Flyback secondary resistance?

[nix85]

Does anyone know what is the approximate resistance of the flyback secondary coil? It would be best if you have the same BSC26 flyback but any model in this range will give good approximation.
Since diode forward resistance is insignificant compared to the coil resistance, if you have one with burned or removed diodes even better.
My guess would be between 50 and 200 Ohms but i may be way off.
I know i could measure the voltage drop with an external resistor, but my 12V PS is not enough to make secondary diodes conduct, that would take at least 18V.

[sm0ky2]

Is that from an LG tv? (or chinese generic similar)


My understanding of this with transformers and
'dynamo style' generation inductors:


Over time the surface of the wires becomes 'pitted'
which increases the surface area
eventually this causes an increase in distance of
the shortest path between the 2 ends.
Thus, the resistance increases over time until
failure of the device or breakage of the wire from heating


I believe a brand new one should read 8 Ohms to ground when not powered.

[nix85]

It's Chinese.

I have no idea what you mean that wires become "pitted", secondary is totally
encapsulated in plastic and resin, nothing like that should even theoretically happen.

8 Ohms? Secondary is thousands of turns of like 0.2mm thick wire.

It must be more. I mean i made coils of 250 turns of 1mm diameter
wire and they're 1.5 Ohm and we're talking 10 times more turns of much thinner wire.

[sm0ky2]

Pitted might be the wrong word, its' a microsurface effect you cannot see
happens over time/use, and is the primary reason coils go out of spec in a
non-short non-fault device failure.


Theres a range of increasing resistance until the device no longer works properly
If there is breakage (but still partial contact or arc-over), you will also see an
increase in resistance.
If there is a short, resistance may become lower than spec.


Theres also a slue of less common deterioration such as non-linear heating
which can result in deformation of windings, cracking of enamel, etc.


If the part is used, its best to find the data sheet and perform a diagnostic
To see if you are still within the acceptable range.

[nix85]

"If there is breakage (but still partial contact or arc-over), you will also see an
increase in resistance.
If there is a short, resistance may become lower than spec."

I think this is self evident, one of those sky is blue things.

Microsurface effect caused by what, heat? Heat can create a short or open but i don't see it creating "pitted surface".

Quote

while flyback transformers can on occasion be blown due to a failure elsewhere in tv or monitor power supplies or deflections circuits,in most cases,they simply expire on their own.why?

flybacks are wound with many layers of really really fine wire with really really thin insulation.the entire assembly is potted with an epoxy resin which is poured in and allowed to cure.

in some ways,these are shorts waiting to happen.

imperfections while mfg these transformer and operating temp are hot insulation and epoxy cracks allow moisture to seep in cracks shorting out windings.once the sparking and arching starts develops usally its terminal time to replace.also windings break down and short out.its amazing they last as long as they do eight to ten years under optimnal operating conditions with outpower surges or outages.so remember in electrical storms unplug your tv for safty reasons if nothing else.