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Power Transformer Questions

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teehowe

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Hello folks! I've got some questions about a power transformer I just obtained recently. This is my first attempt at building a dual +/-9V power supply. I do not have issues building the thing but I have questions on this model of transformer. Take a look at the picture attached:

**broken link removed**

As far as I know, the primary windings usually have two connections, one live and one neutral right? But this one has three and as stated on the plate, it's 230V - 115V - 0V. Does this mean this transformer has two modes (~20:1 and ~10:1)?

How do I connect these wires to a power plug anyway? I assume the signal (red or black) connects to the live and ground goes to neutral?

For the secondary winding, is it safe to assume that it means 12V, ground and -12V?

I could appreciate some safety tips on building a power supply. I personally got shocked by just measuring the DC resistance at primary (230V to 0V) without any connection to mains. I got a 40 ohms anyway.

Any hint would be greatly appreciated! Thanks and cheerio....
 
As far as I know, the primary windings usually have two connections, one live and one neutral right?
Assuming the main power supply is TN system, it's true.


But this one has three and as stated on the plate, it's 230V - 115V - 0V. Does this mean this transformer has two modes (~20:1 and ~10:1)?
Yes.

How do I connect these wires to a power plug anyway? I assume the signal (red or black) connects to the live and ground goes to neutral?
Do not assume things. Not only can it cause damages to transformer, but you running a great risk being harmed with burn or electric shock, or both.

For the secondary winding, is it safe to assume that it means 12V, ground and -12V?
It's AC so you don't have plus or minus on the outputs. Most probably you've 12V AC and a center pin you can choose to call neutral.

I could appreciate some safety tips on building a power supply. I personally got shocked by just measuring the DC resistance at primary (230V to 0V) without any connection to mains. I got a 40 ohms anyway.
The impedance (correct term here instead of resistance) will behave different when you feed the transformer with AC voltage.

Any hint would be greatly appreciated! Thanks and cheerio....
The little thing you have picture of is really not what I would call a "power transformer". If you've ever seen the size of a transformer at your power company, you'll see what I mean ;)
 
Do you want to power it with 120V or 240V? Since you didn't fill in a country when you registered for the forum, I have to ask!

Nothing in the power transformer is "Ground" except the Mounting tabs. You get to decide what to connect the secondary wires to. You can build a supply that is "floating", which is the way most lab supplies are, with no connection at all to earth ground through the third prong on your AC line cord.
 
Thanks for the quick response. I'm from Malaysia (rather follows UK standards), but it's a wee complex. Some places are rated at 230V while some places use 240V. I'll be using only the 230V and the 0V wires it seems. And I suppose I "cap" the 115V terminal since I'm not using it?

Just for finalizing sake, the red or black goes to the live terminal of the plug while the green goes to the neutral?
 
I would guess that BLK is the one that is capped. Check it with an Ohmmeter. The winding resistance across the 240V input will be twice the winding resistance across the 120V input.
 
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