Transformer basics

[Gandledorf]

As part of an effort to increase my knowledge of analog electronics, I've been reading up on the various principles behind the different components, and working out circuits as exercises, then trying to find the parts to build them with. One problem I am having is transformers. I understand the theory well enough, but this really hasn't given me the ability to look at a transformer in a catalogue, and determine what it does.

For instance, say I want to use an audio transformer for whatever reason. The only information given on these devices are Primary and secondary "Matching Impedence" ratings. (Ex: 10,000 CT 2,000 CT)

If I apply a 5Vpp square wave to the primary, what comes out of the secondary? How does choosing taps affect this? Is there a general formula I can use to determine these properties? What if it is rated as 10,000 CT 10,000 CT?

Thanks for all of your help!

 

[Gandledorf]

Ok, I think I've straightened things out, if only a bit. For a transformer with three leads per coil (center tap + two ends), and the following impedence: 1kCT Primary, 2kCT Secondary, the turns ratio is 1:2, and thus the ratio of voltage will be 1:2, and current will be 2:1. So 5Vpp @ 500mA, produces 10Vpp @ 250mA, correct?

Does this also mean that 1kCT, 2kCT is exactly identical to 10kCT, 20kCT, or for that matter, 20kCT, 10kCT? For that matter, if it is 1kCT, 2kCT, this means using the two ends would produce 2kCT, 4kCT, correct? Using the center tap of the primary, and either end, and both ends of the secondary would then produce 1kCT, 4kCT, correct?

 

[B-o-b]

Transformers

Hi Gandledorf,

Actually, impedances in transformers (mostly audio tansformers) indicate a function of voltage and current... that's about it. Altho at 1st, it may be a bit difficult to grasp these concepts, it is similar in a lot of ways to the understanding of DC circuits. Again, DC circuits use voltage and current in certain operating conditions. When it comes to audio (or AC), these fundamental principles still apply, but with certain caveats, such as reactances to certain frequencies and such. That's about the only differences between AC and DC. Remember- audio is considered "AC".

As to yer question about a 1kCT, 2kCT being exactly identical to 10kCT, 20kCT transformer, that would probably only be accurate in terms of what we call "Ratio". Again, impedances, such as the ones U'd mentioned, are different than ratios. Altho we do use them togeher sometimes, as in, to determining the turns ratio of primary to secondary of a transformer.

On the other hand, I may not fully understand yer question as to using the different imedance taps (also expressed in ohms) on a transformer.

Again, I assume U are referring to an audio transformer. I can tell U, that if U have a transformer with an impedance of, say, a 1KCT (1000 ohm with center-tap), either primary or secondary, this would be the impedance (NOT the DC resistance) across the entire winding- disregarding the center-tap. Now if U regard the center-tap in this case, then the impedance would be 500 ohms from either side of the end windings to the center-tap. Clear as mud now? (hehe)

In terms of comprehending how impedance (in ohms) works, it might help U to understand how resistors work. There are basic similarities, but then there are some "Got'chas" too. But both are perfectly explainable... just not in this short of space tho. Hope this helped U a bit. Lemme know.

Cheerz!

Bob
bob4analog@yahoo.com

 

[ukeee]

A transformer will have different specifications depending on its use, an audio transformer is used to match impedances. So the specifications given for it will concern how it matches. I don't really know much about audio/impedance matching transformers so I can't help you with what these particular specifications are. However for more general purpose transistors (for example a mains transformer) there are a few key specifications:-

VA rating this is the maximum total power the amplifier is capable of handling and is simply the voltage multiplied by the current. It is an average value not a peak value

Turns ratio this is the number of turn on the primary compared to the secondary, for example a transformer with a turns ratio of 10:1 with a 10V input would give an output of 1V. The current through the primary will then by ten times smaller than the current through the secondary.

Hope this helps you,

 

[Russlk]

The impedance of a winding is proportional to the square of the number of turns, so the centertap, having 1/2 the turns will have 1/4 the impedance. Similarly, the turns ratio, which is the same as the voltage ratio, will be the square root of the impedance ratio.

 

[jem]

There is a lot more to transformers than simply turns ratio. Impedance ratio is related to turns ratio as Russlk mentioned. One such factor is core meterial (its permeability determines the minimum number of turns needed to get a given magnetizing inductance, and its type and size determined how it saturates. The magnetizing inductance determines (among other things) how it would behave when, for example, you try to pass a square wave. For example, you might find that your transformer which happily transforms a 100Hz sinusoidal wave does not work quite as well when trying a 95% duty cycle 100 Hz square wave. The transformer will show the edges of the wave, but deep into the high portion, ti might consider the signal as DC (unchanging).

Also, in order to get wide frequency range you need to limit parasitics such as leakage inductance and interwinding capacitance. All of these are affected by construction, and selection of core type and shape.

Finally consider that a transformer with 2000 turns primary, and 200 turns secondary (turns ratio 10:1) will happily transform 120V 60Hz to 12 V. Reducing the number of turns to 10 and 1 will probably cause a short circuit on the 120V line, even though the turns ratio is still 10:1.

 

[laroche73]

transformers

this is one of those threads where I'm content to sit back and (re)learn a few things. Thanks to Gandledorf for bringing it up.

 

[Gandledorf]

Thanks for all of the useful information guys. One thing I'm still not clear with, in transformer impedence. I assume this acts as the imput impedence that any load on the transformer will "see"? Does this effect current at all? Say I have a 4.6kCT Primary, 20CT Secondary transformer. This means the turns ratio is 230:1, correct (I think I have this part down). So if I supply a square wave, 5Vpp @ 20mA to the Primary, the secondary should show a 1150Vpp @ .087mA, correct? Thus, if I attach some load to it, it sees an input impedence of 20ohms, meaning it's easy for us to use this source, as we just need an impedence on our load of >> 20ohms. Right?

Is this the effect of the different impedences, or does it also have some effect on the current in the secondary and primary coils?

 

[jem]

Actually, if the impedance ratio is 230, then the voltage ratio is the square root of that, or about 15 in the case you mentioned. So, 5 V at the primary will give you about 0.33V at the secondary. Of course, the current will then be 15 times more on the secondary.

One way to look at it is to assume an ideal transformer. Such a transformer cannot create energy or give you more power then you put in. Since it is ideal, it does not consume any power as well. It simply reflects the load connected to the secondary to the primary. If the turns ratio is N, and your primary voltage is Vp and primary current is Ip, then the secondary voltage will be:

Vs = Vp/N.

Since power is not created nor used, the primary power must be equal to the secondary power. In other words, Vp.Ip must be equal to Vs. Is (Is is secondary current), assuming the proper load on the secondary. Now since Vs is Vp/N, it follows that Is must be N. Ip.

From an impedance point of view, Zp = Vp/Ip.
and Zs = Vs/Is.

Therefore, Zp/Zs = (Vp/Ip) / (Vs/Is) = (Vp/Ip) / [(Vp/N)/NIp]

This works out as: Zp/Zs = N^2 (the square of the turns ratio).