Switching Mode Supply Wave-forms

[KeepItSimpleStupid]

OK my version of back EMF. In general I see it applied to motors, particularly DC motors and it can get confusing.

Take a typical DC motor with a winding resistance Rw.

That DC motor when driven mechanically will produce a voltage proportional to speed. In fact it can be used as a tachometer.

When you start to load that motor we now have V = Vm - (Im * Rw), since we are drawing current. I is non zero. the current is actually in the SAME direction, but it's OPPOSITE to what's being applied. It doesn't reverse polarity. The current does because it' being generated. So this generated current happens because the motor is moving drops across the resistance of the motor.

I*R is and EMF or V and -(Iw*R) is opposite to what's being applied. But the sign of the current is what changed,

You can design a speed regulator by maintaining the calculated terminal voltage using the current and the constant Rw (winding resistance). In theory the resistance changes with temperature a bit.

 

[kinarfi]

As I see the inductor, it is a devise that resists a change in current flow, and generates a magnetic field which opposes that change, if the voltage is applied for enough time for current to become stable, then is is just a piece of wire, remove the voltage, (open the switch) and the magnetic field collapses and generates a voltage of the same polarity with a higher amplitude in an attempt to keep the current at it's present state, theoretically, it could go high enough to arc across the switch if there is no other path for the current.
PS, back EMF is the magnetic field collapsing and opposes the applied voltage by being the same polarity, in motors, where the voltage is not switched, if the RW is 10 Ω and the voltage is 100V the motor would draw 10 amp, but the back EMF might be 90 volt at no load and current would be 1 amp, as you load the motor, the back EMF drops to 80 v and the current rises to 2 amps, more load, more current. In motors, back EMF is created by the rotating armature, slow the armature and it will draw more current, drive it faster and it becomes a generator via higher back EMF.
In SMPS, the collapsing magnetic field turns the inductor into a "generator"
IMHO

 

[Electroenthusiast]

hi Crutschow, Nigel and Dr. Pepper,
Thanks for the replies. The link reads so...

The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This current keeps the bulb lit for a period of time even though the switch is open. In other words, an inductor can store energy in its magnetic field, and an inductor tends to resist any change in the amount of current flowing through it.

My doubts are these.... How will the current through resistor(bulb) start when coil has built-up field? In my opinion, the coil is still has lower resistance than the resister(bulb).
2) Are these same inductors used in mechanical flashlights? Or instead, can a capacitor be used?
3) How can i isolate two batteries from each other, if i use them for the same circuit?

 

[Electroenthusiast]

hi KISS, Kinarfi,

I hadn't read your replies. The point is the back emf is of the same polarity as the emf from the cell. When the switch is open, will there be an arc? I've not heard of any arc at low voltages like that of AA batteries. The flyback diode, which we would use is right. But still have doubts on it's essentiality at low voltages.

Again, i don't know why my school had though me that inductor's stop AC.

 

[KeepItSimpleStupid]

two little rules:
The voltage across a capacitor can't change instantaneously.
The current through an inductor can't change instantaneously.

I'm sure one could intentionally wind an inductor where at 1.5V there would be a huge spike, It's not the battery, but the inductor properties.

 

[kinarfi]

Have you ever heard of ELI the ICE man, E leads current in an inductor, Current lead voltage in a capacitor. In SMPS, there's no contacts to open, it's all solid state and the voltage difference isn't that high. The voltage going high enough to arc is theoretical, not practical. And where I learned the theory was on Current transformers used on power lines, but I never got one of them to arc either.

 

[dr pepper]

An inductor isnt a piece of wire when it doesnt have a magnetic field stored within it, its a resistor, and can be at first a high resistance.

You can isolate 2 power sources using a dc to dc converter, part of these is a coupled inductor, there are 2 inductor windings on the same ferrite, power is transferred between them magnetically so there is no electrical connection.

A torch (flashlight, I'm english), wouldnt have a inductor in it if the light source is a filament bulb, however a lot of torches these days use a led, to drop or boost the voltage from the battery to the led a inductor is often used as part of a switching supply, they make custom chips just for this.

 

[Electroenthusiast]

Thanks for the replies.

@drpepper
I'm unsure how a coupled inductor helps in isolating. Isn't it same as transformer? Transformer only convert AC (?)
regarding the flash light, why cant the capacitor be used?

@KISS, kinarfi,
i need to brush up the working of inductor in depth for those to understand. still unsure why current and voltages have delayed waves.

 

[dr pepper]

I think your trying to run before walking, I suggest you read up on basic switching supply techniques.

However a coupled inductor is exactly like a transformer, only its designed as an inductor, there is a primary and a secondary winding, these 2 are not connected to each other in certain dc to dc converters, power from the source is used to 'charge' the inductor in the form of a magnetic field caused by its windings, this is done by rapidly switching on and off power to the primary using a transistor (this is where 'switching' comes from), this 'charge' is then taken off the secondary end, rectified by 4 diodes back into dc and then supplys the load.

These can be usefull in applications where you need to power a device without an electrical connection to it.

Not sure what you mean about the flashlight and capacitor.