
content: I understood how does large inductive over voltage spikes(the voltage across inductor summed up with the source voltage)generated during turn off of a devicebut now i want to knowHow does large over currents passes through a device during turn-on??If possible please explain with picture

author: fsr
author_rank: Regular Contributor
timestamp: December 31, 2018, 01:23:10 am
content: Well, let's go with a classical one: transformer, rectifier and capacitor PSU. When you turn the device on, the capacitor is completely discharged, so as the current "thru" a capacitor is I = C * (dv / dt) and the voltage potentially goes from 0 to the peak AC voltage of the transformer very quickly, the current is going to be really large. Diodes don't limit current, so it's going to pass thru them into the capacitor.Discharged capacitors are probably the #1 cause of inrush current in circuits, i believe. Incandescent bulbs also have very low resistance when cold, but who uses them anymore?

author: Dave
author_rank: Super Contributor
timestamp: December 31, 2018, 01:30:46 am
content: If we are talking about transformers, residual flux and having the transformer connected to the line at the wrong (read: less than ideal) moment, will cause the core to saturate and the current will go sky high.

author: IDEngineer
author_rank: Super Contributor
timestamp: December 31, 2018, 10:15:42 pm
content: I'm presuming your question is "Why does 'inrush' current occur?" where inrush means a surge of current at turn-on. If so, let's consider the opposite question: Why doesn't the steady-state current remain high after turn-on? An easy, non-math answer is that something changes after the inrush current. If you were dealing with a pure resistance, the current wouldn't change - there would be a single, constant value based on Ohm's Law. So what can we think of that changes with current flow? How about reactance?Let's start with capacitive reactance. As others have answered, current flowing into the total system capacitance (which we presume to be discharged) can, for easy analysis, be thought of as an almost dead short, so the inrush current is limited primarily by the current source's ability to provide it. As the current begins to charge the total system capacitance, its voltage increases, and the difference between the capacitive charge voltage and the charging voltage is reduced, which reduces the current flowing into that capacitance. Eventually the two voltages achieve parity and capacitive inrush current drops to zero, leaving you only with the steady-state current required by the rest of the system. (Again, I must emphasize that this is a simplified explanation.)Next we have inductive reactance. For an easy thought experiment, an inductor can be considered a dead short at DC. So when you first turn on the current, the inductor presents basically zero impedance to the current flow and you have very high inrush current. But the effect of current in an inductor is to create a magnetic field, which in turn (pun intended!) increases the reactance of the inductor and thus its "resistance" to further changes in current. Just as with capacitive reactance above, you thus have an effect that initially permits near-infinite current to flow but then begins resisting that current as its magnetic field builds up.Thus you have an easy two answers to the question "What can we think of that changes with current flow?" If your load were a pure resistance, inrush current wouldn't occur. But since all real-world systems have reactance too, inrush current is very real.Does that help?EDIT: Look up "Cuk Converter" and check out the first inductor. When the converter's switch is closed, that inductor presents a dead short to the input current source. How can that possibly work? The answer is the same as above... inductive reactance limits the current flow once the magnetic field is established. Otherwise that first inductor would act as a very expensive fuse.

author: Jwillis
author_rank: Super Contributor
timestamp: December 31, 2018, 11:06:43 pm
content: Every conductor will have an inherent capacitance and resistance Everything will have a resistance and because when voltage and current pass through any device it will create a magnetic field producing an inherent inductance. In most cases these inherent parasitic properties are small and don't cause problems .As we all know you can not pass current without a voltage and you can not pass voltage without a current. A circuit will only pass as much current as it requires at a specific voltage.Now during turn on a capacitor will draw maximum current until it begins to charge.In the first millisecond of charge it acts like a dead short because of it's inherent ESR (Equivalent Series Resistance). This creates Over Loading. This is important when dealing with semi conductors like transistors because they have an inherent capacitance.During turn on of a circuit ,there can be transient voltages caused by the saturation in inductors,most noticeable in transformers, that can exceed the voltage expected until the magnetic field stabilizes.These Transient Voltages can be several times the except-able rating of the component for an instant.This causes excessive heat for an instant because of Ohms law. Watts = Voltage X Current.Now take a transistor that can pass a peak current at a specific voltage .This is determined by its FBSOA (Forward Bias Safe Operating Area). If the voltage is excessive to its peak current it will over heat and burn out.This can happen in an instant.So how do we control these potentially catastrophic events in a circuit.By controlling the "inrush current". It's much easier to control current by simply limiting it with a resistor .So even if theirs a voltage spike,the current is kept very small during power up , there by limiting the damaging heat produced in those first instances.So to answer your query ,Current can't pass with out voltage and Over current is the same thing as Over Loading.

author: IDEngineer
author_rank: Super Contributor
timestamp: January 01, 2019, 12:49:45 am
content: Quote from: Jwillis on December 31, 2018, 11:06:43 pmThis thread is in danger of getting more technical that I believe the OP was hoping for, but it's worth mentioning that simply inserting a series resistor also increases the output impedance of the source supply, with potential consequences for voltage regulation during transient events. It's Engineering - everything is a tradeoff!

author: Kasper
author_rank: Frequent Contributor
timestamp: January 01, 2019, 04:09:22 am
content: Capacitors delay changes in voltage and inductors delay changes in current.When you turn on your circuit, the caps have 0V across them. It takes time for them to charge. During that time there can be a large voltage difference and low resistance between supply and caps. I = V/R, when V is big and R is small, I is very big.

author: MrAl
author_rank: Super Contributor
timestamp: January 01, 2019, 02:26:00 pm
content: Hi,These currents are usually referred to as "inrush current". That is the initial current that surges into the device when first turned on.The amplitude is related to the device impedance during startup and what phase the AC line is at the time of the turn on.There are various devices that exhibit this phenomenon and is usually considered to be a problem.A few devices:Capacitor input filters (used for power supplies)incandescent light bulbs (about 1/8 the resistance before they get hot)Motors (spin-up current is higher than run current)Each of these has their own properties that cause the inrush current.Steps are often taken to limit the inrush current level. Surge limiting devices are often connected in series to help lower the inrush current level.Sometimes diodes will blow out with repeated inrush surges when charging capacitors. They last for many turn on cycles but then one day one of them blows out and has to be replaced.

author: Jwillis
author_rank: Super Contributor
timestamp: January 03, 2019, 12:15:00 pm
content: "Inrush Current" or "Over Current" are really inaccurate ways to describe whats really going on.Current can't "travel" on it's own .It must have a Closed circuit for it to "travel". What is really going on is Over Loading during turn on. This over loading occurs anywhere in a circuit where there is a capacitance or an inductance. Because both inductors and capacitors act like dead shorts just at start of charge And the load decreases until they are fully charged. At full load the full amount of current available to those components passes through them causing heat.This over loading is what can cause the damage to other components in the circuit. When turn occurs at near or at the 180 degree cycle Of an AC circular sine wave ,over saturation of the main transformer occurs distorting the field of the core creating high voltage transients which pulls up the available current which further increases the heat created at the source of load.All this heating shortens the life of all components in the circuit.At lower voltages and current,the amount of over loading is quite small and usually won't require current control .It becomes more serious with very high voltages and high currents.Since motors are essentially inductors ,the coils also act like dead shorts until a field is formed.Ac motors will act just like a transformer because the field also becomes distorted causing over saturation creating transient voltages there by drawing more current from the source at start up.Reminds me of a 600 volt motor that was started with full mechanical load (a loaded water pump)and a faulty soft start.Blew the thing and the shed it was in to pieces.Light bulbs are also dead shorts until the filament begins to heat creating a higher resistance .A high resistance has low load and low resistance has a high load.

author: MrAl
author_rank: Super Contributor
timestamp: January 03, 2019, 02:45:18 pm
content: Quote from: Jwillis on January 03, 2019, 12:15:00 pmHi there,"Inaccurate" to who? :-)"Inrush current" is a common way to describe this even though current usually circulates. It is like many other terms and phrases we as humans use, it is an abstract concept, and abstract is always so darn abstract :-)If you really want to get down into the mud here though, there are some people who will argue that we cant say "current flow". That's because they recognize the fact that "current" is already something that flows (electrons) so saying "current" is like saying "electron flow" and so saying "current flow" is like saying "electron flow flow" which sounds strange because of the two instances of the word "flow".So this is what terminology does to us sometimes. In spite of the ones that seek to change all this, phrases like "current flow" and "inrush current" and "surge current" are accepted and more to the point are understood to mean the same as if we just said for example "current".