Inductance

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lquadros

Member
I have a question about inductors. From my observations, am I correct in assuming that
1. A ballast used in a florescent tube is to initially start the electron emission. :?
2. An ignition coil in a vehicle is used to generate high voltage sparks. :?
My favorite website below, gives an explanation of inductance. How do I relate it to the above observations? :?:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
"EDIT: and select inductance from side bar to read through."
I know some of you are the very best :) in explaining things. I hope you will do it again.

Thanks in advance for your replies.
 

eric stromberg

Senior Member
Location
Texas
The voltage across an inductor is given as

V = L di/dt

where V is the voltage across the coil (the inductor)
L is the value of the inductor (in Henry's)
and di/dt is the time rate of change of current going through the coil.

So, let's imagine a perfect circuit with ridiculous values.
We have a 1 henry inductor used as the ignition coil of a car.
The battery sends 1 Amp of current through the coil under normal circumstances.
The points are not points at all, they ramp the current down in a controlled fashion.

First case. The points break the current and cause it to go from 1 amp to 0 in 1 full second.

V=(1)(-1) The voltage across the coil is -1 volt

Second case. The points break the current and cause it to go from 1 amp to 0 in 1/10 of a second, making the rate of change -10 amps/second

V = (1)(-10) The voltage across the coil is -10 volts

Third case. The points break the current and cause it to go from 1 amp to 0 in 1/100 of a second, making the rate of change -100 amps/second

V = (1)(-100) The voltage across the coil is -100

Fourth case. The points break the current and cause it to go from 1 amp to 0 in 1/100,000 of a second.

V = (-1)(-100,000) The voltage across the coil is -100,000 volts.

So, in the real world, the 'switch' breaks the current almost instantaneously, making the voltage across the coil almost 'infinite,' but the voltage across the coil is limited by the resistance of real world components so 'infinite' becomes 'real high.'

This is also why 24 V DC relays often create a reverse voltage upon de-energization. This is called 'flyback' voltage and should be mitigated with the installation of a reverse biased diode across the coil or an MOV.

Hope this helps.
Eric
 

rattus

Senior Member
In the ignition system, a capacitor (condenser) is wired across the points to limit the rate of change of current. Without this capacitor, the induced voltage is too high which burns the points prematurely and reduces the ignition voltage.

The fluorescent starter performs a function similar to the points. When the little switch opens, the voltage to the lamp increases to a level which ionizes the mercury vapor. After the lamp is started, the ballast limits the lamp current by means of its inductive reactance,

X = 6.28xfxL
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Good description of the coil developing larger voltages, and I agree that the same effect causes reverse voltages on a relay.

But I've never heard the term "flyback voltage" used for a relay reverse voltage. The origin of the term "flyback voltage" comes from TV sets. A ramp voltage scans the electron beam slowly across the screen from left to right producing the picture. Then the voltage ramps back down very quickly to cause the electron beam to "flyback" to the left side of the screen where it is ready for another scan. (Although the electron beam is actually turned off during flyback).

This voltage is developed by pulses from a transistor applied to a transformer, but it is a very precise controlled voltage.

So I'm just saying I don't think flyback voltage is the best term for a relay reverse voltage. I've always just called it inductive kick, but someone probably knows a better term.

Steve
 

eric stromberg

Senior Member
Location
Texas
You're right about the flyback voltage of a TV. I forgot about this one.
The device connected to the relay coil, either diode or MOV, has been referred to as a "flyback protection" device. I've heard this one for years. Now that you mention it, however, it was probably the wrong term all along. You've got me curious. Now i'm going to have to go back and research where that term came from.

It's kind of like "Dry contacts." This is also a badly misused term. Today, it is being used as "available contacts with no voltage on them." Originally, it simply meant "contacts that aren't mercury wetted."

Eric
 

rattus

Senior Member
Speaking of relays, more than one green engineer has tried to drive a relay with a transistor and has been surprised when the transistor failed immediately even though the current rating was not exceeded.
 

eric stromberg

Senior Member
Location
Texas
OK,
I'm back from checking on the term "flyback."
Thanks for sending me on that mission. I found some great websites dealing with do it yourself high voltage experiments, most starting with the flyback transformer from a tv. One shows a picture of a guy standing on a plastic paint bucket, with a pie tin on top. He is standing on the pie tin with bare feet and the pie tin is connected to his 40,000 Volt generator. If i can find the picture again, i'll post it.

In any event, i found several usages of the word 'flyback' that simply indicated the transient caused by the disconnection of an inductive device. There was even an article that was about 'flyback' voltage present when a USB cable is disconnected from an inductive device.

So, it appears as though 'flyback' is an appropriate term in this case.

Google the phrases:
Flyback transformer
Flyback protection

Eric
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
I never knew that the term flyback applied so broadly. I guess I learned something new. But I still feel a little uneasy about calling it a flyback voltage. But using Rattus's example, I guess I can say its the voltage that can make a transistor flyback to the trash can :)
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
steve66 said:
But I still feel a little uneasy about calling it a flyback voltage.
"Inductive kick back" describes this phenomenon with motor switching.
 
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