Remember...reflected impedance figures are CALCULATED...not "measured". Turns ratio never changes, therefore impedance ratio never changes either. Yet in your chart the turns ratio keeps changing. Why is this?
You have a voltage transfer ratio of 24.35, which constitutes a turns ratio of 24.35. Your impedance ratio would be -
24.35 ^ 2 = 593
Now...impedance ratio is the turns ratio squared.Therefore, your chart should have looked more like this -
Unless you know something about valve amplifiers, it would seriously be in your best interests to refrain from posting on this thread. Seriously.Code:Secondary Load Reflected Primary Impedance 1R 593R 2R 1186R 4R 2372R 8R 4744R 16R 9488R 32R 18976R 64R 37952R 128R 75904R
Also, you seem to be saying both that the transformer impedance ratio is constant, but also that it is not constant. So which is it?
Note that if in the above example if we increase the output load to 16 ohms we can only use the transformer for 4 watts not its rated 8 watts, so we can no longer use the transformer as effectively. Also, if we try to use it at the full 8 watts we'd have to increase the input voltage which would either burn up the transformer or at best create very bad odd harmonic distortion.
did you notice that at 8 ohms, the results of the impedance ratio calculation closely matched the measured voltage ratio? the reflected primary impedance with a secondary load of 8 ohms was also very close to 5k.
Not sure where I appeared to state that impedance ratio was not constant. Zratio = Tratio ^ 2. Since turns ratio is constant, so is Zratio.
Now...this "ideal vs real world" stuff really needs to go away. While there is SOME variance between the two, it's not really enough to cause huge concern and create the confusion that it does.
That's not the point. The point is that you said that the wattage wasnt important and that you can change the impedance to whatever you wanted it to be. I showed you that there are negative consequences to this and that we need to know the wattage of the transformer.If we double the load impedance on the secondary, the load impedance also doubles on the primary so theoretically this will halve the output power. However...the load line and where it crosses the plate/grid characteristic curves on a given valve type also comes into play on this. If the power transfer ratio is not kept the same, it won't be exactly halved.
In regards to how "effectively" we use a given transformer, this all depends on what kind of power transfer ratio you're after.
The point is not to "accurately halve the output power". The point is to use the transformer core metal mass and winding copper content as effectively as possible. By doubling the output load impedance on a transformer designed for non doubling (ie 8 ohms vs 16 ohms) we can not use the transformer as effectively. We can not ignore the Transformer Equation.A common trick we do on parallel push-pull valve amps is to double the load impedance, then remove either the outer or the inner pair of valves from the circuit. This accurately halves the output power as it keeps the load line crossing the plate curves at the same place it crossed it with both valves installed and the correct load impedance so the power transfer ratio remains the same.
Remember...reflected impedance figures are CALCULATED...not "measured". Turns ratio never changes, therefore impedance ratio never changes either. Yet in your chart the turns ratio keeps changing. Why is this?
You have a voltage transfer ratio of 24.35, which constitutes a turns ratio of 24.35. Your impedance ratio would be -
24.35 ^ 2 = 593
Now...impedance ratio is the turns ratio squared.Therefore, your chart should have looked more like this -
Code:Secondary Load Reflected Primary Impedance 1R 593R 2R 1186R 4R 2372R 8R 4744R 16R 9488R 32R 18976R 64R 37952R 128R 75904R
Ok then i guess i can toss real vs ideal out the window and use one of those audio transformers on the input to my scope which has a 1 megohm input impedance, so i can boost the input impedance to 600 megohms for testing audio circuits, right? That's nice i think i'll hook up by 1000watt audio transformer with 8 ohm characteristic output and 600:1 impedance ratio right now so i can start testing my audio pre amp stuff with a nice near 1 gigaohm probe
That's not the point. The point is that you said that the wattage wasnt important and that you can change the impedance to whatever you wanted it to be. I showed you that there are negative consequences to this and that we need to know the wattage of the transformer.
The point is not to "accurately halve the output power". The point is to use the transformer core metal mass and winding copper content as effectively as possible. By doubling the output load impedance on a transformer designed for non doubling (ie 8 ohms vs 16 ohms) we can not use the transformer as effectively. We can not ignore the Transformer Equation.
We dont care about valve amplifiers here, we only care about transformers
You're completely missing the point. Stop thinking impedance and start thinking VOLTAGE & CURRENT. It will make lots more sense that way.
In no post did I EVER state that wattage was not important. While load power consumption can be run BELOW what the transformer is rated at, you never want to exceed the transformer's power rating.
Depends on the application.
So true or false, i can use the 1000 watt audio transformer on the input of my scope so i can perform tests on my audio pre amp stages now having a 600 megohm input probe?
Now you are saying that wattage is 'somewhat' important? The wattage is important not ONLY because of the transformers power rating. It's also about induction limits.
Depends on the application? What does? Transformer cost is or at least should be a concern in most but maybe the lowest power applications.
**broken link removed** Originally Posted by colin55
I am just saying you need to know the wattage of a transformer to see if it is suitable for an application.
It looks like he wants an output transformer.
Unless you know something about valve amplifiers, it would seriously be in your best interests to refrain from posting on this thread. Seriously.
I used my vacuum tube amplifier for two years. It played very well for two or three months then its output tubes needed replacement over and over to keep the distortion low. I still have the solid state receiver (HH Scott) I replaced it with about 47 years ago and it still works perfectly.
I found this in a book. "The primary impedance of a transformer as it appears to the source of power is determined wholly by the load connected to the secondary and by the transformer turns ratio."
Np/Ns = Square root of Zp/Zs
Np = number of turns in the primary.
Ns = number of turns in the secondary.
Zp = impedance of the primary.
Zs = impedance of the secondary.
Example.
Zp = 5000 ohms and Zs = 8 ohms.
Ratio = Square root of 5000/8 = 25
Turns ratio is 25 to 1
This means the transformer can have 25 turns on the primary and 1 turn on the secondary OR 300 turns on the primary and 12 turns on the seconday OR many other choices.
The book says, the design of the transformer has to be determined by its needs and by testing to get frequency responce, distortion, etc. that you want. It says, if a transformer is designed to work best at mid range frequencys it will not work as well on low frequencies or high frequencies. The number of turns on the primary determines the frequency and frequency range of the transformer.
===============================================================================
Lets assume you want to find the impedance of an unknown audio transformer. Put 10 VAC on the 8 ohm secondary and you get 250 VAC on the primary.
Turns ratio = Np/Ns = 250/10 = 25 which means a ratio of 25 to 1
Primary Impedance = Ratio x ratio x secondary ohms = 25 x 25 x 8 = 5000 ohms.
It should be understood that this statement, without further qualification, is only true for an ideal transformer.
The last part in red is something that ideal transformers can do without any effect on the transformer's performance, and that real transformers cannot.
This contradicts the earlier statements which I pointed out only apply to ideal transformers. If those statements applied without limitation to real transformers, then why would it be true that "if a transformer is designed to work best at mid range frequencys it will not work as well on low frequencies or high frequencies. The number of turns on the primary determines the frequency and frequency range of the transformer."?
The book goes into great detail explaining this I did not want to type a novel so I made it short. I will try to explain this better but you need to read the book. The number of turns on the primary determines a lot of things, you can make the transformer work better at low frequency, better a mid range or better at high frequency but you can not have your cake and eat it too. The ideal transformer would work best at any frequency but the real transformer can not do that. If there are formulas to determines the number of turns on the primary to get a certain frequency the transformer operates best at I have not read that yet. The way I understand it is you wind a transformer with 300 turns on the primary then test it to see how it works. Then build 2 more transformers one with 275 turns and another with 325 turns and test them and so on. You may have to build several more transformers and test them to find the one that suits your needs for the application. I would assume if your needs involves a lot of low frequencies then you would want the transformer to operate best at low frequency. The ideal transformer would be 3 totally seperate transformers each one designed to operate best at low, medium and high frequency with 3 seperate sets of speakers for low, medium and high frequency.
It is imprecise to use the phrase "the impedance of an unknown audio transformer" in engineering discussion. In the situation you have described, what you want to find is "the apparent impedance at the primary winding with a specified impedance connected to the secondary."
An unknown transformer is a transformer that you removed from some old junk equipment and you know nothing about it other than it is an audio transformer, you will probably also know how it was used and the tubes it was used with but the RCA and GE tube manual is not going to tell you the impedance of the transformer. Now you need to do some tests to determine what this transformer turns ratio and impedence is.
As I showed in the measurements of post #12, it is not true for all load impedances that "The primary impedance of a transformer as it appears to the source of power is determined wholly by the load connected to the secondary and by the transformer turns ratio." if the transformer is a real transformer.
There is a load impedance "range", where the turns ratio squared closely determines the impedance ratio, and when the load impedances are greater or less than this optimum range, the impedance ratio is no longer as close to the squared turns ratio. The further the load impedance is from the optimum, the further the actual impedance ratio deviates from the square of the turns ratio.
You have described a situation where the rated secondary load impedance is known. In that case, the voltage ratio can be measured and the apparent primary impedance will most likely be determined by the square of the voltage ratio when the rated load impedance is connected to the secondary. We assume that the manufacturer has designed the transformer so that the optimum impedance range is the one specified. The optimum range is the range where the impedance ratio is nearly the same as the square of the voltage ratio.
But, suppose you have a completely unknown transformer? How can we determine the optimum impedance range at primary and secondary for that transformer? This is a problem not well understood by non-specialists. Many people think the DC resistance of the windings is approximately equal to the "rated" impedance of the windings. This is a topic that is probably worthy of a separate thread.
An unknown transformer is a transformer that you removed from some old junk equipment and you know nothing about it other than it is an audio transformer, you will probably also know how it was used and the tubes it was used with but the RCA and GE tube manual is not going to tell you the impedance of the transformer. Now you need to do some tests to determine what this transformer turns ratio and impedence is.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?