# Finding start of ferrite transformer

1. May 27, 2007

### microman

Hello ,

I have a ferrite core transformer with two separate windings. I was hoping if someone can tell me if I can locate the start of each winding relative to each other.
I have a scope, and am comfortable with micro based circuits.

Cheers
Thomas

2. May 29, 2007

### Staff: Mentor

It sounds like you want to determine the polarity of the two windings with respect to each other? If so, put a sine wave of the appropriate frequency (do you know what frequencies this transformer is designed for?) into the 1st winding, and measure the output on the 2nd winding. That will give you the polarity and turns ratio.

Welcome to the PF, BTW!

3. May 29, 2007

### waht

There is even a cheaper method if you don't have a sine wave generator. Just manually pulse a DC current in one coil and observe on the oscilloscope whether the spike in the other coil goes positive or negative.

4. May 29, 2007

### microman

Hi Berkeman , Waht,

Thanks for the response to my post and the welcome to PF. I am attaching a pdf ,would the output waveform be as it is there ? Would there be any difference between a Sine and a square wave for this check ?
This flyback txfer is for a dc-dc low power converter ,square wave at 60kHz,to convert from 9V to 90V at 1mA.

Thanks again.

5. May 29, 2007

### microman

oops no attachment,so trying again

#### Attached Files:

• ###### waveform.pdf
File size:
22.5 KB
Views:
93
6. May 29, 2007

### Staff: Mentor

That's not how you would generally drive a flyback transformer. The secondary output winding (with more turns for the step-up function) would be grounded on the dot polarity end, and the non-dot end of the secondary would have a rectifying diode connected with an output smoothing capacitor.

You drive the flyback converter by putting a positive supply and capacitor on the dot end of the primary, and using a low-side transistor switch to pull down the bottom of the primary for the energy-storage phase. You would choose the drive frequency (or on-time pulse width) at the primary to stay short of saturating the core.

I guess the way you've shown a "test drive" circuit to verify the polarity dots is probably okay, it just gives me a bit of vertigo when used in the context of a flyback converter circuit.

EDIT -- Ack, I was brain dead. I've corrected a polarity error in the secondary part of this post, and sent the OP a PM to be sure they know about my error. Jeeze, sorry for any confusion. I realized my error sitting in a waiting room this afternoon, with no Internet access. Here's a summary page from wikipedia.org about about flybacks:

http://en.wikipedia.org/wiki/Flyback_converter

Last edited: May 29, 2007
7. May 30, 2007

### microman

Hello Berkeman,

Thanks for your post.Yes I am aware of the flyback configuration.This was purely for verifying the start end of each coil.So i guess the waveform should be ok to verify the start ends of the txfer.

In case anyone is more interested here is a link to a nice site on power converter topologies and a design methodology.