Hi,
You could drive the primary with a small test voltage, like 1 volt, then measure the output voltage Vout. The primary impedance is then the output impedance times the square of the turns ratio.
TR=TurnsRatio=Vin/Vout
Rp=PrimaryImpedance=SecondaryImpedance*TR*TR
For example, say you drive the primary with 10 volts and the output is 1 volt, and you measure that 1 volt on the 8 ohm output of the transformer. This means the turns ratio is 10, and so the primary impedance is:
Rp=8*10*10=800 ohms.
This is an approximation and you may need to apply some resistive load on the secondary to get better results.
You also need to know the wattage of the transformer.
If you double the load impedance on the secondary, the reflected primary impedance ALSO doubles. If you halve the load impedance on the secondary, the reflected primary impedance also halves.
It really is that simple.
Load Resistance Primary Impedance Square Root of impedance ratio
(measured)
1Ω 1190Ω 34.50
2Ω 1642Ω 28.65
4Ω 2854Ω 26.71
8Ω 4842Ω 24.60
16Ω 8068Ω 22.46
32Ω 12215Ω 19.54
64Ω 15710Ω 15.67
128Ω 17896Ω 11.82
Apparently it really isn't that simple.
I got an output transformer and measured a voltage transfer ratio of 24.35 @ 1kHz.
The square root of the primary to secondary impedance ratio should be the same as the voltage ratio
I then connected various load resistors to the secondary and measured the impedance at the primary side with an LCR meter, also @ 1kHz, obtaining the following results:
Code:Load Resistance Primary Impedance Square Root of impedance ratio (measured) 1Ω 1190Ω 34.50 2Ω 1642Ω 28.65 4Ω 2854Ω 26.71 8Ω 4842Ω 24.60 16Ω 8068Ω 22.46 32Ω 12215Ω 19.54 64Ω 15710Ω 15.67 128Ω 17896Ω 11.82
The measured results aren't following the simple rule you gave, so apparently there's more to it than the simple rule.
Apparently it really isn't that simple.
I got an output transformer and measured a voltage transfer ratio of 24.35 @ 1kHz.
The square root of the primary to secondary impedance ratio should be the same as the voltage ratio
I then connected various load resistors to the secondary and measured the impedance at the primary side with an LCR meter, also @ 1kHz, obtaining the following results:
Code:Load Resistance Primary Impedance Square Root of impedance ratio (measured) 1Ω 1190Ω 34.50 2Ω 1642Ω 28.65 4Ω 2854Ω 26.71 8Ω 4842Ω 24.60 16Ω 8068Ω 22.46 32Ω 12215Ω 19.54 64Ω 15710Ω 15.67 128Ω 17896Ω 11.82
The measured results aren't following the simple rule you gave, so apparently there's more to it than the simple rule.
Secondary Load Reflected Primary Impedance
1R 593R
2R 1186R
4R 2372R
8R 4744R
16R 9488R
32R 18976R
64R 37952R
128R 75904R
colin55 said:I am just saying you need to know the wattage of a transformer to see if it is suitable for an application.
It looks like he wants an output transformer.
The output level from the output transformer of a vacuum tube amplifier changes when the speaker impedance changes. But the output level from a solid state amplifier does not.
Wow...if THIS doesn't defy every aspect of Ohm's Law.
Seriously, I think it would be in your best interests to retract this statement. I know you know better than this.
When an excellent output transformer with low phase shift is used in a vacuum tube amplifier then it can have some negative feedback to reduce its fairly high output impedance (and also reduce its fairly high distortion). Then changes in load impedance by changes in a speaker's impedance vs frequency will make less difference in output level.
A solid state amplifier has a lot of negative feedback that reduces its output impedance (and distortion) to almost zero so its output level does not change when a speaker's impedance changes.
Nigel Goodwin said:Don't forget, as the impedance of the speaker changes with frequency, the impedance of the transformer changes as well.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?