**This Ible was written to answer the questions of fellow Ibler, Electorials. I hope it helps.**The flyback circuit is a classic method of generating moderately high voltages. It is not, by any means, the ONLY way, but it was once a very common circuit, and was employed in EVERY CRT based TV or monitor.

It is a VERY clever circuit, because it solves two design problems in the system with one circuit.

The scanning electron beam in a CRT needs to be a.) created and b.) scanned.

All pictures bar two are taken directly from Wikipedia.

**Signing Up**

## Step 1: Creating an electron beam

Wikipedia

SO, we need a high voltage.

## Step 2: Scanning the beam

Well, an electron beam is a current, a current passing through a magnetic field experiences a force at right angles to the direction of both the field and the current.

So if we create a field, at right angles to the beam, we can cause the beam to bend, and scan.

If a current passes through a coil, it creates a field, if the current increases linearly, the field follows, if the field follows, the bending in the electron beam follows linearly too.

....we need a linearly ramping current

Oh, and at the END of the scan, it would be really nice if the trace flew back VERY quickly, so fast you can't see it happen.

**That's the "Flyback"**

Wikipedia: Raster scanning

## Step 3: How ? Charging the primary

Here is the topology:

The flyback TRANSFORMER is something of a misnomer. In a classic transformer, changing currents in the primary induce changing currents in the secondary. In a flyback they specifically do NOT

WHY not ?

See the diode in the secondary ? Current can ONLY flow in the secondary....when none is flowing in the primary.....EH ?

So, lets start the circuit, and switch on the transistor under the primary - here its a switch, S, what will happen ?

Current will start to flow in the primary (Is). Since the secondary has a diode in series with it, NO CURRENT FLOWS IN THE SECONDARY....and current ramps.....linearly.....

Put the coils to deflect the electron beam in series with the primary, and that beautiful LINEAR ramping current (see step 7) will cause our beam to scan left to right smoothly.

Score +1 - we have a linear ramping current.

Oh, and STORED ENERGY IN THE MAGNETIC FIELD = 1/2 x L x I^2 Remember that bit.

## Step 4: NOT charging the primary....

NOW turn OFF the current. What happens ?

WHERE DOES THE ENERGY GO !!

## Step 5: MAGIC !

The great trick is the collapsing field will now induce a voltage in the secondary - moderated by the turns ratio - usually very high, and aided by the fact that because we are taking energy from the field, it collapses very quickly.......

.....so the current in the primary FALLS very quickly, and the electron beam jerks suddenly to the left.....

Score +2

## Step 6: ZAPPED !

## Step 7: Gotchas. Saturation.

TYPICALLY, it would be ~64kHz in a 1024 x 768 monitor.

So, why not slower ?

SATURATION.

???

Just like an electric circuit, where we say that a voltage creates a current: I=V/R, there are magnetic circuits, where we say B=uxH. B is called FLUX DENSITY and its units are Teslas (T) or more likely mT or uT.

H is the field strength, and its related to current by N (number of turns of wire) x I (current)

UNLIKE the electric circuit, there is a maximum limit of H a material can withstand and still give you more B. This is called "Saturation flux density"

The RATE of ramping of the current is fixed by two things: the drive voltage and the INDUCTANCE of the primary.

The inductance is given by the formula L = A

_{l }x n

^{2}

B supports the inductance of the device. So long as B can increase, I's growth is restriced by L.

V= L di/dt. - or the voltage ACROSS the coil is restricted by L and the rate of change of current IN the coil

We can also say that V= n d phi/dt - or the rate of change of FLUX in the coil is proportional to the voltage across it, but flux phi is related to B by phi = B x core area. B is a KEY property.

Flyback operation is VERY carefully adjusted to be in the NON-saturated part of the curve.

**What happens if the core saturates ?****YOU HAVE NO INDUCTANCE !**

*What happens then ?*

What limits the current ???What limits the current ???

**NOTHING LIMITS THE CURRENT, apart from the DC resistance of the switch and the coil !**

So, apply a voltage for too long to the core and the flux rises PAST saturation.....and you say goodbye to your switching transistor.

Very educational instructables!

Keep up the good work!

so... i took a flyback out of an old tube tv, and i'm having a few questions... it's got ten pins, 3 wires, and then one other pin that's wider and flat, i think it might be a ground or sometyhibng. pins 1 and 2 are not connnected to anything as far as i can tell, so i think they're part of the secondary coil(voltage out). pins 3 and five are connected together, about .5 ohms between them, 4 is connected to 6, 7, and 8 and all 4 have about .5 ohms between them. so like... tertiary coils or something? multiple secondaries. probably used to power other things in the television. pins 9 and ten seem to be the primary, have about 1.4 ohms, and are connected. the wire that had the suction cup on it should have been part of the secondary, but my basic multimeter can't read the resistance. it's too high, seems to be connected to nothing. in fact the other 2 wires don't really read as anything either. one of the wires has less insulation surrounding it than the other 2, so i'm assuming it's lower voltage, but the second wire has almost as much as the secondary wire. i think, logically, that the pins 1 and 2 are connected to the secondary wire and the thicker insulated one, and the thin wire is connected to the supposed ground pin. the trouble is, i'm doing a lot of assuming and supposition, which i am NOT willing to rely upon for as high of voltages(and possibly high current if i screw it up) as this can produce. do you have any suggestions upon finding these? ANY AND ALL HELP IS APPRECIATED. thanks everyone.

raise the frequency or limit the current going in to the driver

^{2}If L = 0, because your saturated, you get no energy storage

Running a higher frequency will help greatly - remember they are DESIGNED for 64kHz typically.

Carefully watching the current in the core, and then cutting it off at the maximum value, before saturation is what is needed. This 'ible has the maths you need to work out the F for a given V and L.

Will i be able to power up anything with the give output ?

Thanks for this, it explains a lot.

Is this picture somewhat correct?

it's what I now understand about how it works.

http://www.instructables.com/files/orig/FCU/X6WR/GUQRR77M/FCUX6WRGUQRR77M.jpg

The flux is not proportional, but more like di/dt.

This means it will be zero if the current doesn't change. (a constant 200mA current is also a non changing current)

It's negative if the current Decreases, and positive if it Increases.

If the current increases Linear, the flux will have a constant value

If the current increases quadraticly, the flux will be linear

(Each time one power lower)

Flux is the amount of stuff moving past a point, or more typically through a plane, or through a Gaussian surface.

Guass' Law = Divergence Theorem.

Current , I, is a rate A/s, that is a flow and thus as long as the electrons move throw the surface you have chosen, there is flux.

Flux is positive by convention, if the flow is OUT from the origin and negative if the flow is IN .

Since the flow can be a function of time or some other parameter, we typically integrate to find the total flux. Flux is also a function of space, and we often want to add up all the field lines passing through a plane or sphere.

We almost always use symmetry arguments when integrating flux because fields get too complicated with funny shapes or edges of planes.

good luck !

yeah duh...field lines ....

[:dunce cap:] again

charge not electrons ...I still get this confused obviously...

Is charge carried by the holes ?

NO !! FLUX is ALWAYS present if I is present !!

Are you happy with differential calculus ?

V = L di/Dt or V = n d (phi) /Dt

It all comes from that really. But flux is effectively proportional to NxI

dI/Dt produces dPhi/Dt.

My flyback has 10 pins. I found out which pins are connected by testing if there was a resistance between two pins with a multimeter.

Pins 1, 5, 9 are connected; pins 2, 8 are connected; pins 3, 4, 6 are connected; pin 7 is by itself; pin 10 is by itself.

This seems problematic because there are only two coils on a flyback, but I have 5 different groups of connected pins. Does this mean that something is disconnected inside my flyback?

Do you have any other ways to determine the primary coils besides this http://lifters.online.fr/lifters/labhvps/tht.htm

Steve

where the only limit on current is the small DC current path resistance.

A path that includes the switch, the coil and the power source resistance, which will fuse a silicone semiconductor in an instant and heat the wire insulating encapsulation into a chard coil scrap.

A

Seen anything you'd change ?

Steve