Friday, June 17, 2016

Understanding how fly-back transformer works

Fly-back transformer is probably the biggest component in CRT television or CRT monitor.

In this article I am going to highlight few things about his component.

Technically speaking FBT is simply a transformer just like any other transformer and the mode of operation is the same (It has the primary side and the secondary side)

below you can see a diagram of internal structure of a fly-back transformer.


For the FBT to work it needs the support of other components and these includes:

1.Horizontal output transistor (H.O.T) Usually all transformers require alternating current (A.C) to work, In FBT this is accomplished by the H.O.T transistor.

In this circuit the H.O.T act as a switch which open and close very fast and by doing so current flow through the primary of the fly-back to the ground and this will cause voltages to appear on the secondary of the FBT.

These voltages includes, HV, focus, and screen voltages among others.  

2. Snubber capacitor. Basically the snubber capacitor is used to suppress (snub) induced high voltages by collapsing magnetic field in the fly-back transformer primary winding which could otherwise lead to destruction of the horizontal transistor (H.O.T).

That is why sometimes this capacitor is also referred to as “safety capacitor” if this capacitor is open or is removed from the circuit it will cause instant destruction of the H.O.T transistor.

So next time you find the H.O.T shorted you should replace it together with the snubber capacitor.

3. Damper diode. Usually this diode is inside the H.O.T transistor as you can see in the internal diagram of the H.O.T. and sometimes it is used separately.

This is usually Fast recovery (FR) diode and therefore if it is used externally should never be replaced with a normal diode.

The damper diode is usually used to protect the H.O.T from reverse breakdown current.

Besides these components, FBT also require two essential inputs in order to function. These includes:

4.B+ supply voltage. This is usually the main voltage from the secondary output of the power supply (S.M.P.S) and it is usually DC voltage. This voltage (B+) enters the FBT and out through the H-out pulse pin to the middle (collector) pin of the H.O.T transistor.

This is usually called the primary coil of the fly back transformer.

5. Input signal. As I mentioned earlier transformers are not supposed to work with DC and therefore that is why we need to change this DC to AC.

To accomplish this we need a switch, in this case the H.O.T which switch this DC on/off very fast to generate AC.

This switching is not done manually but via horizontal signal pulses which switch the H.O.T on/off very fast and this in turn generate voltage on the secondary side of the fly-back transformer.

That’s all for now friends


  1. hi Sir,thank you for your post. I have a problem with the crt tv, it has lost channels and even if I do auto search,the problem still remains. I only got channels only if I play the tv using av connections/dstv. If I am using antenna for analog signalss, I don't get channels. I have already changed EPROM; tuner, polarised caps around the tuner; resistors around the tuner are still testing good; . The screen only shows menu and blue back ground as if there is no arial on it, all the buttons are working perfectly; but no sound; no picture. Any help please sir, this is a Chinese tv.

  2. Hi Rasta, check if the tv is set to blue background "YES" and select "NO"

    Regards Humphrey

  3. Have horizontal lines on the screen

  4. Thanks for the great lesson,, and what should be the value of this B+ voltage in Crt Tvs?