|
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. |
|
Thread Tools |
15th Jan 2008, 12:37 pm | #1 |
Octode
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
|
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 |
15th Jan 2008, 12:45 pm | #2 |
Heptode
Join Date: Nov 2004
Location: Ripon, N.Yorkshire, UK.
Posts: 782
|
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 |
15th Jan 2008, 1:08 pm | #3 |
Octode
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
|
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 |
15th Jan 2008, 1:10 pm | #4 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,193
|
Re: Transformer question
What's the waveform on the primary like?
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
15th Jan 2008, 1:41 pm | #5 |
Nonode
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,571
|
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 |
15th Jan 2008, 4:35 pm | #6 |
Octode
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
|
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 |
15th Jan 2008, 5:18 pm | #7 | |
Retired Dormant Member
Join Date: Oct 2004
Location: Bristol, UK.
Posts: 837
|
Re: Transformer question
Quote:
Pete. |
|
15th Jan 2008, 6:10 pm | #8 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
|
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 |
17th Jan 2008, 10:35 pm | #9 |
Hexode
Join Date: Apr 2007
Location: Tonyrefail, Rhondda, South Wales, UK.
Posts: 337
|
Re: Transformer question
Hi,
Normaly the kit data sheet should give an equasion for adding extra turns for load regulation. Daniel. |
18th Jan 2008, 2:12 pm | #10 |
Octode
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
|
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 |