Guys,
Inject 6VAC into the output transformer Primary winding and I get the readings as follows:
Primary winding = 5.8V
secondary winding = 0.25V
turn ratio = 5.8/0.25
= 23 : 1
impedance = 23X23
= 529 X 8 ohms
=4.2K
But the output transformer stated as 3.5K. Is the calculations or steps correct?
Inject 6VAC into the output transformer Primary winding and I get the readings as follows:
Primary winding = 5.8V
secondary winding = 0.25V
turn ratio = 5.8/0.25
= 23 : 1
impedance = 23X23
= 529 X 8 ohms
=4.2K
But the output transformer stated as 3.5K. Is the calculations or steps correct?
That's pretty close, so OK measurements.
Its difficult to get better 'accuracy' unless there is a separate winding to use for 'driving'.
Its difficult to get better 'accuracy' unless there is a separate winding to use for 'driving'.
Resolution and accuracy at 0.25VAC will be limited, try a higher voltage and see if you get the same ratio - reversing the transformer is OK if you're careful (> 120V!).
Winding resistance is ignored in this method - to include its effect, you can terminate the load side (which means you'll need to deliver POWER to it - probably OK at 60 Hz, but may not be practical with an audio generator as source). Or you can just add twice the primary DC resistance to your calculated impedance (which assumes that the reflected secondary loss is similar to the primary - a reasonable assumption).
Winding resistance is ignored in this method - to include its effect, you can terminate the load side (which means you'll need to deliver POWER to it - probably OK at 60 Hz, but may not be practical with an audio generator as source). Or you can just add twice the primary DC resistance to your calculated impedance (which assumes that the reflected secondary loss is similar to the primary - a reasonable assumption).
Measuring low voltage A/C has always been 'hard' with simple meter systems. Back in the analog meter days, one couldn't measure A/C below about 0.6 V at all, because of forward diode voltage drop. VTVMs (Vacuum Tube Volt Meters) could get down to 0.05 V or better because they not only amplified the input, but also used vacuum tube rectification, which does substantially better at low voltage. Modern high-input-impedance meters (even fairly cheap ones…) can do pretty well, because they too use an amplified input architecture. Nevertheless, we learn simply to seldom trust low voltage A/C readings, unless some calibration has been done before measurement (of the meter!)
Since your output transformer is likely rated for watts of output, using the formula
P = I E combined with
E = I R then
P = E²/R (with some substitution and rearrangement)
Now, substitute in an appropriately conjured P … say 10 watts? and R … say 8 Ω
10 = E²/8
80 = E²
9 = E
OK, now using your presently found 23:1 ratio
9 × 23 = 200+ volts
So, theoretically - even with this output transformer - one could use 110 VAC wall current to drive the primary side. Be careful! At least though you will get a much higher secondary A/C voltage. Transformer damage likelihood is near-zero.
GoatGuy
PS: One could have used the 3,500 ohms as well with the power equation, and gotten nearly the same answer:
10 = E²/3500
35,000 = E²
187 = E
Since your output transformer is likely rated for watts of output, using the formula
P = I E combined with
E = I R then
P = E²/R (with some substitution and rearrangement)
Now, substitute in an appropriately conjured P … say 10 watts? and R … say 8 Ω
10 = E²/8
80 = E²
9 = E
OK, now using your presently found 23:1 ratio
9 × 23 = 200+ volts
So, theoretically - even with this output transformer - one could use 110 VAC wall current to drive the primary side. Be careful! At least though you will get a much higher secondary A/C voltage. Transformer damage likelihood is near-zero.
GoatGuy
PS: One could have used the 3,500 ohms as well with the power equation, and gotten nearly the same answer:
10 = E²/3500
35,000 = E²
187 = E
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Nothing wrong here. The apparent discrepancy between the measurement and the spec is due to the fact that an "8 Ohm" OPT (or secondary tap) is frequently designed for a 6R0 load. For speakers, "8R" refers to the DC resistance of the voice coil winding. In actual operation, the impedance is likely to be anything but 8.0 + j0 Ohms. It can go much higher, lower, and with widely varying phase. For dealing with that, OPT secondaries are frequently designed for a 6.0R load. Sometimes, the spec sheet will say so, and sometimes it won't.
That's just one of those things about audio design: those nice simple straight loadlines are very much approximations and over simplifications. You only see this in RF design where you can always tune out any reactive components of the load, and you're operating over a narrow band of frequencies.
Actual measuring + calculating beats any "label" any time of the day.
That said, AC low voltage measurement is in general not too precise.
If you used one of those cheap but popular meters which typically have only 2 AC scales (say 200V and 700V or so) they can't be trusted below, say, 12VAC.
Those having scales reaching 200mV full scale are better, because they incorporate internal preamps and active rectification.
In any case, try to apply a higher voltage to the primary: anything from 24VAC to, say, 60VAC from some SS transformer available is better.
Mains frequency (50/60 Hz) is fine.
That said, AC low voltage measurement is in general not too precise.
If you used one of those cheap but popular meters which typically have only 2 AC scales (say 200V and 700V or so) they can't be trusted below, say, 12VAC.
Those having scales reaching 200mV full scale are better, because they incorporate internal preamps and active rectification.
In any case, try to apply a higher voltage to the primary: anything from 24VAC to, say, 60VAC from some SS transformer available is better.
Mains frequency (50/60 Hz) is fine.
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