OT impedance

[jestaudio]

You still refrain from getiing the point?

i,m not quite sure what i,ve done to offend you, maybe its because im a bit thick and am trying to get a handle on things before i chuck a few hundred volts at anything, if thats a problem well i'd say thats your problem not mine mate, i don't bother with sarcasm and certainly don't ask for srcasm when i'm trying to learn

[ToneMerc]

You still refrain from getiing the point?

i,m not quite sure what i,ve done to offend you, maybe its because im a bit thick and am trying to get a handle on things before i chuck a few hundred volts at anything, if thats a problem well i'd say thats your problem not mine mate, i don't bother with sarcasm and certainly don't ask for srcasm when i'm trying to learn

Actually, he was being very truthful. No offense, but 2 other persons also said that you don't measure impedence with a DMM, yet you are still harboring on it. Please go back and review the very most basics concerning impedence and once you understand the difference between impedence and DC resistance, ask your question again.

TM

[Aurora]

This has nothing to do with sarcasm or offence. You have several times been told that measuring DC resistance of the windings tells you nothing about windings or turns ratio, which is what sets your impedance levels, - yet you seem to ignore this, and go on with your DC measurements- which still tells you nothing.

From your profile, I see you list yourself as an electrician. Impedance transformation is probably not what an average electrician meets on a dayly basis, but concider this:
Take any average job site transformer - 240 in / 120 V out. One would expect the same power level in KVAs in and out. This of course means that the secondary will have to be wound with a rather heavier gauge wire than the primary, - as roughly said, I would like to pull twice the amperage from my secondary as I put into the primary, given equal power levels in and out ( not counting iron loss ).
In theory ( saving myself from digging out an exact data sheet), I could measure the same DC resistance in both windings, given the fact that my secondary would have to be a heavier gauge than the primary, since secondary current will be twice the primary current? No?
Does that make my winding ratio 1:1, since DC resistances are equal?
Of course not, - as I certainly think you must know.
BTW - since it has not been said explicitly - voltages transform equal to the winding or turns ratio - impedances transform by the square of the turns ratio. DC measurements can only tell if the transformer has shorted, burnt off or seems OK.

No offence implied or intended - and no offence hopefully taken..

EDIT: Well, this is roughly what a glass of Glen XXX can do to one's brain on a friday night... of course the secondary of my example would have a lower resistance than my primary - both because of having only half the turns of the primary, but ALSO being of a heavier gauge wire. In fact - if I was really "unlucky" , I might even measure a 4:1 ratio ( being highly unlikely), which would be exactly the impedance ratio.........
But still - to the fact - DC measurements are not valid for impedance ratios.
Unfortunately, I'm not able to quickly find a real OT datasheet that also lists winding DC values.....

[martin manning]

http://ampgarage.com/forum/viewtopic.ph ... highlight=

[Aurora]

Oh- thanks, Martin.... just what was needed.........
( I'd forgotten that thread..... )

[tubeswell]

Transformers don't have a fixed primary load resistance - they have an impedance ratio. If you change to the load on the secondary, then the load on the primary changes accordingly - determined by the proportion of the impedance ratio. E.g.: 1000:1 impedance ratio will give a primary reflected load resistance of 8k if you hook up an 8R speaker to the secondary (or a 4k reflected load if you hook up a 4R speaker instead and so on).

Furthermore, in order to get the transformer to work, you have to use alternating current. Direct current won't tell you anything (so measuring the DC resistance of each winding with your R-meter won't divulge anything about the impedance ratio). You need to use a VAC source to stimulate the transformer and then measure the VAC on each winding in order to determine the Pr:Sec voltage ratio. The impedance ratio is the square of the voltage ratio. (The voltage ratio is the same as the Pr:Sec turns ratio). So an OT with a 1,000:1 impedance ratio has a voltage ratio of the square root of 1,000:1 (i.e.: 31.62:1). So a measurement of 1VAC on the secondary and 31.62VAC on the primary will confirm a 1,000:1 impedance ratio.