Can you measure the current through the winding with 10Vrms across the winding? Do not add any resistance to the 10V supply winding (apart from say a 1 ohm current sense resistor). That will then give you the inductance via the impedance of the winding and ohms law.
No... the DUT is a 115+115:5+5... the signal used to measure came from a 12.6v 20VA transformer hooked up to a variac.
Ok... now I am looking at 63.92H... seems a bit more reasonable. Was expecting to be higher from other reports of the larger VA models. I will see if I can do a resonance measurement with this number in mind.
Are you able to report the winding test voltage, current, and ESR ?
PS, and your mains frequency 🙂
PS, and your mains frequency 🙂
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I just realized I am sure I got the 63.92 value wrong. I keep forgetting to measure the winding voltage separately from the resistance. Correct me if I am wrong, please... (10/13.5ma)/(2π60)= 1.964H on the secondary. Multiply by the turns squared... 1.039 kilo Henries... I am really beginning to believe the inductance is this high. I can't seem to get away from these results. But I could be doing it all wrong. It has been nearly a decade since learning about reactance, and I haven't touched it since.
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I get 1.96H, Assuming you are applying 10Vrms across the total primary winding (115+115 windings in series) and have left the 5+5 secondary windings unconnected.
10V/13.5mA gives an impedance of 741 ohm. At 60Hz, that impedance relates to an inductance of 1.96 H.
If you have been measuring the secondary (5+5V) windings, then I suggest you need to measure the primary side instead, as that is what is seen by the output stage valves in class A PP operation.
10V/13.5mA gives an impedance of 741 ohm. At 60Hz, that impedance relates to an inductance of 1.96 H.
If you have been measuring the secondary (5+5V) windings, then I suggest you need to measure the primary side instead, as that is what is seen by the output stage valves in class A PP operation.
This is 10 volts across the secondary with the primary open. I will measure the primary tomorrow evening. As I understand, the inductance of the primary is the turns ratio squared... am I wrong here? The application is parallel feed single ended keeping the voltage swing below 240 volts to avoid saturation.
Parasitic capacitance seems to be in the ballpark around 500pf, but I haven't done any concrete measurements. I am rather green in the transformer department.
Do you mean 'para-feed' single ended ?
A transformer is a voltage (turns) ratio device. It isn't appropriate to translate an inductance parameter from one side to the other.
With respect to your amplifier it is driving one side (the primary side), and the other side of the transformer is loaded, and the primary winding inductance is a key parameter for determining LF response when assessing an equivalent circuit of your setup. Primary inductance is best measured directly from the primary winding terminals.
A transformer is a voltage (turns) ratio device. It isn't appropriate to translate an inductance parameter from one side to the other.
With respect to your amplifier it is driving one side (the primary side), and the other side of the transformer is loaded, and the primary winding inductance is a key parameter for determining LF response when assessing an equivalent circuit of your setup. Primary inductance is best measured directly from the primary winding terminals.
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The lumped shunt capacitance Cp can be measured using an RC LPF -3dB frequency (F-3dB) test setup, where R is ~5x the driving (plate) impedance of your amp, and R is inserted in series with signal generator driving the winding, with other windings open, and one side of windings connected to ground (0V side of signal generator), along with the transformer core (if accessible).
Measure the winding voltage as the signal generator frequency is increased, which should rise to a maximum in the kHz range, and then reduce frequency back to where voltage is 71% of maximum (the F-3dB frequency).
- Measured capacitance Cp = 1/(2π.F-3dB.R)
- Subtract measurement voltmeter or oscilloscope probe capacitance off the measured capacitance to get the bulk winding capacitance.
Measure the winding voltage as the signal generator frequency is increased, which should rise to a maximum in the kHz range, and then reduce frequency back to where voltage is 71% of maximum (the F-3dB frequency).
- Measured capacitance Cp = 1/(2π.F-3dB.R)
- Subtract measurement voltmeter or oscilloscope probe capacitance off the measured capacitance to get the bulk winding capacitance.
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Yes, parafeed... I have an isolation transformer that can be wired for 240 volt output. I will try measuring the primary directly tomorrow evening. I certainly appreciate the help.
I'd suggest you aim to use just a simple transformer with 5 to 12VAC secondary supply as the signal source for measuring the primary winding inductance. There is no need to use a 240Vac signal source, which is what I think you intended to do.
The inductance will vary with excitation voltage. To provide a comparison with other output transformers, a benchmark voltage level of typically, 5V, or 10 or 20Vrms is commonly used.
Well, at least those are realistic voltage levels for a transformer which is feeding a few Watts to a loudspeaker, which is the intended use I presume.
Measuring at a few hundred millivolts passing a few mA is simply not realistic, measured values may vary WILDLY and yet not describe working performance.
Measuring at a few hundred millivolts passing a few mA is simply not realistic, measured values may vary WILDLY and yet not describe working performance.
Test the primary only, if you measure from the secondary you generate lethal voltages in the primary (so be careful), and see the substantial stray capacitance of the primary transformed to a much larger capacitance viewed from the secondary (in fact the secondary can look capacitive with an unloaded primary even at fairly low frequencies).
Looking from the primary sees only tiny reflected capacitance from the secondary, and its own unmagnified capacitance which won't dominate till much higher frequencies.