Question about a flyback's polarity

Hello,


I am confused with this flyback transformer I have.
I have a pulsed DC input on the primary coil (pulses from 0 to 15V) and that creates high voltage in the secondary coil.

When I connect that input in the opposite direction (wrong polarity) I don't get high voltage any more.
The possible explanation for this is that the primary coil has a diode in series with it.

Now, I'm confused by this because I tested if there actually Was a diode in there (by connecting a DC supply and a DC motor, in series with the coil) and the motor always works!
(also tested with a halogen light)

I put the coil in series with the motor to test if there is a diode inside. If there would be one inside, the motor should not work in one of the two cases, when I switch the pins of the primary coil.

So, as the results of the test tell me: there is no diode in series with the primary coil.

could it be possible that there is a diode in parallel with the primary coil?
If that's not possible, how come the flyback is not working when I connect the polarity incorrectly?


Could someone please help me out with this?

Thanks,

Electorials

Comments

[steveastrouk]

There's often a diode in the SECONDARY
Steve

[Electorials]

An example: on a Regular transformer with k=1 (primary windings = secondary windings):

An AC input voltage of 10Vptp
this will go from -5 to 5 volts.
Will create a -5 to 5v secondary voltage

An input voltage of 10Vptp with 30V offset (DC)
this will go from 20V to 40V
will create a -5 to 5V secondary voltage, because the DC offset doesn't matter.


I base myself on this example (that I was told at school) to try and figure out what the flyback does.



Now, with the real flyback transformer:

When the primary voltage changes from 0 to 15V that will create a positive flux, creating a positive voltage on the secondary windings. (also if there is a diode there)

If I connect the primary voltage with the wrong polarity,
There will be a primary voltage change from -15V to 0V, which is also a raising voltage, so that will Also create a Positive flux, creating a positive voltage on the secondaries.

They told me at school that no matter what polarity the input voltage has, it only matters if that voltage is increasing or descending.

so this would mean that it doesn't matter if there is a diode at the secondary windings, for connecting the primaries with the wrong polarity.


I'm not Really sure if this is all correct, but please correct me if I'm wrong :)
Thanks for the fast reply!

[steveastrouk]

DC in a transformer will NOT have no effect - it will cause the core to saturate. You MUST NOT have standing current in the transformer.

[iceng]

Or acrid smoke will rise up and smite thy lodgings.

[Electorials]

We all put standing DC on a flyback because a 0 to 15v blockwave is the same as a 7.5Vptp blockwave with +7.5v DC offset

[Electorials]

But I didn't mean that much DC offset.

[iceng]

Still much fun,
But really the varnish on a small pulsed transformer for residential use must dampen mechanical and magnetostrictive events not just quiet but silent..

The varnish does the job and has a low visible acrid stink if burned.that can
permeate a room..  Avoiding this smelly possibility the fly black transformer
is over designed which explains why they are a favorite among hobbyists .

[steveastrouk]

No we don't because we switch it off pretty quick, or we WILL saturate the core.

Steve

[iceng]

Yea that's the case Lol

[steveastrouk]

EXACTLY !

Actually, di/Dt= V/L UNTIL THE CORE SATURATES.

Would you like to see my unfinished notes on FLyback action ?

[steveastrouk]

I don't live on IRC - if you see Lizzyastro on there, try pinging her.

The "instructable" is a work in progress - no pictures.

Steve

[Electorials]

So the flux doesn't raise as fast as the primary voltage does?.

Now, this still doesn't answer why I'm not able to use a different polarity.
there is no primary diode, and a secondary diode shouldn't be a problem like I said in my other post.

[Electorials]

So the flyback is your favorite transformer because it doesn't smell bad when you're destroyinhg it? haha :D

[Electorials]

ok ^^

[Electorials]

Ok, thank you
and yes please, I would love to see it.

Is it possible to chat somewhere with you? it would be a lot easier.
It seems you are not on that IRC channel you told me about.

I have lots of questions haha ^^

[Electorials]

Ok, no problem.
If you'd like too, I would want to read the instructable to get some more information on how the flyback works.

[Electorials]

Oh, I just got your PM =) thank you!