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Old 9th March 2011, 07:11 PM   #10881
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Hey Mike!

Quote:
Originally Posted by hitsware View Post
> But of course an ideal transformer has no reactances.

Are you sure about that ?
Quite sure. The key word being "ideal."

An ideal transformer simply reflects impedances.

If you take an ideal 1:1 transformer and put a purely resistive 1k load on its secondary, then when looking into the transformer's primary, you'll see a 1k, purely resistive load. No reactances.

The reactances seen in a realworld transformer are parasitic. Namely, the non-infinite primary inductance, leakage inductance (due to non-ideal coupling between primary and secondary) and winding capacitances.

se
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Last edited by Steve Eddy; 9th March 2011 at 07:16 PM.
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Old 9th March 2011, 07:12 PM   #10882
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Quote:
Originally Posted by janneman View Post
There'll be another chance
Oh alright. But don't take too long.

se
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Old 9th March 2011, 07:15 PM   #10883
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Quote:
Originally Posted by Steve Eddy View Post
An ideal transformer simply reflects impedances.

If you take an ideal 1:1 transformer and put a purely resistive 1k load on its secondary, then when looking into the transformer's primary, you'll see a 1k, purely resistive load. No reactances.
Even at DC, I would guess
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Old 9th March 2011, 07:21 PM   #10884
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For everyone who might what to understand Q in the context of transformers. Let me go through what I learned in found, so many decades ago, when at Ampex.
I was assigned to evaluate tape heads and transformers that were used with them.
It is easiest to start with a conventional analog magnetic tape head.
This tape pick-up is composed of some sort of mumetal like material, often slightly compromised toward more hardness to keep wear to a minimum. Then a coil is wound on the core of the head usually between 1mH and 1H. More than one H is exceptional, because the wire gets really tiny and the head will self resonate below 20KHZ typically.
1mH is only used for record heads, in general, because the output is so low, that a high ratio transformer is mandatory to get the level up for best S/N.
For this example, let us take a 1H head or equivalently a 10mH head and a 10:1 transformer. These two examples are remarkably equivalent.
Now, let us first concentrate on the 1H head, and let us say that we find an equivalent series resistance of 1Kohm. What is the Q of the head at 1KHz?
Well: 6.28K/1K = Q = 6.28. What about 10K? Then 62.8K/1K (ideally) = 62.8
20K? 125.6K/1K = 125+ Wow!
Now what do we usually measure? Instead of 125, we might get 5 on a good day, with a good head. What happened? Eddy current loss. AND what happened to the self noise of the tape head? It increased significantly with frequency proportional to the difference between the IDEAL Q and the REAL Q of the head.
Now, what IF you made a transformer of essentially the same material in the same way? After all, a transformer is made of 2 coils, the secondary usually being very much like the 1H head. Could there be a similarity in behavior? I should think so, and I measured it, long ago.
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Old 9th March 2011, 07:23 PM   #10885
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Quote:
Originally Posted by PMA View Post
Even at DC, I would guess
Theoretically DC doesn't exist as the life of the universe is finite. Theoretically on average if you have one foot on fire and the other frozen in ice you are comfortable.

Ain't theory wonderful! Shame reality intrudes, then we actually have to use knowledge and skill to get the results we aim for.
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Old 9th March 2011, 07:33 PM   #10886
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"
Wattage 0.5W
Input Maximum Voltage 10Vrms
Turns Ratio 1:1
Primary (input) Impedance 600 Ohms
Primary (input) DCR 64 Ohms
Primary (input) Inductance 3H
Secondary (output) Impedance 600 Ohms
Secondary (output) DCR 68 Ohms
Frequency Response 20~20K Hz., <1dBu
THD+Noise <0.05% @ 1K Hz.
Plot Frequency Response/THD+N
Insertion Loss 0.5 dB
Bobbin Material Glass Filled Nylon 6/6
Flamability Rating Class B 130C
Core Material M6 29 ga. line grain oriented steel
Termination 0.187" (3/16") quick disconnects
Mounting Zinc plated steel channel/frame
Weight 0.4 lbs.
"
3 Henries ...... That's a leaky one alright.
And 64 Ohms leaked up to 600 Ohms ...
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Old 9th March 2011, 07:51 PM   #10887
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Quote:
Originally Posted by PMA View Post
Even at DC, I would guess
Sure.

They work in Washington the same as they work anywhere else.

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Old 9th March 2011, 07:53 PM   #10888
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Quote:
Originally Posted by DF96 View Post
This is not excess noise, as it is all thermal in origin. I am still waiting to hear of any sources of excess noise in a transformer.

When I test coils around high permeability laminated cores, my HP 4284A calls eddy current losses "resistance" I use the Ls-Rs model.

So if those losses are resistive by nature, then the noise generated by the resistive losses of eddies would be a modulated noise. No signal, no noise..

Cheers, John
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Old 9th March 2011, 08:08 PM   #10889
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Quote:
Originally Posted by hitsware View Post
"
Primary (input) Inductance 3H.....

3 Henries ...... That's a leaky one alright.
...
Why is 3 henries "leaky"? I must have missed something..

What about 680 henries?

Cheers, John
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Old 9th March 2011, 08:22 PM   #10890
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JN, what does this have to do about the discussion in hand?
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