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deepfried

Mon Jun 27 2011, 01:37AM

Registered Member #3933 Joined: Mon Jun 06 2011, 04:45PM
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Posts: 4

Seems like sometimes I understand (or think I do) turns ratios - and then I don't. I need to calculate a transformer with a 7.5v primary and a 2000volt secondary. 2000/7.5 gives me a ratio of 266.66 to one I think. i also think that's the turns for 1 volt on the primary. So if I want a transformer with a 10 turn primary.. and then I get lost.
Vp/Vs = Np/Ns
Can anybody help me - thanks in advance

H8erade

Mon Jun 27 2011, 01:57AM

Registered Member #3451 Joined: Sun Nov 28 2010, 11:13PM
Location: United States
Posts: 100

The voltage ratio is 1/266.66 and the turns ratio is 10/x.
Using proportions, we can find that the number of turns on the secondary should be 2666.6 (about 2666). Is that what you were going for?

EDIT:
1/266.66 = 10/x
10(266.66) = 1(x)
x = 2666.6

Ash Small

Mon Jun 27 2011, 02:12AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

smcerm wrote ...

.Using proportions, we can find that the number of turns on the secondary should be 2666.6 (about 2666). .

I hate to say this, but 2666.6 is approximately 2667, not 2666.

You'll probably want more turns than this, due to losses.

H8erade

Mon Jun 27 2011, 02:40AM

Registered Member #3451 Joined: Sun Nov 28 2010, 11:13PM
Location: United States
Posts: 100

Ash Small wrote ...

I hate to say this, but 2666.6 is approximately 2667, not 2666.

Ah. I like to round down (truncate).

deepfried

Mon Jun 27 2011, 03:02AM

Registered Member #3933 Joined: Mon Jun 06 2011, 04:45PM
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Posts: 4

Thank you so far ..
here is what is throwing me the wrench.
I took apart a transformer from a throw away camera.
I found two primaries, both about 6 turns.
Then I started counting the secondary. As I approached the one thousand mark I dropped the bobbin and it escaped somewhere in my shop.
BUT 6 turns on the primary, >1000 turns on the secondary, 1.5volts on the primary is charging the strobe's capacitor to around 300v.
Also, I plan to add an additional 4-5% to my secondary for losses, etc.
So when I try to relate that to my need for 9V in and 2000v out???

Ash Small

Mon Jun 27 2011, 03:35AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

6/1.5=4,

>1000/4=>250,

300>250,

It seems to add up.

Dr. Slack

Mon Jun 27 2011, 06:09AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Transformer turns ratios tell you the ratio between volts on the the two sets of turns.

However, if you're dealing with a flyback, which is what a camera flash transformer is, you need to be careful how you define the primary volts. During the primary charging phase, the voltage is your battery voltage. When the primary turns off, the voltage rises (the clue is in the name of the device) to an order of magnitude more, or so. It's this enhanced voltage that does the work.

And the ratio doesn't tell you the absolute number of turns. A large number of turns is usually OK, apart from copper losses, and self capacitance, and winding cost, so how few can you get away with? For this you need the core area and the operating frequency.

radiotech

Mon Jun 27 2011, 09:14AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Forget the ratio. Just wind enough turns to give you the flux you need in the core at the frequency. Once this is settled, wind enough turns to give you
desired loaded secondary voltage.
Thats how transformers are made and thats why the turns ratios never
work out as a np/ns ratio.
There are many degrees of freedom on the worksheet, you can
trade amongst. The method is to find a starting point and core flux
is a good one.

Pinky's Brain

Mon Jun 27 2011, 01:40PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

You could get a higher output voltage than input with a flyback even if you put more turns on the primary than the secondary (it would be a silly thing to do for various reasons, but it's still possible).

Antonio

Mon Jun 27 2011, 03:53PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

radiotech wrote ...

Forget the ratio. Just wind enough turns to give you the flux you need in the core at the frequency. Once this is settled, wind enough turns to give you
desired loaded secondary voltage.
Thats how transformers are made and thats why the turns ratios never
work out as a np/ns ratio.
There are many degrees of freedom on the worksheet, you can
trade amongst. The method is to find a starting point and core flux
is a good one.

This is a very extraordinary information. Transformers -are- designed having the turns ratio as one of the parameters. There are other considerations, but an unloaded transformer, with good coupling, and operating at not too high frequency will transform voltage and current as determined by the turns ratio.

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