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Transformer wiring to mains
Transformer wiring to mains
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Old 15th July 2010, 01:17 PM   #1
AudioMike is offline AudioMike  United Kingdom
diyAudio Member
Join Date: Jul 2010
Location: UK
Default Transformer wiring to mains

Hi guys,

I'm new here and this is my first post after lurking around for a while...

I have a question. I'm relatively new to electronics, but am having a go at constructing a chipamp based on old STK080's and circuits that I have already. These are old and obsolete so I'll probably change to LM3886's if it works out.

Anyway, my question is this. In the UK, we have 240v AC and Live, Neutral, Earth (ground) but when I check the Live and Neutral wires they read continuous (beep) with my digital MultiMeter. I am reading the continuous beep off the rear of the IEC socket across the Live/Neutral tags. There's a fuse in the Live line between the IEC socket and transformer.
Is this simply because the transformer (center tapped 20-0-20v) is connected in circuit, or is this a short i.e dangerous. Am I being too cautious or is it a silly question? Apologies if silly question, but I haven't wired up a mains transformer before and don't really want a weirdo hairdo!

Your input is greately appreciated...


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Old 15th July 2010, 01:46 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
Read lot's on mains Safety. Start with ESP's site and Decibel Dungeon's site.

Build yourself a mains bulb tester. DD shows a complicated version with extra switches.

Never ever plug in new mains projects direct to the mains until you have proved they are correctly wired.
Prove this by using the bulb tester.

A mains transformer has a primary winding that is electrically insulated from the secondary winding/s. This allows isolated power on the secondary side that is less dangerous than using Mains power.

That primary winding is just a big coil of copper wire wound around a magnetic core. The magnetic core gives enhanced AC performance.
If you measure the resistance of the primary winding using a DC meter you will find somewhere between 100ohms and 1ohm.
That is what your continuity setting is revealing.

If you feed DC into your primary it will draw current according to ohm's law.
10Vdc into a 10ohm primary will draw 1Ampere. The secondary will show nothing. The power going into the primary is I*V = 1A * 10V = 10W. The primary is going to heat up fairly slowly.
Now apply 100Vdc to the primary. The power dissipated in the primary is now 1kW. It will heat up so rapidly that you will probably damage it before you can cut the power off. If you miss-wire the dual primaries of a 115/230 Vac universal transformer, you will end up with a situation that will pass similar currents through your primary that are of a similar order to applying the same DC voltage, i.e. 240Vac applied to a 10ohm primary that is miss-wired could dissipate ~6kW until the mains fuse blows. Damaged transformer and maybe damaged self.

Be careful!!
If you are unsure, always ask, Never take risks by guessing. Be informed.

We are here to help.
regards Andrew T.
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Old 15th July 2010, 08:23 PM   #3
jrgoswick is offline jrgoswick  United States
diyAudio Member
Join Date: Jul 2010
Location: Arkansas
Pull the fuse and ohm it out. Should be open(no beep). Put the fuse back in and try it again. What you're reading is the primary of the transformer, fuse, wiring, switch, ect... THIS IS NORMAL! Being paranoid with mains wiring WILL GET YOU HURT! Being cautious will keep you alive.
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Old 15th July 2010, 09:46 PM   #4
AudioMike is offline AudioMike  United Kingdom
diyAudio Member
Join Date: Jul 2010
Location: UK
Thanks for the advice guys. Much appreciated.

I checked and double checked the transformer primary mains wiring. Yes, realised it was the primary winding coil reading continuous, so I powered it up, all OK :-) Success! Works fine.

I can now proceed to the next stage to wire the STK080 boards in.


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