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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Jan 2008, 12:37 pm   #1
dominicbeesley
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Default Transformer kit problem.

I recently bought one of the transformer kits from http://www.livinginthepast.demon.co.uk/xformer.html and have wound a hefty heater transformer following the instructions on the page.

This doesn't quite give the voltage I was expecting coming out at about 5.8V for 27 turns of 1.5mm enamled with no insulation between the three layers first layer 11 turns, second 10, third 6.

When looking on a scope the crests of the waveform have been chopped off causing the measurement to be wrong - it would be about right if the waveform were not distorted.

I've been using it to drive a scope tube and an SN7GT and they seem be giving roughly the emission I would expect?!?

What is going on? Is the core saturating too much? Is it a fault of the primary or something I've done? I'm new to all this transformer lark! I managed to stull all but one of the E laminations in to the core and that required a certain amount of hammering!

Cheers

Dom
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Old 15th Jan 2008, 12:45 pm   #2
jim_jobe
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Default Re: Transformer question

How are you measuring the output voltage Dom? If it's with the scope is it the peak value?
Are you measuring with a load connected or just putting the scope across the secondary winding?
Jim
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Old 15th Jan 2008, 1:08 pm   #3
dominicbeesley
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Default Re: Transformer question

Hi yes, sorry wasn't clear.

Measuring with a scope (peak scaled) and a DVM give the "wrong" sub 6V answer.

I've tried with and without a load, both give similar results but voltage drops when loaded (as expected with the regulation factor given). The waveform stays the same though!

Dom
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Old 15th Jan 2008, 1:10 pm   #4
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Default Re: Transformer question

What's the waveform on the primary like?
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Old 15th Jan 2008, 1:41 pm   #5
KeithsTV
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Default Re: Transformer question

I assume you are connecting to to the 240v mains directly. If this is the case it is quite normal to see the top of the waveform distorted due to hundreds of TVs, PCs etc all drawing current at the peak of the waveform.

The current drawn by a typical TV, PC etc is not sinusoidal but a pulse as the rectifier conducts to recharge the main capacitor. It can only conduct when the input voltage is greater than the voltage on the capacitor. Multiply this by the number of TVs switched on and its no wonder the waveform is distorted.

You could try late at night/early morning when most of the TVs etc are turned off and the peak load is less. You should see a difference in the waveform and the RMS level may be higher.

Have you measured the mains voltage as this can vary throughout the day? Generally the daytime voltage will be lower than the evening voltage as industry and people at work will be using power.

Note that most meters AC ranges assume a sinusoidal waveform. True RMS meters actually calculate the RMS value.

Hope that helps

Keith

Last edited by KeithsTV; 15th Jan 2008 at 1:43 pm. Reason: Adding sentence on RMS
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Old 15th Jan 2008, 4:35 pm   #6
dominicbeesley
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Default Re: Transformer question

Thanks all, the mains waveform is indeed similarly distorted - I am an idiot and should have checked the bleedin obvious before posting!

I've wound a few extra turns on for good luck and am now up to 6.2 volts - which leads me to suspect that the transformer instructions are wrong and it should be 4.61 turns per volt instead opf 4.16!

Cheers

Dom
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Old 15th Jan 2008, 5:18 pm   #7
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Default Re: Transformer question

Quote:
Originally Posted by dominicbeesley View Post
I've wound a few extra turns on for good luck and am now up to 6.2 volts - which leads me to suspect that the transformer instructions are wrong and it should be 4.61 turns per volt instead opf 4.16!
It sounds like a misprint as you say, but I'd be inclined to drop an email to the suppliers and check.

Pete.
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Old 15th Jan 2008, 6:10 pm   #8
Ed_Dinning
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Default Re: Transformer question

Hi Dom, the current waveform into the tranny is the one to look at (use a 1R resistor) tocheck for saturation.
If you read the instructions closely you may find it gives details of compensating turns to add to the sec to get the correct voltage. This is due to volt drops in pri and sec wire as well as leakage reactance between the two windings. If it is one winding on top of another the leakage reactance is not too bad, if, as is the case with many kits they are side by side (much "safer" but a lot less effecient), the leakage reactance is much higher and needs even more compensation.

Ed
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Old 17th Jan 2008, 10:35 pm   #9
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Default Re: Transformer question

Hi,

Normaly the kit data sheet should give an equasion for adding extra turns for load regulation.

Daniel.
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Old 18th Jan 2008, 2:12 pm   #10
dominicbeesley
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Default Re: Transformer question

I'd done all the extra "regualtion" turns but it just looks like the turns ration was plain wrong (numbers transposed). Not heard back from the website yet for confirmation - as it was my first go at winding a tranny thought I'd done something weird but it looks like a plain mistake!

Wasted me about a pound of copper wire so far!

Dom
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