Measure impedence of OT?

[azatplayer]

I got a new output trannie recently. I dont want to go in to the details, but ive been told by the guy that it was 4k. Then i was told after that it was indeed 4.8k which is what was ordered. Thing is ive seen the 4.8k OT and the label, and the number was different..
I wanna make sure what i have is right and that he was not blowing me off.
Ive googled it and gotten confused with the different interpretations on how to measure an unknown OT's primary with a low AC voltage.
Can i git a little help?
Cheers!

O, im not gonna diss on the maker, they are great, but i was told 2 different things so.. i can use this OT if it is indeed 4K, for another build i have planned, but want the actual proper one for this build.

[tubeswell]

Some homework for you

Hook the OT secondary up at a small AC source (the 5V rectifier winding of 6V heater winding of a PT may be convenient). Make sure that you (carefully) measure the VAC across the secondary and the VAC across the primary.

The ratio of Pri:Sec VAC squared, will be the impedance ratio.

Say you have 5.4VAC from your 5V winding on the secondary, and 108VAC across the primary - that is 108:5.4 = 20:1 (Pri:Sec) VAC ratio.

The square of 20 is 400. (So the impedance ratio would be 400:1)

So if you put a 8R speaker onto a 400:1 OT you would get 8 x 400 = 3k2 load resistance.

[azatplayer]

Some homework for you

Hook the OT secondary up at a small AC source (the 5V rectifier winding of 6V heater winding of a PT may be convenient). Make sure that you (carefully) measure the VAC across the secondary and the VAC across the primary.

The ratio of Pri:Sec VAC squared, will be the impedance ratio.

Say you have 5.4VAC from your 5V winding on the secondary, and 108VAC across the primary - that is 108:5.4 = 20:1 (Pri:Sec) VAC ratio.

The square of 20 is 400. (So the impedance ratio would be 400:1)

So if you put a 8R speaker onto a 400:1 OT you would get 8 x 400 = 3k2 load resistance.

So, basics, what am i hooking up? Ive read this info, but as its AC, theres no plus and minus. Take me thru it blow by blow. 5V winding to secondary, is there a ground? Singly a 5V tap nothing else?
Thats the bit that confuses me!

[KT66]

So, basics, what am i hooking up? Ive read this info, but as its AC, theres no plus and minus. Take me thru it blow by blow. 5V winding to secondary, is there a ground? Singly a 5V tap nothing else?
Thats the bit that confuses me!

No, there is no ground, transformers only work with AC. As Tubeswell stated, you need to find the transformers turns ratio in order to find the impedance ratio. So, if you input an AC voltage into the secondary of an OT, it will induce a current in the OT's primary which will often be around 20 to 30 times as big as the voltage that you put into it. He is suggesting to use a small voltage, such as 5V AC or 6.3V AC because it makes the voltage that you will be measuring on the primary manageable - for example, if you input the wall voltage of 117V AC into the OT secondary of tubeswell's example you would get 2340V AC out of the other side.

Another way to do it if you have a Variac is to put the input voltage on the OT's primary and adjust it until you measure 1V AC on the OT's secondary. Again, using Tube's example of an OT with a 20 to 1 turns ratio, the Variac would be imputing 20V AC into the OT primary.

[azatplayer]

OK, BUT, apply an AC voltage to the sec, and read the corresponding voltage on the primary, how can you read an AC voltage without the neutral wire being connected, or earth? If i stick a meter in the wall socket, it wont read 230V without the other lead on the neutral or ground tab.
I know im being slow, but walk me thru the connections. place 5V AC rec winding on sec wire, nothing on the common wire? How can i measure across the winding without a reference?
I have just had a 6 pack of Peroni, so forgive my brain if its not gettin it

[rp]

http://www.radioremembered.org/outimp.htm

Use two meters if you got, and be careful! Even 5VAC can become 100VAC or way more. Use alligator clips with boots. Shut the Variac off each time you move connections around - you know the usual stuff.

[tubeswell]

You can also measure the AC across each half of the primary if you like. The total AC across the primary will be the sum of the 2 halves.

[KT66]

I think the confusion is coming from the fact that you know that the AC that comes out of the wall has a hot and a neutral ( essentially ground ) wire, but that fact does not imply that all AC currents have a neutral connection. For example, the 240V AC mains connections in the US are hot on both sides, although you can complete a circuit with either side and a connection to ground, which is what a 120V AC wall outlet basically is. The 5V AC that comes out of the secondary of a filament transformer is hot on bot sides also, not that it would matter if it did have a neutral - it would still be 5V AC. In fact, someone please correct me if I am wrong about this, if you had a 10V AC secondary on a power transformer with a center-tap connected to ground, you would measure 5AC between the CT and either leg of the secondary which would be indistinguishable from that measured on the secondary of a 5V AC filament transformer as we are suggesting to use to find the turns ratio. SO, don't worry about polarity or ground references, there isn't any - it's AC. Just connect both terminals of the output of a 5V, or 6.3V filament transformer to the secondary of your output transformer and measure the AC that develops on the primary.

[tubeswell]

In fact, ... if you had a 10V AC secondary on a power transformer with a center-tap connected to ground, you would measure 5AC between the CT and either leg of the secondary .

correct

[Reeltarded]

Another way to do it if you have a Variac is to put the input voltage on the OT's primary and adjust it until you measure 1V AC on the OT's secondary. Again, using Tube's example of an OT with a 20 to 1 turns ratio, the Variac would be imputing 20V AC into the OT primary.

Duuhh.. one more question? How many times does 20 go into 1?

Just the best question I could think of, thanks for the tip. I never thunk of that before. Thanks!

[KT66]

In fact, ... if you had a 10V AC secondary on a power transformer with a center-tap connected to ground, you would measure 5AC between the CT and either leg of the secondary .

correct

Ya, I know that it's 5V of course, the correct me if I'm wrong reference was to the statement that a CT to one leg of a transformer voltage measurement looks the same to a measuring device as if it was taken from a source where both terminals are hot.

[tubeswell]

Another way to do it if you have a Variac is to put the input voltage on the OT's primary and adjust it until you measure 1V AC on the OT's secondary. Again, using Tube's example of an OT with a 20 to 1 turns ratio, the Variac would be imputing 20V AC into the OT primary.

Duuhh.. one more question? How many times does 20 go into 1?

Just the best question I could think of, thanks for the tip. I never thunk of that before. Thanks!

1/20th times (or .05 x).

20:1 is a ratio. and in this sense we are talking about an example of 20 volts on the primary winding for every 1 volt on the secondary winding. The voltage ratio is equal to the turns ratio. So in this example, the OT would have 20 turns on the primary winding for every 1 turn on the secondary winding.

However, the impedance ratio is the square of the voltage (or turns) ratio - in our example case 20 x 20 = 400. So the impedance ratio of our example OT would be 400:1 (Pri:Sec). So when you put an 8 Ohm load onto the secondary, is is transformed into a 3,200 Ohm load on the Primary. There you go.

[azatplayer]

Well feeling a bit more with it this morning
I used an old parts chassis with the PT still in it as my 5V source.
And attached the 8ohm sec lead.
So i got a turns ratio of 24.106. Yielding 581.11 x 8ohm= 4.648K
So as a 4K8 OT i guess thats about right, and its not 4K. Confrontation avoided!
Cheers guys

[overtone]

Is there a good reason why the AC voltage is applied to the secondary / speaker side of the OT, rather than the primary plate side?
Could this be for accuracy? I have always applied the AC (usually around 11V) to the primary.

[Phil_S]

Often, with low source voltage applied to the primary, readings less than 1 volt on the secondary make it difficult to be certain about the turns ratio. In theory, there's nothing wrong with applying the a/c supply to the primary, and it is much safer in terms of preserving the transformer. Sometimes, when I'm not sure what's up with a transformer, I start with voltage on the primary to get a rough idea, but 5V on the secondary almost always gives numbers that I feel better about with respect to accuracy.