i used a 12.6 vac filament transformer with center tap. With line voltage at approx 122.vac the secondaries are about 15vac and 7vac.
Can someone explain why its bucking 15 volts instead of approx 12.6?
Can someone explain why its bucking 15 volts instead of approx 12.6?
A 12.6Vac or other xfmr will deliver the rated voltage only when the specified current is being drawn from it. Draw less current, and the voltage will rise.
Yes, agreed. The load versus no load is the main issue.
But read on . . .
Was this filament transformer rated for primary voltage of 115V?
122/115 = 1.06
12.6 x 1.06 = 13.36V, but only when it is loaded with the specified current.
The loaded current spec is usually into a resistive load.
A brute force solid state diode and extremely large cap power supply, will reduce the voltage more than a resistive load will.
This is the kind of load that will be presented to the Buck supply, not a resistive load.
What are all the ratings and conditions the manufacturer gave for the filament transformer?
Just remember, if you use the filament transformer to buck voltage on a tube amplifier that has the majority of power used by the B+, and the B+ filter is cap input, it will reduce the peak voltage of the buck secondary by more than a resistive load will.
And now we have a measurement problem.
VOM are Average responding, calibrated on a pure sine wave (but a load that is acapacitor input B+ causes truncation of the sine wave).
DMM, some are Peak responding, calibrated on a pure sine wave (but a load that is a capacitor input B+ causes truncation of the sine wave).
DMM, some are True RMS responding, calibrated on a pure sine wave (but a load that is a capacitor input B+ causes truncation of the sine wave).
Due to the 3 above types of meters, and the final shape of the voltage on the filament secondary of the buck transformer that is caused by the amplifier's power supply that is loading the buck winding . . .
You may get 3 different readings if you use all 3 types of meters.
Oh, did I forget, most Power Mains are already slightly truncated, even if you measure them with no load.
Not all is as it appears
But read on . . .
Was this filament transformer rated for primary voltage of 115V?
122/115 = 1.06
12.6 x 1.06 = 13.36V, but only when it is loaded with the specified current.
The loaded current spec is usually into a resistive load.
A brute force solid state diode and extremely large cap power supply, will reduce the voltage more than a resistive load will.
This is the kind of load that will be presented to the Buck supply, not a resistive load.
What are all the ratings and conditions the manufacturer gave for the filament transformer?
Just remember, if you use the filament transformer to buck voltage on a tube amplifier that has the majority of power used by the B+, and the B+ filter is cap input, it will reduce the peak voltage of the buck secondary by more than a resistive load will.
And now we have a measurement problem.
VOM are Average responding, calibrated on a pure sine wave (but a load that is acapacitor input B+ causes truncation of the sine wave).
DMM, some are Peak responding, calibrated on a pure sine wave (but a load that is a capacitor input B+ causes truncation of the sine wave).
DMM, some are True RMS responding, calibrated on a pure sine wave (but a load that is a capacitor input B+ causes truncation of the sine wave).
Due to the 3 above types of meters, and the final shape of the voltage on the filament secondary of the buck transformer that is caused by the amplifier's power supply that is loading the buck winding . . .
You may get 3 different readings if you use all 3 types of meters.
Oh, did I forget, most Power Mains are already slightly truncated, even if you measure them with no load.
Not all is as it appears
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yes thats the issue too, the primary is rated at 115vac @ 4A.
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edit, I replied before the rest of your explanation. Thank you for it, I have more to consider now.
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edit, I replied before the rest of your explanation. Thank you for it, I have more to consider now.
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freddy1,
I had more edits in my Post # 4, after your Post # 5.
You may want to read my Post # 4 again.
The edits:
There are some helpful design considerations there.
There are some helpful measurement considerations there.
There is something helpful about Power Mains there.
Much of this is ignored by the masses.
I had more edits in my Post # 4, after your Post # 5.
You may want to read my Post # 4 again.
The edits:
There are some helpful design considerations there.
There are some helpful measurement considerations there.
There is something helpful about Power Mains there.
Much of this is ignored by the masses.
Because the bucking is only a few volts, it's worth it to use a transformer with a well-oversized low voltage winding. I mean that if you draw 1A at 120V for the bucked load, then use a transformer with a 3 to 4A secondary if space and finance permits. The regulation over the load range should be better.