Primary impedance values for OTs

[jameshanly]

Hello, I have a quick question...when a manufacturers states the primary impedance of an OT do I assumed that figure was derived from an 8ohm load on the secondary? Is that a convention? It's often not mentioned how the PI value was derived from the impedance ratio (ie, what secondary load was used as the multiplier) for OTs with multiple taps on the secondary.

TIA!

[Roe]

no its just a reflected primary impedance or ratio, dependent on the load on the secondary

[jameshanly]

Thanks Roe, think I got myself confused with some measurements because I was measuring the taps unloaded

Appreciate the response.

[martin manning]

Hello, I have a quick question...when a manufacturers states the primary impedance of an OT do I assumed that figure was derived from an 8ohm load on the secondary? Is that a convention?

With an 8 ohm load on an 8 ohm secondary, the primary impedance will be the spec value. That relationship holds if the secondary impedance is something other than 8 ohms, or if there are multiple secondary taps, i.e. the primary impedance will be as specified regardless of which tap the load is on, as long as the load matches the stated impedance for the tap.

When determining primary impedance by applying a test voltage and measuring primary to secondary voltage ratio, you can do that “unloaded” (the input impedance of your DMM is a load), just use the stated impedance for the secondary tap on which you’re measuring the secondary voltage as the multiplier on the square of the voltage ratio. You can get a better result by connecting an appropriate load to the secondary. Doing this changes the measured voltage ratio slightly due to the current flowing in the coils.

[Helmholtz]

.when a manufacturers states the primary impedance of an OT do I assumed that figure was derived from an 8ohm load on the secondary? Is that a convention? It's often not mentioned how the PI value was derived from the impedance ratio (ie, what secondary load was used as the multiplier) for OTs with multiple taps on the secondary.

Primary impedance will be the same no matter which secondary you use.
Reason is that each tap has its own impedance ratio to the primary, so that the different secondary impedances reflect to the same primary impedance.

[R.G.]

They're right - it's about ratios, not resistance.

Transformers don't have impedances, not in the way we normally think of it. They have only ratios and frequency responses.
The manufacturers "simplified" what they say about transformers so people didn't have to understand as much. Instead of saying "this transformer will give a low frequency response down to X Hz when loaded with a Y ohms resistor load; and when this is done, the primary end to end impedance will b Z ohms", they just shortened it over time to saying "Z to Y" and more recently just Z ohms.

As Martin notes, you can find the voltage ratio by carefully measuring the primary volts to secondary volts ratio with the secondary unloaded, then squaring that ratio, and you have the impedance ratio. That's the impedance ratio for all frequencies above the low end rolloff, and below the high end rolloff caused by leakage inductance and interwinding capacitance.

For a multi-tapped output, the primary impedance is as stated ONLY when one and only one resistor of the designed output impedance loads the secondary. Examples:
A 4K:8ohm transformer presents 4K to the primary only when there is an 8 ohm resistor across the secondary.
A 4K primary with a secondary with 4, 8 and 16 ohm taps is 4K when a 4 ohm is from the 4 ohm tap to common and the other two are unloaded. It's 8K if you put an 8 ohm resistor on the 4 ohm tap.
Same transformer, 4K to 4,8,16 taps, has a 4K primary with a 16 ohm on the 16 to common taps. An 8 ohm resistor from 16 ohm tap to common will present a 2K load on the primary.
Bottom line: it's only the specified primary impedance when the correct, matching resistor load is on the correct secondary tap. It's not always 8 ohms.

[Ten Over]

For a multi-tapped output, the primary impedance is as stated ONLY when one and only one resistor of the designed output impedance loads the secondary.

What if you loaded the eight Ohm tap with a sixteen Ohm load while also loading the four Ohm tap with an eight Ohm load?

[R.G.]

What if you loaded the eight Ohm tap with a sixteen Ohm load while also loading the four Ohm tap with an eight Ohm load?

Off the top of my head, four ohm tap would reflect a 16 ohm load to the primary, and the eight ohm tap would reflect a 16, appearing in parallel. So.. nominally right for the primary load. I will need to think some about this to be sure for all cases. There are several ways "taps" could be wound and presented outside the trannie.

Push-pull output stages are pretty forgiving of 2:1 mismatches either direction, so it's unlikely that it would be disastrous.

[martin manning]

I think Ten Over is impishly providing an example of a configuration that presents nominal primary impedance while violating your claim that "For a multi-tapped output, the primary impedance is as stated ONLY when one and only one resistor of the designed output impedance loads the secondary."

[R.G.]

Probably.
I was, admittedly, oversimplifying for the audience.

[solderhead]

For a multi-tapped output, the primary impedance is as stated ONLY when one and only one resistor of the designed output impedance loads the secondary.

What if you loaded the eight Ohm tap with a sixteen Ohm load while also loading the four Ohm tap with an eight Ohm load?

You have to follow good engineering practices. If you intentionally violate them and load the 8R tap with 16R you can still get the right answer -- you just have to fix the math in your calculations. If you account for your deviation in test conditions you will still get the right answer. OTOH if you deliberately set yourself up with data that's garbage-in, you should expect to get garbage-out.

[Ten Over]

...garbage-in, you should expect to get garbage-out.

I believe that Dr. Emmett Lathrop Brown proved that one to be a total fallacy.

[B Ingram]

...

[SoulFetish]

...garbage-in, you should expect to get garbage-out.

I believe that Dr. Emmett Lathrop Brown proved that one to be a total fallacy.

Mr. Fusion?