Tesing secondary impedance.

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RockinRocket
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Tesing secondary impedance.

Post by RockinRocket »

How do I test the secondary impedance of a output transformer. Just want to know and make sure they are what they are supposed to be.
Also all I have is multimeter.
Thanks fellas
R.G.
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Re: Tesing secondary impedance.

Post by R.G. »

I think you've found another of those things that seem like they ought to be simple, but really aren't.

First, contrary to the common notions, transformers don't have impedances, they have RATIOS. They transform (doh!!) one impedance into another one. They do this by transforming both voltages and currents, and wasting nearly no power in doing this.

Let's do an example. Take some hypothetical OT that purports to transform the 4400 ohm plate to plate for a pair of 6L6s to an 8 ohm speaker. What it actually does in transform the primary voltage and current to a much lower voltage and much higher current. Let's say it's rated at 50W. 50W into 8 ohms is the result if we have enough voltage to make the 50W. We know that P = Vsquared times the load impedance, so the voltage needed to do this is the square root of 50 times 8, or the square root of 400, which is 20V rms.

We know the same power is sent in on the primary side, but at 4400 ohms, So 50W equals Vsquared times 4400, and V = 469Vrms.

What we need to make a 4400 to 8 ohm OT is a transformer that makes 20Vrms out of 469Vrms, or one that has a voltage ratio of 469/20 or 23.45 to 1. Notice that the impedance ratio (4400:8) is the square of the voltage ratio of 23.445:1.

And that's how you tell what impedance ratio your OT has: you put a voltage into a winding, measure that input voltage accurately, and then accurately measure the output volltage. The results are then divided - ratioed - and squared. And the result is the impedance ratio of the transformer.

So you can't measure the output impedance of a transformer. But you can measure the impedance transformation ratio if you have a multimeter and a source of AC voltage to measure.

It's also worth mentioning that a transformer impedance as seen at one end depends on what is loading the other end. If you had our hypothetical 4400 to 8 ratio transformer, it is only 4400 on the primary side if you have 8 on the other. If you put 4 on the other, you get 2200 on the primary side; if you put 16, you get 8800 on the primary side. It's a ratio, not an impedance.
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Phil_S
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Re: Tesing secondary impedance.

Post by Phil_S »

What RG says...
On a practical level, use the 5VAC or 6.3VAC filament supply from a power transformer. Feed that to the secondary winding. Measure the measure voltage on the primary winding. Remember to measure the actual voltage on the secondary winding -- don't just assume. Do the math to find the voltage ratio. To find the impedance ratio, it is necessary to make an assumption about the secondary impedance (or you could assume the primary.)

The problem, as I see it, is that it can be difficult to determine the power rating. To really know the answer, you'd want to know the wire gauges used and the actual number of turns. If you can't find the manufacture's record, you have to destroy the transformer to find out and there's no sense in doing that. So, for example, you determine based on voltage ratios it is either 4K:4 or 8K:8. What is it for? You are probably safe with a pair of EL84 at 8K:8 (15-18W), but will that same OT be good for a pair of 6L6 at 4K:4 (30-50W)? The size of the laminations and maybe the weight are the telling factors. You will have to make an educated guess. I'd base that guess on comparison to a known transformer.
R.G.
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Re: Tesing secondary impedance.

Post by R.G. »

What Phil says...
On that power rating thing, though, the only real way to figure the power rating of a transformer is by how hot it gets. Well, or how hot it doesn't get.

For AC mains power transformers, the design space is so driven by economics that you can generally simply weigh them. Manufacturers don't put in any more iron and copper than they have to, so simple weight is a good indicator.

It's more complicated for OT's.Outputs have to be wound funny ways based on audio needs, and they can't always use the iron to the fullest extent, especially for SE designs. So they are not in general so optimized for minimum iron and copper.

For **all** transformers though, internal heating of the wire and layer insulation is a limit.There are well-defined insulation temperature class systems. Like for fuses, the insulation class of the internal materials guarantees that the material will not die up to that temperature. Above that, it will die ... somewhere. See http://www.hammondpowersolutions.com/fa ... nsulation/

Audio output transformers are wound from power transformer materials for all but the most la-de-da hifi snob types. The big boys determine internal temperature by measuring the change in the resistance of the copper wire in the windings. They heat up the trannie by pumping power through it, then take it loose and measure the wire resistance with a high-precision meter. Copper's resistance rises by 0.393% per degree C, so it makes a decent thermometer material if you want to take the time and effort to do that. But audio transformers are limited by considerations of fidelity, not just power handling. So they tend to reach their audio design limits of how much distortion it generates inside the transformer. Expecially for SE designs, the iron must be only partially used ( in terms of its magnetic swing), so it contributes less heating than in full-swing Class AB designs.

It's really quite difficult to put a number on an OT's power rating knowing nothing much about what the manufacturer says about it. About the best you can do is to run audio through it and see how much it heats up. You gradually increase the audio power until you find it's nudging a 105 C temp rise - as Class A/105C insulation materials are the most commonly used ones. Some newer OTs may use Class B/130C materials.
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xtian
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Re: Tesing secondary impedance.

Post by xtian »

R.G. wrote: Fri Jun 23, 2017 2:36 amwasting nearly no power in doing this
Are you talking only about OTs when you say "wasting nearly no power" ? Wondering, because POWER transformers seem to waste "a fair bit" of power. As a typical example, the ClassicTone PT I bought for my #183 was consuming 13 watts all by itself, plugged in to nothing but wall voltage.
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pompeiisneaks
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Re: Tesing secondary impedance.

Post by pompeiisneaks »

I think he's talking about power losses, the circuit you're using, to test it, has to have some kind of load on the other side, at a minimum the volt meter's got some kind of resistor (I think it's often like a 10M resistor) This means it can drop some amperage as it's reading it, and therefore you burn some wattage just testing it. The actual magnetic translation, as I understand it as well, has very little loss, no heat generated etc. The heat generated from transformers is the amount of current being pulled through the copper by the load in the amp itself.

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pdf64
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Re: Tesing secondary impedance.

Post by pdf64 »

xtian wrote: Fri Jun 23, 2017 4:19 pm
R.G. wrote: Fri Jun 23, 2017 2:36 amwasting nearly no power in doing this
Are you talking only about OTs when you say "wasting nearly no power" ? Wondering, because POWER transformers seem to waste "a fair bit" of power. As a typical example, the ClassicTone PT I bought for my #183 was consuming 13 watts all by itself, plugged in to nothing but wall voltage.
What is the full power rating (ie sum of all secondaries at max rated load) of that PT though - 250watts?
5-10% loss seems reasonable.
RockinRocket
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Re: Tesing secondary impedance.

Post by RockinRocket »

Yikes! Probably over my head.
I've always been curious if the manufactures goofs up the secondary wiring codes and wanted to test for my own piece of mind.
Thanks for the great info gentleman.
R.G.
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Re: Tesing secondary impedance.

Post by R.G. »

As they said, "nearly no power" is relative. The no-load power losses in a transformer are composed of
(1) Eddy current losses in the transformer iron; this is minimized by using thin laminations of iron alloyed with silicon to increase its resistivity. Thick laminations of low-silicon iron yield higher eddy current losses. Just-right iron alloys are more expensive, and so is the labor to stack many thin laminations.
(2) The flux density/voltage selected by the designer; if this runs into the soft beginnings of saturation B-H curve, the primary inductance goes down at high densities, and increased magnetizing current flows for portions of each cycle.This is expressed as core heating and as resistive losses in the wires
(3) The normal, non-excessive magnetizing current in the core. This can be reduced to a nearly arbitrarily small level by using huge cores of high quality iron and many, many turns of copper to run the magnetizing current down. Of course, you pay for big iron and big copper, just as in (1).

So transformer makers strike a balance between cost of manufacture versus low losses. 95% to 98% efficiency based on full load is pretty reasonable. Switching power supplies have issues getting to that. Of course, the no-load efficiency is zero, just as it is for any power supply.

Two to five watts per hundred transformed isn't all that bad.
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martin manning
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Re: Tesing secondary impedance.

Post by martin manning »

RockinRocket wrote: Sat Jun 24, 2017 12:13 am I've always been curious if the manufactures goofs up the secondary wiring codes and wanted to test for my own piece of mind.
That's not hard... put some AC on the primary and measure the voltage on the primary and on the secondary from the common lead to each tap. You should see increasing voltage from 4 to 8 and from 8 to 16, by a factor of 1.41 each time the secondary impedance doubles. A reasonable check of the primary impedance can be made by computing the voltage ratio Vpri/Vsec, squaring that and then multiplying that result by the listed value of the secondary tap on which you measured Vsec.
tubeswell
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Re: Tesing secondary impedance.

Post by tubeswell »

When testing OTs (or, for that matter, reverb transformers) in an amp, you can simply unhook all the transformer windings and use a couple of insulated gator clip leads to hook the secondary winding ends up to the 6.3VAC heater supply on your PT. Then flip the power switch on and measure the VAC across the primary winding ends. (Take care to make sure the winding ends don't make contact with each other, or with anything else, when you do this test).

The (ratio of) (Pr:Sec VAC)^2, = the Pr:Sec impedance ratio
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