Determine Polarity of a Transformer?

Thread Starter

TheLaw

Joined Sep 2, 2010
228
I found an old unused transformer in my parts bin. I want to hook it up but it has no markings on it.

Is there a way to find out which wire is is live and which is neutral?

Thanks.
 

bitrex

Joined Dec 13, 2009
79
Do you have a signal generator and a dual-trace scope? Put a small amplitude sine signal in on the primary side of the transformer with one channel of the scope connected across the terminals, and put the other channel of the scope across the secondary winding you want to determine the polarity of. The proper position of the transformer "dots" should be apparent from the relative phases of the two signals displayed on the scope.

Be careful to turn off the signal generator first before disconnecting the leads, as even though the voltages involved are small the windings can give a mighty inductive kick when the current stops flowing. (you think this happened to me? :D) Hopefully the output of the generator is protected against such spikes.
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
Wish I did, but do not. Oh well, just a fleeting thought that I could use it. It's probably only worth like $3-4 anyway.

I appreciate the input. I'll write that down in my head for next time.
 

Wendy

Joined Mar 24, 2008
23,457
Are you trying to figure which is the primary vs. secondary?

There is no polarity in a transformer, not hot, nor ground.

A transformer is an AC device, and it isolates the two sides, and is not connected to the core.
 

spinnaker

Joined Oct 29, 2009
7,830
Or is this one of those power packs (wallwarts) like for laptops, cell phones and such? In which case it is not a transformer.
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
Maybe the real term isn't polarity, but, I just meant...how would you determine what is hot and what is neutral?

Thanks for the link. I missed that article it seems. I don't have an O-Scope...but it seems that getting one seems more and more logical.

I'd rather keep my life than accidentally soldering up the wrong wires for the sake of cheaping out.

Thanks to all.
 

#12

Joined Nov 30, 2010
18,224
Scopes are getting cheaper, too. In 1977, a 20 mega hertz, dual trace scope cost over a thousand dollars after sales tax and an extra 10x probe. Remember that when you look at todays prices and you won't be so shocked at spending two, three, four hundred bucks.
 

Wendy

Joined Mar 24, 2008
23,457
Again, there is no Hot or Neutral, there is a primary and secondary. The assumption in this case is the primary uses line current (it doesn't matter which is hot or neutral, the wires on the primary are interchangeable), and the secondary is the low AC voltage out side.

Transformers are usually (but not always) reversible, put low voltage AC in the secondary and out pops line voltage, something that is often used in inverters. I would strongly suggest you read the link in post #6. It is the AAC book explanation of transformers and how they work.

Care to post a picture?

Given we still aren't sure what you are talking about, there is a good chance this is a wall wart or variation thereof. A picture will help with that.
 

jpanhalt

Joined Jan 18, 2008
11,087
I thought the OP is talking about the phasing, which is widely referred to as polarity. Hence the AAC link. Of course, post#7 doesn't make much sense in that regard, unless he is considering making one lead on the secondary common with the primary.

I agree, a picture might help enormously.

John
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
The red wires are for the primary 120V side, and the blue wires are on the secondary (unknown VAC). So...what I was trying to ask is. How do I know which wire on the primary side goes to live (black) and which goes to neutral (white). Then, I guess I could use my multimeter on the secondary...but I don't want to solder up the wrong connections.

 
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#12

Joined Nov 30, 2010
18,224
Here's an idea: Put a 33k 1/2 watt resistor in series with one winding and power that series combination with 120VAC. Then measure the AC voltage on each winding. If the unused winding shows higher voltage, it's the primary...lower voltage means its the secondary.
 

jpanhalt

Joined Jan 18, 2008
11,087
The blue wires are for the primary 120V side, and the red wires are on the secondary (unknown VAC). So...what I was trying to ask is. How do I know which wire on the primary side goes to live (black) and which goes to neutral (white). Then, I guess I could use my multimeter on the secondary...but I don't want to solder up the wrong connections.

Actually, you have the red wires labeled primary.

If you are really, really, really sure which is the primary. It doesn't matter which wire is connected to the white wire and which is connected to the black wire of 120 VAC. That's why they are the same color. :D


John
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
Yes I'm dead positive red is the primary.

So....I can wire live to whatever I want and neutral to whatever I want?
 

#12

Joined Nov 30, 2010
18,224
That's what post 14 said. Black to any red and white to the other red. Of course a fuse would be a safe thing to minimize the smoke if anything goes wrong.
 

Thread Starter

TheLaw

Joined Sep 2, 2010
228
Then for the secondary side, how do I determine which one is positive and which one is negative with a multimeter? I'm not too good with multimeter theory.
 
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